Your Bike’s Charging System – Regulator/Rectifiers

*** UPDATED JAN 2021 ***

Updated to include views and opinions on the newly launched Tri-Spark MOSFET reg/rec.

Table of Contents

This article is broken down into sections – you can skip to them by clicking below:


How a charging system on a bike is specced out and works seems to be a really emotive topic – certainly on a par with wetsumping and engine breathing.

There are a lot of strong opinions out there – unfortunately many of them are incorrect as they have been formed over many years based on assumption, hearsay and sales spiel.

For the record, I am not a salesman – I am just a techie – I trained as an electro-mechanical engineer, before moving into electronics and communications and finally settling in computing.
I am a member of the IEEE (the Institute of Electrical and Electronics Engineers, who are the world’s largest professional association for the advancement of technology) and the IET (the Institute of Engineering and Technology who were formed when the Institute of Electrical Engineers were merged with the Institution of Incorporated Engineers in 2006). Importantly, I guess, I can drive a soldering iron and diagnose an issue rather than look it up on YouTube and declare I’m an expert. I’m not an expert, I’m learning every day!

I have never made a cent from the time I spend helping other people or from the hundreds of hours I spend documenting and drawing diagrams for our community and for individuals.

I do not sell products and am not linked to any manufacturer – any recommendation I make is based on my own research, findings and experience.

What you will find in this article is based on information I have obtained from several manufacturers, their specs, their documentation and discussions I have had with their staff. Sadly, some are no longer with us, such is the way with this industry.

Where I do share my personal opinion, it is based on my own findings either on a bike, on the bench or through helping others – so I would like to think I am quite well informed.

I have obtained information from the following, and pass on my sincere thanks for putting up with my incessant questioning:

In addition, I have used data from the following:

Hopefully these sources will help put together an interesting article, that clarifies some points.


This article sets out to provide some information around regulator/rectifier options for British bikes – where possible I have tried to break things down to make them simpler to understand, and explain terminology.

There is no information here that I haven’t presented or discussed at length across the various groups or forums before, but I guess this is the first time I have put it all in one place.

We are discussing charging systems that are built around permanent magnet alternators, so this does not cover alternators with field wound rotors or bikes that use a dynamo.

This also does not cover more modern (often Japanese) bikes where many are either using field wound rotors (so can reduce the alternator output depending on the demand) or are switching in and out additional alternator stator windings based on the power demand (just like Lucas used to do with their ET alternators on the light switch back in the early days)

The article is quite lengthy, but my guess is that most people will want to skip to the diagrams.

My drawings of regulator/rectifiers shows how the regulator and rectifier circuits work and how the components are configured to perform those functions.

The ‘control circuit’ that I reference in some diagrams will vary from one manufacturer to another, and also over different periods of time. This can range from a simple zener diode (in the case of something like a Tympanium, to quite an advanced integrated circuit in the case of the latest and greatest MOSFET designs.

Different manufacturers will use slightly different chips, some will use resistors and capacitors, others have RFI chokes to help with interference (some don’t but maybe should have) and some even have thermal protection circuits integrated.

In addition, some manufacturers offer units with a larger capacitor integrated into the unit, to allow a battery to be eliminated from a bike if you wish.

More Power Back Then

Most brit bikes of the era used the Lucas RM21 10-amp single phase alternator stator.

Grant Tiller

The majority of coil ignition bikes of the time consumed about the same amount of power, so this was a good fit in most scenarios – I will cover power, load and consumption in more detail further in to this article.

However, when bikes with Electric Starters started popping up, it was clear that a more powerful charging system was required.

It is worth mentioning a couple of higher power systems from the time:

MK3 Commando

The MK3 Commando had electric start, and with the starter solenoid using 3 amps and the Prestolite starter motor peaking at 250 amps, it meant that the charging system was in desperate need of uprating, otherwise you’d never fully recharge your depleted battery during the course of your ride.

So, the new and improved RM23 16-amp single phase stator was fitted (which Norton said in the factory manual produces 15 amps at 6,000rpm, but was 14.4 amps according to Lucas)

This was an interesting implementation, as Lucas were already running the zener diode very, very close to it’s maximum rating (100 watts) which meant that they needed to do something different for the MK3

The solution was to downgrade the rectifier to a half wave one LU49181 (2DV406)

Grant Tiller

They then used two zener diodes on the AC side to bring rectification up to full wave, but also to bring the voltage back down to something that wouldn’t boil the battery.

The benefit of doing it this way is that they didn’t have to perfectly match the two zeners, and that the regulation could be split between both zener diodes knowing that both would be used – the zeners were mounted on both z-plates to help with heat dissipation.

The drawback of this was the fact that although the alternator output was higher, the voltage was in fact a little lower than the older RM21 10-amp system it was replacing – 0.7 volts lower in fact.

Here is the diagram from the Norton MK3 Commando workshop manual which illustrates how the components are arranged:

Grant Tiller

Many people assume that because the MK3 Commando has two zener diodes, it is a three-phase charging system.

That’s a myth – it’s still just a single-phase system.

Triumph T140E

The next system to note is the Triumph T140E. In 1979 they introduced a slew of updates – including major changes to the charging system.

This was a little after the MK3 Commando, and on the grounds they were both under the same management at this point, I would suggest they took some of the lessons learnt with the Commando, and applied them to the Triumph.

The T140E used a three-phase alternator stator the 10.5-amp RM24 coupled to a three-phase rectifier the LU83539

Because they chose the lower output stator, it meant they could stick with one zener diode.

The T140E was negative earth though, so neither the rectifier nor the zener diode were useable on positive earth bikes.

Here is the diagram from the Triumph T140E workshop manual which illustrates how the components were arranged:

Grant Tiller

Lucas Powerbase

The final system to cover is the Lucas Powerbase kit.

This was an aftermarket kit put together by Lucas and Mistral Engineering and was sold as an upgrade often alongside the excellent RITA electronic ignition system (which craved a solid, stable and consistent 12 volts)

The Powerbase was a three-phase system, built around the high output version of the RM24 stator and they proudly offered 180 watts of power and 85% of full output being produced at only 2,400 rpm in the sales blurb.

The kit was supplied with a matched pair of Zener diodes – matched by hand at the factory to be within 0.1 volts of each other. There was a need for all the cables in the install to be exactly the same length and gauge (same resistance in the wires on both sides) so that both zeners would work equally.

Unfortunately, as the zeners burned in to service, there was often a slight discrepancy between the two, meaning that one zener would often take most of the load.

Also, with owners installing their own kits, they often shortened the cables to keep things tidy, not knowing they were affecting the way the kit operated. As a consequence, the kit was fairly short lived.

Here is the diagram from the Lucas Powerbase kit which illustrates how the components are arranged:

Grant Tiller

Factory Standard Charging

The factory standard charging system on a Norton Commando (and most other brit bikes of the time) was built around the 10-amp Lucas RM21 stator and consisted of a separate Zener diode (the regulator, that brings the voltage down to a safe level for charging the battery) and Rectifier (the bit that converts AC to DC).

Zener diode Lucas Part Number LU49345:

Grant Tiller

Silicon full wave bridge rectifier Lucas Part Number LU49072 (2DS506):

Grant Tiller

Contrary to rumour, Lucas stopped using selenium rectifiers in around 1964 – so they never made their way on to the Commando.

The old selenium rectifier Lucas Part Number LU47132B is very recognisable by it’s square fins.

This is a googled the pic, as I don’t have one of these:

Thanks to Classic bike electrician and all round guru Al Osborn at AO Services for keeping me honest and correcting me with this information, as I had posted the wrong picture previously.

It is the job of the rectifier to convert the AC output from the Alternator stator into DC.

Here is the AC output:

Grant Tiller

And here is the rectified DC output:

Grant Tiller

A battery or a capacitor or (both) are then used to smooth some of the peaks out – to give the bike a nice clean, stable supply of power.

The problem with the alternator being attached directly to the engine’s crankshaft is that the output will vary massively according to how many rpms the engine is doing – if this output was not regulated in some way, your lamps would blow, and your battery would boil.

So, the job of the zener diode is to make sure that the DC voltage that comes out of the rectifier does not get dangerously high.

The zener diode starts clipping at around 14.3 volts – it does this by dumping anything over that setpoint to ground.

The zener is attached to the bike’s z-plate – a nice lump of aluminium stuck out in the flow of air, so it acts as a superb heatsink.

Lucas were cutting it fine with the specs of the zener – it is rated at handling 100 watts. The standard RM21 alternator is rated at 120 watts (they always based their nominals on 12 volts) so the zener doesn’t have any spare capacity to handle an uprated alternator stator.

Here is what the rectifier and zener look like as a simple circuit diagram:

Grant Tiller

You should note that the diagram shows the rectifier and zener connection to ground.

Although on a Commando, the positive feed off to these components is technically by the red wire that is part of the harness, both are attached electrically to ground by their mounting bolts.

This means, you cannot use them if you change the polarity on the bike from the standard positive earth to negative earth.

This standard charging system is absolutely superb and actually very robust. The only real thing to watch out for is on the rectifier with the soldered joints on the diodes between the cooling fin ‘wafers’ – these joints can eventually shake themselves loose with vibration.

This can be easily repaired with a soldering iron, or alternatively you can fit a modern rectifier which has been encapsulated in resin – these are robust and are much better at resisting vibration.

These modern rectifier units are less than five dollars to buy and work perfectly!

Grant Tiller

Shunting Regulator/Rectifier

In the mid-seventies and early eighties, a popular aftermarket “upgrade” was to swap the standard rectifier and zener for a combined regulator/rectifier.

The very early shunting reg/recs (and even now still the very low-cost ones) are basically functionally a direct equivalent of the original rectifier and zener setup. And there is nothing wrong with this – they work totally perfectly for most people and will provide many years of trouble-free service. Just don’t upgrade your stator to a higher output one, or introduce LED lighting. For good measure you may consider riding with your headlight on, as that continual 3-4 amp current draw acts as a great place for any excess charge to go.

These units are nothing more than a pair of thyristors that are clipping the AC input from the alternator at around 14 volts. The rectifier is then converting the clipped AC supply into DC.

In this example, the ‘control circuit’ that I reference elsewhere in my diagrams is nothing more than a zener diode – it is effectively switching the rest of the unit on or off depending on the voltage in the battery.

However, the DC output was very poor quality and choppy indeed, so over the years the subsequent designs, redesigns and iterations have been refined a little – although you still see these used extensively on lawn mowers, and small tractors to this day.

The simplicity of these is very attractive, and it is the nearest you’ll get to the original rectifier and zener setup.

You should note that a battery and/or a capacitor is an absolute must, as the output needs a lot of smoothing and conditioning.

It is also worth noting that many of these are polarity sensitive – i.e., they are grounded to their heatsinking enclosure electrically as well as thermally, so can only be used for negative earth (if the flylead is red) or positive earth (if the flylead is black) This is particialrly the case for the agricultural ones, so be careful if you are repurposing one for use on your bike.

Also watch out – some of the agricultural ones are rectifiers only – they don’thave regulation built in. Confusingly, they look the same as a reg/rec, so please make sure you know what you’ve got before you fit it.

This type of unit would probably not be recommended (in my opinion) for sensitive electronics like some of the digital electronic ignitions and electronic speedos/tachos.

Short-Type SCR Regulator/Rectifier

SCR is silicon-controlled rectifier, and it refers to the type of technology used under the covers.

In fact, the original Lucas rectifier on the bike is based on the same tech, thus it is well proven, reliable and low cost.

The most common and most sold units these days are short-type regulator/rectifiers.

These are often identified as Shunt reg/recs, but that is a confusing and incorrect term to use because actually every type of unit available (including the factory standard zener) shunts to a certain extent, and as you’ll find when you read on, how these work is very different to the basic functionality covered in the shunting type described above.

I feel that it is good practice to refrain from using “shunt” when describing a reg/rec type to prevent further confusion.

A short-type regulator/rectifier works in the following way:

  • The AC output from the alternator stator is rectified (converted) to DC
  • When the battery reaches it’s target charge voltage, the alternator stator input legs are dead-shorted by the control circuit, effectively closing the gate to the introduction of more incoming power.
  • Any remaining over voltage (i.e., if the DC voltage of the battery is a little over the target voltage setpoint) is then dumped out as heat.

The reason the AC input legs are dead-shorted is mainly down to keeping heat out of the reg/rec.

To be clear, at no point are the AC input legs shorted to ground – they are shorted to each other. The alternator stator winding still remain electrically separated from ground.

Think about the original setup:

  • the zener and the rectifier are kept separate and away from each other
  • the zener is attached to the z-plate which is acting to wick the heat away from the component
  • the zener is positive earth only, so the mounting stud will always be connected to the positive feed – this can be used to dump excess voltage and for heat transfer

Now compare this to the new combined reg/rec:

  • everything is in one enclosure
  • they are (most commonly) manufactured to be dual polarity – meaning that the same unit can be connected to a positive OR negative earth bike. So the manufacturer cannot rely on electrically connecting the components to the heatsinking enclosure for grounding
  • the gauge of the wires used would not take a single high amperage current dump
  • although most instructions specify that these should be installed in cool airflow, most manufacturers are aware and accept that these will be hidden out of sight in an enclosed area with zero airflow

I feel it is really important to understand this mode of operation, and what is actually going on here.

So, with the factory original charging system, when the target voltage of the battery is met, the alternator is working like this:

Grant Tiller

The whole output from the alternator, independent of the engine rpm is being converted from AC to DC by the rectifier.

If that DC is not required because the battery is fully charged and at it’s target voltage, the excess power is dumped to ground via the zener diode. This means that the alternator is constantly working at it’s full potential, and the load is outside of the alternator stator.

The load (and hence the heat) is kept away from the stator windings, even though they are constantly running at full load.

When a short-type regulator/rectifier is used, the alternator will be working more like this when the target voltage of the battery is met:

Grant Tiller

The control circuit inside the reg/rec dead shorts the AC input, which is effectively exactly the same as crossing the two wires on the alternator stator together (or three wires, if you have swapped your stator from the standard single-phase model to a three-phase one). As a reminder, this is not shorted to ground (it still remains floating).

At this point, the alternator stator becomes it’s own load. The gauge of the stator windings and their interlinking wires are considerably lighter weight than the gauge of the wiring outside the unit, so it is the stator that gets hot, because it is dissipating the excess power as heat.

If the battery voltage is then still a little higher than the target voltage, the control circuit can dump any of that overvoltage out to ground (this is where it wrongly gets the name shunting reg/rec from) – it is this constant clipping action that makes the thyristors (the components that are used instead of zener diodes in these units) get hot – the better quality regulator/rectifiers use a thermal paste to attach these to the aluminium heatsink casing, prior to the whole thing being encapsulated in a decent quality thermally conductive epoxy resin. This will act to protect the unit against vibration, as well as assist with thermal transfer.

The cheaper, poorer quality units available from eBay usually cut corners in this area, as it is an obvious place to bring the cost down. Not using thermal paste at all, and using a lower cost potting compound like an acrylic or polyurethane are popular.

The analogy I often use here is:

Put a small lightbulb across the terminals of a 9-volt battery – the battery will remain cool to the touch and the bulb lights up and gets hot. The lamp is the load.

Now put a table knife across the terminals of the same battery – the battery will get hot and the knife will stay cool. The battery is the load.

Running dead-shorted, is not the same as running under full load – another mistake that is often made. These alternators are designed to run at full load, and provided the power has somewhere to go (an external load) it is no problem at all. However, when dead-shorted, the stator windings become the load – which is not a good thing, and not what the alternator stator was designed to handle.

This methodology is absolutely fine, and in normal circumstances no big deal – it was how a short-type regulator/rectifier was designed to operate and there are hundreds and thousands of units out there operating in this exact way with no issue at all.

The reason it is fine is because of the way the standard charging system was originally designed.

Let’s look at the power consumers on the bike:

1Ignition (running) – approximate2.5
2Main Beam (60 watt or 45 watt)5.0 or 3.8
3Dip Beam (55 watt or 40 watt)4.6 or 3.3
4Pilot Bulb (5 watt)0.4
5Tail Light Bulb (5 watt)0.4
6Speedo Bulb (3 watt)0.3
7Tacho Bulb (3 watt)0.3
8Ignition Warning Light + Assimilator0.1
9Main Beam Warning Light (¼ watt)
10Turn Signal Warning Light (¼ watt)
11Turn Signal Bulb pair (21 watt each)3.5
12Stop Lamp Bulb (21 watt)1.8

Now let’s consider that the standard RM21 alternator stator that is fitted to the bike is rated at 10 amps.

The 10 amps it outputs is at 6,667 rpm.

At engine idle (1,000 rpm) the alternator stator is putting out 1.5 amps.

With a wasted spark electronic ignition system, two 6-volt coils wired in series, and doing away with the condensers and ballast resistor, you are consuming a little more than the points ignition as quoted in the above table. In reality, it is nearer to 3 amps.

All this means you are actually consuming more than you are producing at idle (this is the reason the red warning lamp flickers on and off at engine idle). This is by design, and is a good thing – something that people struggle to get their head around.

During the course of your ride, you are busy recharging a depleted battery, which means there is no need at all for the short-type regulator/rectifier to be dead-shorting your alternator stator.

Happy days, and working as designed – over the course of your journey, the power you have produced roughly equates to the power you have consumed, leaving you with a battery that’s in a decent state of charge, and ready for your next outing.

The reg/rec was designed as a direct replacement for the zener and rectifier on the bike – and if you change nothing else, there is no technical reason at all why it won’t provide you with years of good, reliable service.

Here is a simple circuit diagram of a single phase short-type SCR regulator/rectifier:

Grant Tiller

And here is a circuit diagram of a three-phase short-type SCR regulator/rectifier:

Grant Tiller

Where the short-type regulator/rectifier can become an issue on a bike is when you mess with the balance of the charging system.

An example is when you fit a more powerful alternator stator to your bike.

Lucas have available the RM23 high output stator – which is rated at 16 amps (compared to the original RM21 at 10 amps)

You are now producing an additional 6 amps of power, but are not consuming any more than you were previously.

This means that your battery will be charged significantly faster, and the short-type regulator/rectifier will dead-short the AC input legs when that target battery charge voltage is met.

With the battery charged sooner into your ride, you’ll now be spending more of your ride passing more current through your stator windings compared to the original setup.

This can result in the temperature of the stator racing up, and failure occurring – this most commonly can be seen as the resin around the stator cracking out, and eventually breaking down.

A second scenario is when you fit LED lighting to your bike.

LEDs consume significantly less power than their filament lamp counterparts, which means once again you are upsetting the balance of the charging system.

Instead of consuming around 6.5 amps across your headlamp, tail light and instruments when you turn your lights on, with a full complement of LEDs onboard, you could be consuming less than 1 amp.

So much less depletion of stored battery energy, meaning that once again, your short-type reg/rec will be spending most of it’s time dead-shorting the AC input legs.

Unfortunately, I frequently see that as part of a restoration or upgrade project, a bike owner will follow the advice of a salesman, or the manufacturer’s websites and install a new short-type regulator/rectifier in lieu of the factory original rectifier and zener diode, and they will add a new high output 16-amp stator and they will upgrade all their lighting to LEDs.

It is at this point, I often see cracked out stators, bikes no longer charging and in some extreme occasions I have seen people reporting melted resin too – one in particular was a pretty catastrophic foul up inside the primary case (although I have a theory this is down to physical contact between rotor and stator).

Don’t forget that inside the primary case is not very well ventilated, runs really hot in there, and there is a splash of oil to lube the primary chain, but not enough to cool anything down. It’s a pretty hostile place for anything electronic.

I am really keen that people understand that bigger does not always mean better – despite what we are told by the manufacturers and the salespeople looking to sell their latest and greatest kit.

The charging system on a bike should be balanced – take out around the same amount of power that you are putting back in over the course of your ride. Making changes to produce more, consume less or both will upset that balance and cause problems.

Ride with your headlight on all the time – as I said with the shunt-type reg/rec that extra 3-4 amp current draw over and above the igintion electronics acts as a great place for any excess charge to go during a ride will make sure that you are constantly recharging a depleted battery. Your alternator stator will run a lot cooler, as will the regulator/rectifier itself.

Short-Type MOSFET Regulator/Rectifier

These are relatively new to the world of classic bikes, however have been available on Japanese sports bikes and the like for many years already.

MOSFET is an acronym, and it stands for Metal Oxide Semiconductor Field Effect Transistor – it is several decades more modern than SCR technology, so in comparison is considered state of the art.

The bit that is of interest to us is that MOSFETs switch much, much more quickly than SCRs – which means the quality of the output is much better.

Here is an example of the DC output of a MOSFET regulator/rectifier:

Grant Tiller

As a matter of interest, compare that to the DC output of an SCR-based regulator/rectifier:

Grant Tiller

You can see that the MOSFET-based regulator/rectifier is switched several times during each cycle (i.e., each revolution of the crankshaft) which means the output is a lot cleaner and more precise in contrast to it’s SCR counterpart.

These reg/recs were designed for modern bikes that have a lot more electronics onboard – complex electronic fuel injection systems with high pressure fuel rails, and a multitude of sensors monitoring engine temperature, inlet and exhaust gases, water and air temperature/pressure etc…

Each sensor is basically a point of analogue to digital conversion, so can be prone to radio frequency interference (RFI) and electromagnetic interference (EMI), plus these low voltage electronic systems require a very stable and consistent power supply for maximum reliability.

The MOSFET-based reg/rec mitigates against a lot of the issues that were linked to the older SCR-based technology and they are being fitted as standard on many of the newer Japanese bikes.

It is well worth noting that the MOSFET regulator/rectifiers that are currently on the market are all short-type.

That is to say, they deal with a fully-charged battery that has met the target voltage in exactly the same manner as the short-type SCR reg/recs discussed above.

The AC input legs from the alternator stator are dead-shorted by the control circuit and any over-voltage caught the other side of the gate (i.e., at the battery) is shunted to ground.

However, because the MOSFET reg/rec is switching so much more frequently, the reg/rec itself runs significantly cooler, a really nice benefit of using this technology, since we are all well aware that these are often not installed in the cooling airflow that the manufacturers ask for in the install guides!

In the dead-shorted state though, the alternator stator will still be getting overly hot, and exactly the same applies with regard to making sure your charging system is balanced.

Make sure that you produce as much power as you consume, and regulate it yourself by using your headlight as a load.

Here is a simple circuit diagram of a three-phase short-type MOSFET regulator/rectifier:

Grant Tiller

To this date, I have not come across any manufacturers making a single phase MOSFET reg/rec – therefore I have not drawn one.

Most manufacturers say you can use a single-phase alternator stator with a three-phase reg/rec with no problems.

Open-Type (also known as Series-Type) SCR Regulator/Rectifier

Like MOSFET, this is the new kid on the block for classic bikes, although the technology itself is certainly not new – it has been used in aeronautic applications for many years.

Some manufacturers refer to these as series-type reg/recs, but I personally try to avoid using that term, as I feel it can confuse people.

(It’s the same reason as I try to avoid describing short-type SCR reg/recs as Shunt reg/recs as discussed above, some terminology can make things unnecessarily confusing for others, and I feel it is my role to try and help people get their heads around it all)

Open-type reg/recs have been adopted on some adventure bikes, ATVs, jet bikes, snowmobiles etc. – vehicles that may have a lot of additional electrical load like big banks of lights, heated clothing, additional pumps and heaters. The reason for this is that these vehicle types will typically have bigger batteries and larger alternators to cope with the additional electrical load. However, this additional load may not always be required – no heated clothing and no big spotlights needed on a hot sunny day, could easily equate to needing 300 watts less power, and dumping that back through the alternator stator windings like a short-type reg/rec does would very quickly cause damage.

The most common way to deal with wide range of loads is to use a different type of alternator – one with a field-wound rotor instead of permanent magnets. That way, you can easily dial down the amount of power that the alternator actually produces, taking away the issue of trying to deal with excess power. However, this type of alternator is significantly less efficient than a permanent magnet alternator (especially when you factor in the compact dimensions that we need for our bikes), and will also put significantly more load on your engine (so less engine power gets to the rear wheel).

So, the open-type regulator/rectifier differs from the short-type one in the way it handle things when the target voltage of the battery is met.

Instead of the control circuit dead-shorting the input legs of the alternator stator, it actually opens the circuit (or disconnects the alternator stator input legs completely.

This totally takes the load off the alternator stator, allowing it to run a lot cooler, without the potential of causing damage to the windings.

A complete tangent, but there is also the fact that at 4,000rpm the effect of taking all load off your alternator will free up around half a horsepower at the rear wheel. That’s nearly one percent on a 750cc Commando (based on the 58bhp the Workshop Manual quotes and depending on whose numbers you are working to), but nevertheless it’s still worth having!

Norton engineering powerhouse Jim Comstock runs a non-standard alternator on his Commando as he has some additional, power-hungry electronics on his bike:

  • data logging (from when he was experimenting with his fuel injection setup)
  • heated vest and gloves
  • 3 HID headlamps

Jim’s alternator is an Excelsior-Henderson external alternator rotor (no longer in production) and a homemade stator – the spec is 38 amp and 450 watts.

He runs the Compu-Fire open-type reg/rec and very interestingly saw a gain of around 1 ½ horsepower at the rear wheel on his dyno.

For me, this absolutely qualifies the claims I have read about in the past, which is why I am including it in this article.

If you take the rule of thumb permanent magnet alternator efficiency of 50%, Jim’s alternator would be taking 900 watts out of his engine.

Using the accepted figure of 745 watt equals 1 horepower, Jim’s dyno observation is about right.

There is also the matter of factoring in power loss through the drive train, which I haven’t done here, as it is far too complex for my little brain – but I have a feeling that if it were factored in, Jim’s numbers would be spot on.

I think there is also the matter of what you are comparing, as to what the horsepower gain actually is.

I have done some testing with the bike’s electrics running on a big, external, fully charged battery which meant I could rule out the variable of bad electrics affecting the ignition system from the equation.

As an example, when I was testing how hot an alternator stator gets when running dead shorted, I found that with the two AC legs of an RM21 stator dead shorted, it was nigh on impossible for me to kickstart the bike. The resistance it introduced was massive. I liken the feeling to having too long bolts holding the primary inner chanincase on, and it obstructing the flywheel (don’t ask me how I know)

If I set the idle to 1,000rpm with the stator wires disconnected, if found that when I shorted the stator windings with the bike running, I could very easily stall the bike each time. Don’t forget, the alternator isn’t electrically connected to anything on the bike so the stall was being caused by the braking effect introduced by shorting the stator.

If you compare this behaviour to that of an open-type reg/rec, then you can certainly appreciate where the numbers are coming from, as you can actually and physically feel the difference.

The open-type reg/rec still uses the archaic SCR technology, so is slow switching and gives a choppy output versus the MOSFET design, which means it is by no means the holy grail of regulator/rectifiers on the market.

However, for me personally, I feel it is the best of a bad bunch and in terms of being kind to the other components on the bike, it is my personal choice, what I use on our own projects, and what I feel comfortable recommending to others.

To date, I have never had an open-type reg/rec fail on our bikes, or any one that I’ve assisted over the years – plus it certainly puts an end to alternator-related issues which for me makes it the weapon of choice.

Here is a simple circuit diagram of a single phase open-type SCR regulator/rectifier:

Grant Tiller

And here is a circuit diagram of a three-phase open-type SCR regulator/rectifier:

Grant Tiller

Utopia would undoubtedly be an Open-Type MOSFET regulator/rectifier, as it would offer the best of both worlds. Watch this space, as sooner or later, I have no doubt that one will become available.

But they are not here yet, which means you have to make a choice between nice, clean and precise power or kind to your alternator.


There are a huge number of manufacturers in the market, so I am just going to pick out a few of the more well-known or notable ones and share my opinion.

You should note that many of the manufacturers have a few different models available. In particular, both Boyer Bransden and PODtronics offer units that are marketed as battery eliminator, batteryless or battery-free. These include an internal capacitor, which means you can run the bike without a battery.

If they are not marketed with this capability, it is safe to assume that the functionality is not there, and a battery MUST be used. This is important – the battery serves as a load, which makes the reg/rec work as intended. Also, the circuitry inside a reg/rec is driven by DC – they are a power consumer just like a headlamp, so they need to get their power from somewhere, be that a battery or a capacitor.

The other point to note is the compatibility of a reg/rec with the Warning Light Assimilator (WLA) also known as the Ignition Warning Light/Headlamp Warning Unit.

Whilst most units these days are protected against reversed polarity (i.e. accidentally connecting to the wrong battery terminals) every unit I have come across to date will not tolerate any of the AC input legs being grounded. Doing so simply fries the reg/rec.

The warning light assimilator, because it is monitoring the AC output directly from the alternator stator can have a similar effect to grounding an AC input leg. Boyer Bransden explicitly say in their fitting instructions “If your Norton has a charging warning light simulator it must not be used with the power box, remove it” for example.

Boyer sell a different model of PowerBox that includes it’s own circuit for the charge light, however I would recommend that you use a seperate Charge Warning Light instead, as it will provide you with a lot of useful information like state of charge, battery state, reg/rec failure etc…


The Podtronics units are short-type SCR regulator/rectifiers

Grant Tiller

The “POD” in Podtronics stands for Prince Of Darkness and was a quip on what some see as Lucas’ poor reliability by the founder Bob Kizer (an opinion I don’t share)

Bob is sadly no longer with us, but he was responsible for designing and making what has undoubtedly become the most popular reg/rec in the classic bike world – he was a great (and funny) guy, who kindly provided me with a lot of his time. This is also the most imitated reg/rec.

In Bob’s design of the Podtronics reg/rec his primary goal was that he “wanted to dumb it down so that any idiot could fit one” (his words)

Always bear this statement in mind, and consider that this design was not necessarily technically the best solution.

The Podtronics was designed in the late seventies/early eighties to directly replace the existing Lucas rectifier and zener diode on the bike.

The intention was for it to be fitted as is, using the existing alternator stator, with no other changes – no high-power stator upgrade and no LED lamp conversion.

In this scenario, the Podtronics works absolutely perfectly, and there are many thousands out there that prove it. Riding with your headlight on, as mentioned already, helps things even further!

I have noticed in recent years that there has been a seemingly higher failure rate of Podtronics units than there used to be – more inline with standard figures. Several years ago, production of the units was outsourced to Taiwan, and I cannot help but feel the higher number of DOA (dead on arrival) and ELF (early life failures) tallies with the change in production.

Although, most of the charging issues I help people with are running Podtronics, I must remind everyone that this is because there are far more Podtronics units out there – they have been sold since the eighties, and they are the most common, most well known and biggest selling brand.

Bob Kizer used to make them in relatively small batches, and test them as he went – I think the quality was higher back in those times – industry average for classic motorcycle electronics failure rate is about 2%, and I know that Bob prided himself on a much lower figure.

I have also noticed a ‘rationalisation’ in the product line – the single-phase unit is now identical to the three-phase one, it just has the third yellow wire cut off.

Grant Tiller
Grant Tiller

I do wonder if this is what the documented issue with the Tri-Spark electronic ignition is all about, since it apparently only manifests on the single-phase units – just thinking out loud on my side, but if only one phase is ever used on a three-phase rectifier is it electrically noisy?

For those that haven’t heard about the Tri-Spark issue, you can read more about this topic by clicking here:

Grant Tiller

The final thing I have noticed is that some Podtronics units I have been testing seem to hold a voltage internally over and above most others – these have not been labelled as battery eliminator “POD-1P-MAX” models, so this should not be the case. Definitely some inconsistency there.

The great John Healy from Coventry Spares Ltd in Middleboro, Massachusetts took over Podtronics from Bob Kizer some years ago, so I am hopeful that in due course we will see a resurgence in quality of these units again.

Boyer Bransden

The Boyer Bransden units are short-type SCR regulator/rectifiers

Boyer are well known for their electronic ignitions; however, they have also manufactured the Power Box reg/rec for many years.

Grant Tiller
Grant Tiller

Just like Podtronics, they make a high-power variant, single-phase and three-phase plus a battery eliminator version to enable you to go batteryless if you wish.

A few years ago, these units didn’t have reverse polarity protection, and I saw a few people connect them up wrong, causing irreparable damage to the reg/rec units. To my knowledge, that issue has now been resolved.

The other thing to be aware of is that in all of their documentation, Boyer point out that the reg/rec units must not be used in conjunction with the Norton Commando warning light assimilator.

They point you toward their reg/rec that has a charging light – however, I would advise you use a battery status monitor or charge warning light from the likes of SparkBright or ICM instead.


I don’t see or hear much about Tympanium these days, but back in the day they were very common indeed!

Grant Tiller

Once again, they are short-type SCR regulator/rectifiers – however there are a couple of things to watch out for.

Some Tympaniums are shunt reg/recs – the most basic of the types available.

Tympanium made units for lawnmowers and small tractors and some were rectifiers only.

Some were regulators only.

Some were grounded to their heat sinking enclosure, so have only a red or black lead depending on whether they are negative earth or positive earth.

All of these different types look more or less the same, as they are in similar looking casings, so make sure that you know what you’ve got before you use it!!!


Shindengen are a massive manufacturer, and happen to be the manufacturers I side with.

Grant Tiller

They make short-type MOSFET, short-type SCR and open-type SCR reg/recs.

The FH020AA is their MOSFET model and it is used extensively on Yamaha, Honda and Triumph sports bikes – they have gained a very strong reputation for reliability over the many years that they have been available.

The SH775 and SH847 are their open-type models (30-amp and 50-amp rated) – the SH775 is my preference, and I have used and recommended them many times. The 50-amp version is of course total overkill for a Norton Commando, however sometimes that maybe the only model you’ll be able to get your hands on. It works absolutely fine, but is very slightly larger in size (don’t forget, that is it’s maximum rating – the actual output is governed by the size of the alternator stator)

The issue with the SH775 and SH847 is supply chain related. The designs and the rights of these models is wholly owned by Polaris, who make ATVs, snowmobile and watercraft. So, the units are only distributed as Polaris spares to Polaris parts distributors.

This makes it difficult (and expensive) to pick them up for use on classic bikes.

A good source in the US is Jack Flemming at Roadster Cycle – he has struck up a good relationship with a Polaris parts distributor and usually keeps a pretty good stock.

Here in Europe, MTP Racing are a good source.

One thing to watch out for is the huge number of counterfeit Shindengen units on the market – if you are paying less than $100, you can safely assume the unit is a fake one.

Jack Flemming at Roadster Cycle has done a great little YouTube video where he shows you what to look out for, and how to make sure you have the genuine article.

You can find it by clicking here.

Grant Tiller

Cycle Electric Inc

Cycle Electric is a US manufacturer that make a very high quality open-type regulator/rectifier (which they are keen to market as series-type)

Grant Tiller

These are aimed at the Harley Davidson market, so are available in all sorts of different shapes and sizes to blend in on a Harley.

Some are a little on the large side, which is great for cooling, however can be a challenge to hide them away somewhere inconspicuous.


Another make of open-type regulator/rectifiers is Compu-Fire

Grant Tiller

The Compu-Fire 55402 comes highly recommended, however one word of warning:

This would be suitable only for a negative earth application, since it is grounded through the heatsinking enclosure.


RMSTATOR are another company worthy of mention – I’m not sure whether these guys actually manufacture their own units, or if they source and white label others.

Grant Tiller

However, they have a huge selection to choose from – open-type SCR as well as short-type MOSFET (also traditional short-type SCR, if after this article you still choose to go up that route)

Rick’s Motorsport Electrics

These guys are quite prolific at the moment – lots of advertising and social media activity.

Grant Tiller

Rick’s Motorsport Electrics have introduced a regulator/rectifier that has been tweaked for lithium-based batteries.
Their voltage setpoint is 14 volts ± 0.2

It is nice to see someone has picked up on the fact that lithium-based battery needs to be treated differently to lead acid ones.

After all, with a petrol tank by our nuts, a battery under our butts, and a red-hot engine making sparks between the two, we need to be careful!


The final manufacturer I wanted to mention in this space are Tri-Spark

Grant Tiller

Tri-Spark used to sell and recommend Podtronics, however, when they saw quality issues and more recently the issue of their Tri-Spark Classic Twin electronic ignition units misfiring between 3,000 and 4,000rpm, they stopped selling and recommending them.

Tri-Spark are now selling a short-type MOSFET regulator/rectifier which has been very well received, and I am pleased to see it is becoming available through the likes of Andover Norton, RGM Norton and Rex’s Speedshop (as well as directly from Tri-Spark’s website)

I have always liked Tri-Spark – they are by far my preference on the electronic ignition side, and I think they get an unfair beating on the reliability side (as I mentioned in a previous article)

It is awesome to see Stephen Kelly (the owner of Tri-Spark) keep a keen ear on the community and innovate based on market demands and passionate discussions.

He has brought a new ignition model to market – the FireBox Pro which is a quality piece of kit:

  • a separate electronics box to satisfy those that don’t like the idea of having everything in the heat of the points cover (although it has never been an issue for me)
  • programmable ignition curves from your PC
  • either wasted spark or separate triggers for each cylinder
  • great cable management and strain relief
  • plus retaining the anti-kickback and idle stabilisation benefits of the Classic Twin model.

He has introduced oil-filled coils – much better quality, better cooling and improved longevity over the resin potted ones.

And now, he has introduced his short-type MOSFET regulator/rectifier too.

Very impressive!

Recently, a video of the new Tri-Spark reg/rec was posted on YouTube

You can find it here:

Grant Tiller

The video is a comparison of the new Tri-Spark short-type MOSFET regulator/rectifier “versus” a Podtronics short-type SCR regulator/rectifier.

It is very interesting, and clearly shows the superb, clean quality output of the MOSFET unit – indeed a major advantage of it’s counterparts.

However, in the interest of being fair, and trying to be impartial (which is hard, as I am a big fan of Tri-Spark) there are a couple of things I would like to point out:

  • Although a load bank is used in the video to simulate the power consumers on the bike (lights, ignition etc.) there is no mention of whether or not a battery is used in the test. I don’t know about the new Tri-Spark, but the Podtronics MUST be connected to a battery, or a large capacitor at the very least, otherwise the output will be erratic. Podtronics have always detailed this requirement in their installation instructions.
  • The RPM of the motor that the alternator is connected to in the test probably goes up to no more than 3,000rpm – it would be interesting to see higher speeds, as this single-phase alternator’s output is rated at 6,667 rpm.
  • Most bikes will be using a 10-amp stator, not the 16-amp high-power one used in the video.
  • We are very interested in temperature, and of course the MOSFET unit runs a lot cooler than the SCR one – a major benefit in terms of where the unit will be hidden, and probable lack of airflow. However, for me, the more important temperature reading would be one around the alternator stator, not the reg/rec – particularly with the reg/rec attached to a fully charged battery.

Thumbs up to Tri-Spark though, this is a major step in the right direction and as I said before, I commend their innovation.


So, there you have it – my overview, findings and opinions about charging systems on our beloved bikes.

I have tried to put as much data in here as I could, and as many diagrams and pictures to help explain things as possible.

There are two sister articles, which may also be of use to you:

  • Alternator Issues – where I dig into how an alternator can be damaged, how to spot an issue and what it is usually attributed to.
  • Choosing a New Alternator – if you are set on buying a new alternator, this will walk you through some considerations and hopefully help you make the right purchasing decision.

Hopefully this article has helped squash some of the rumours and bust some of the myths out there, while making it easier for people to understand how things work and what you should be thinking about when planning to change anything in and around your bike’s charging system.

Let me know your thoughts and of course reach out if you need any help!

This piece is part of a series that covers the charging system in general.

Hopefully these will fill in some of the gaps and questions you may have, but as always don’t hesitate to reach out if you need any help or advice.

17 replies

  1. Thank you for this great article.
    Far too much inaccurate/ misleading information on the web for classic bike owner do it yourselfer lay persons.
    Petrol tanks, nuts, butts, sparks and hot engines…….we must be crazy:)
    Just curious…what is your bike and ignition/ charging set up

    • Hi Des, glad to hear the article was of some interest to you!

      We have a few “keeper” bikes here:

      Dommie 650ss
      Modern BT-H magneto (it is actually a CDI self-generating ignition)
      Lucas LU47244 3 phase alternator
      Shindengen SH775 open type reg/rec

      Dommie 650ss Cafe Racer
      Tri-Spark Classic Twin electronic ignition
      Lucas LU47244 3 phase alternator
      Shindengen SH775 open type reg/rec

      Commando MK3
      Tri-Spark Classic Twin electronic ignition
      Lucas LU47244 3 phase alternator
      Shindengen SH775 open-type reg/rec

      Slimline Featherbed Commando 750 (Combat)
      Power Arc programmable electronic ignition (I will probably change this to the Tri-Spark FireBox Pro in the future)
      Lucas LU47244 3 phase alternator
      Shindengen SH775 open-type reg/rec

    • Hi Grant
      This sorted a lot of things for me, correcting some previously held beliefs and confirming others.
      Very well written and easy to understand, thank you!


        • After reading your article I’m thinking of using the Shindengen 775 on a Norton 920 project. In addition to the stator benefit, the free 1.5 rwhp @ 4000 is a nice bonus.
          If I have it right, 1 HP is equal to 746 watts of electrical power. 1.5 rwhp equates to about 1100+ watts rear wheel or 1280 watts at crankshaft.
          Is this large number due to the inefficiency of the alternator or are my numbers off?


          • Hi Glen,

            I can certainly recommend the SH775 from a reliability standpoint – I now use and recommend them exclusively, and have never had one fail.
            The downside here in the UK is that they can be a real pain in the butt to get hold of.
            I tend to order from the USA, as the UK distributors never, ever have stock.

            I don’t know if it is the placebo effect or not, but I do feel that when I fit a SH775 to a bike, it feel more free revving – like a bike or car feels when you take weight out of the flywheel.
            It’s not something I talk about much, as I don’t know for sure that it makes a difference in this way – I am perfectly willing to accept that this is “all in my head”

            However, on the numbers side – Jim Comstock installed a Compu-Fire open-type (AKA series) reg/rec on his bike and ran it on his dyno.
            The downside of these particular units is they are negative earth only, so not suitable for a Commando in standard trim.
            Jim saw an additional 1 ½ hp at the back wheel – although do bear in mind that he was running a different alternator to the standard Lucas offering.
            For me, this does reinforce the claims though and gives them far more credibility.

            The permanent magnet alternators on our bikes are actually pretty efficient in comparison to the alternative – the field-wound alternator.
            The general rule of thumb is that a permanent magnet alternator can put out 1 ½ times more power than a field-wound alternator of the same size.
            As power is not used to energise a field, and there are no brushes, they are generally declared to be more efficient.
            In terms of actual efficiency i.e., how much of the energy put in is gotten out, the generally accepted rates are permanent magnet alternators are 50% efficient versus field-wound alternators which are 30% efficient.

            Using the published figure of 745 watts to one horsepower, I once tried to get my head around it all.
            But being totally honest with you, I couldn’t make it all add up.
            I could only assume at the time that I was not correctly factoring in any kind of loss via the drive train.


  2. Great article Grant, has been a huge help working on my commando mk3, converting to lithium and tri spark r&r + cwl

    • Happy some of the info here has been of use!

      Keep an eye on the reg/rec – it’s charge threshold is at 14.5 volts, which is perhaps a little higher than i’d personally like to see for a lithium-based battery.
      I’d be much more comfortable with something a bit nearer to 14 volts.

      However, using a decent battery status monitor or charge warning light from the likes of these guys will make sure that you keep everything in check.

  3. Hi Grant.

    Thanks for your input to the Commando Facebook page after I posted the question about LED headlights. I have a modern voltage regulator fitted so I could be in for trouble if I fit the LED headlight and do nothing else?

    I think I will go back to the original system I had earlier. I have a good Zener Diode. I will get a rectifier locally to replace the one I have. It is probably fine but the modern equivalent looks a better item. Is this one I can buy in NZ OK?

    Do I still need the capacitor? I still have that component too so it can also be refitted.

    If I go back to 100% original, the lower load problem created by using LED bulbs should be alleviated?

    Really appreciate your help. Like most British bike owners, electrics is still a bit hazy for me. Your article about rectifiers and regulators has been a great help in my grasping the basics!



    • Hi Graeme,

      That looks like it will work just fine!

      The main course of failure of the zener diode is when they get dirty.
      Keep it nice and clean, and I see no reason why it would be good for another fifty years!

      …don’t hesitate to reach out if you get stuck, I’m here to help.

  4. Hello, I have a T140es which is fitted with as standard a high out put alternator.
    Which reg/ret would you recommend. These bikes are fitted with the rm24 alternator and three zenors .

    • Hi Mark,
      I may have the wrong bike, but I was under the impression that the T140ES had the three phase RM24 stator, three phase 3DS5 rectifier, but still only one zener diode.
      If I am not mistaken, this is the model that offered the Lucas RITA electronic ignition as standard circa 1979?

      Other than the soldered joints inside the rectifier shaking themselves apart with vibration, this is actually one of the best and most robust charging systems of the lot.
      A good idea is to replace the 3DS5 rectifier with a modern equivalent like the 36MT60 which should cost you under 15 quid.
      The connections are the same as the original 3DS5, and technically it is an exact equivalent. It is just potted in resin so is more robust in terms of vibrations.

      I did a short article about it here, if it is any interest to you:

      The nice thing about these bikes, is they took a lot of the mistakes of the MK3 Commando, and sorted them for the T140 – so lots of good bits on there.
      One thing to watch is that it is negative earth (pretty much all the other brit bikes of the era were still positive earth, but even with the cars, Triumph was among the first to change)
      The good thing about being positive earth is that the zener diodes are still readily available!

  5. Grant,

    Thank you for the detailed explanation and insight. It helped me avoid the mistake of switching to LED lights with the current setup I have on my ’71 Commando with podtronics reg/rec, Boyer electronic ignition, stock lucas single phase stator, and lead acid battery. If I was to want to switch to LEDs is there a viable configuration that does not stand the potential to damage the charging system components?

    Thanks again for all you share on your site! Truly helpful.

    • Happy to hear that some of the content on the site is of use to you!

      In all honesty, the lowest cost option is actually a good one if you want to replace your lights with LEDs

      Fit the original zener diode to the z-plate – that big lump of cast aluminium is a superb way to get rid of excess power!

      Swap the original Lucas LU49072 (2DS506) rectifier for a modern equivalent like the KBPC3504


      One of these will set you back about £5 and is functionally a direct equivalent of the original.
      However, the componets are resin encapsulated, so the joints don’t shake themselves apart like the origginal rectifiers do.

      Couple of things to watch for.
      1) keep the zener diode nice and clean – a good connection for heat and electricity is important.
      It’s great they are out in the airflow, as that’s what keeps them nice and cool, but it does mean they will pick up road grime along the way.
      Most ‘failures’ are simply down to a bad connection.

      2) if you have turn signals and plan to swap these to LEDs, you will probably need to replasce the flasher unit at the same time.
      The originals rely on using 4 x 21 watt lamps – one on each corner.
      Lowering the load will make them flash slower, and will eventually weld the little bi-metallic strip inside, rendering it useless.
      I did a little article on swapping in a modern flasher unit not so long ago:

      Hope that helps!

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