Motorcycle Shunt Regulator

Thread Starter

Reloadron

Joined Jan 15, 2015
5,456
I have a few questions. First please look at the below image:
Shunt Reg.png

This is one of many versions of a typical 12 volt system motorcycle shunt regulator. Unlike an automotive regulator where a field es excited a motorcycle regulator takes a voltage off a 3 phase stator, rectifies it and applies it to the battery using a shunt design to keep the voltage down. This is apparent in the above drawing. The circuit uses PWM from a simple 555 circuit to drive the MOSFET.

A little history here. Last year I was happily tooling along and went to do a lane change. No turn signals! So I mess around when I get home and notice my battery gauge is showing something like 7 volts. So I remove the stator to regulator connection and start measuring phase to phase, turns out my bike has a 3 phase system. Yank regulator and trot to my local Harley shop. Surprise, the bike did not have original OEM Harley system. Doing some homework I discover someone upgraded the stock charging system to a Thunder Heart Performance 3-Phase Charging System. The entire system cost about $500 and all I need is the regulator. At 1,000 RPM I get about 20 VAC phase to phase and it increases 20 V / 1,000 RPM just like it should. The manufacturer sells only the full kit so I get a brand new stator and rotor I don't need and the regulator which I really need. A tiny little silver box which takes 5 min to swap out. So I bought the system. This was almost a year ago.

So a few says ago I am out for a ride and turn signals quit working so here comes that sinking feeling as I look at the voltmeter. It was erratic but did return to about 13.6 volts. This is exactly like deja vue all over again. I called JP Cycle and they were no problem and gave me a link to a return UPS label, I figured I was screwed but nope so in a week or so I should have another new kit. I returned the brand new rotor and stator from last year still in the packaging. This setup is a 38 amp charging system.

Then I started thinking about these regulators and as can be seen there isn't much to them. A three phase rectifier 100V 50A isn't much then I thought rather than a 555 for PWM and a IRF540 I could use an IRL540 N Channel Mosfet and drive it using a uC like an Arduino which I have a few chip only with sockets, Maybe use a 10 turn 100K pot as a divider into an analog input and generate a PWM out to drive my uC. Analog in and PWM out. The IRL 540 should hack the shunting duty. Can anyone think of why this approach won't work? Really not much to it and thought I would kick it around for something to do. I think I came in just under the wire on that exchange.

Ron
 

Dodgydave

Joined Jun 22, 2012
9,169
It would probably work, you will have to provide feedback to the 555 on pin 5. .
you don't need the 12V zener, replace it with a 1K resistor, and you need a Shottky diode in series with the battery feed from the alternator output, to prevent the battery from being shorted by the fet, better still if you can get acess to the rotor windings and short them out.
 
Last edited:

Thread Starter

Reloadron

Joined Jan 15, 2015
5,456
Thanks Dave and good points I had not thought about. The rotor windings are very accessible too. When I get the new replacement parts I'll likely use them but future plan I want a more robust regulator. On a long road trip a broken motorcycle isn't much fun.

Thanks
Ton
 

Wolframore

Joined Jan 21, 2019
1,748
It would be interesting to take apart a bad regulator to see where the weak link is. I wonder if it’s the diodes from the stator
 

crutschow

Joined Mar 14, 2008
24,987
I don't see how your circuit can regulate properly since there's no feedback from the output voltage to the 555 so its duty-cycle is unrelated to that voltage.

Many regulators use SCRs to shunt the excess current, and the high forward drop of the SCR's creates a lot of heat.
The regulator on my Triumph gets so hot you can easily burn your fingers on the heat-sink and it's that heat that likely cause regulator failure.
Since the regulator is behind a fairing, which tends to trap the engine heat, I added a muffin fan with a shroud to a fairing opening to reduce the heat buildup.
Some regulators now use MOSFETs instead of SCR's to reduce that dissipation.

As an exercise I came up with a MOSFET/Schottky-diode design (below) to minimize the dissipation.
It clips the peak of the alternator waveform when the voltage exceeds the desired charge voltage, as determined by U1.
It's operation is discussed here.
Note that the design has been simulated, but I'm not aware of anyone building it to verify its operation.

1589289950355.png
 

Thread Starter

Reloadron

Joined Jan 15, 2015
5,456
It would be interesting to take apart a bad regulator to see where the weak link is. I wonder if it’s the diodes from the stator
Wanted to try that. The things are well potted. Once I get the replacement I may try warming it up and getting in there.

Ron
 

Thread Starter

Reloadron

Joined Jan 15, 2015
5,456
Crutschow, many thanks.

I don't see how your circuit can regulate properly since there's no feedback from the output voltage to the 555 so its duty-cycle is unrelated to that voltage.

Many regulators use SCRs to shunt the excess current, and the high forward drop of the SCR's creates a lot of heat.
The regulator on my Triumph gets so hot you can easily burn your fingers on the heat-sink and it's that heat that likely cause regulator failure.
Since the regulator is behind a fairing, which tends to trap the engine heat, I added a muffin fan with a shroud to a fairing opening to reduce the heat buildup.
Some regulators now use MOSFETs instead of SCR's to reduce that dissipation.

As an exercise I came up with a MOSFET/Schottky-diode design (below) to minimize the dissipation.
It clips the peak of the alternator waveform when the voltage exceeds the desired charge voltage, as determined by U1.
It's operation is discussed here.
Note that the design has been simulated, but I'm not aware of anyone building it to verify its operation.

View attachment 206942
Many thanks for that input. Yes, in the circuit I posted I was considering similar but going with a uC to drive the MOSFET. I looked at a few designs using SCRs and while I was never fond of using a MOSFET as a load I guess it is done. I also like your approach of using 3 MOSFETs, one per phase.

Ron
 
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