I am restoring a late 1940s Velocette Mac 350 motorbike. I have a few working dynamos for this bike, but the original regulators are hard to find in working condition. I bought a regulator on Aliexpress, but I am pretty sure now that this regulator is designed for an AC input (Alternator), rater than the DC output of a dynamo. Back to square one. I intend to use Led lights, but the original horn is a 6V one, so if possible I want to regulate the dynamo output to 6V DC, it will produce higher voltages depending on RPM, but I'd like to keep the output voltage to a standard 6V if possible. I was going to use 2 x ultra capacitors on the output of the dynamo instead of a battery, because even I will be losing some capacitance having by the caps in parallel I think the caps will soak up any slight variations in the power output from the dynamo and provide adequate lighting for a while when the bike is stopped. This brings me to my question, but feel free to pick holes in my idea if you think it won't work, I was thinking that instead of the standard regulator, I would use a buck / boost converter to keep the voltage around 6V. The Leds will accept from 5v to 30v so it's really only the horn that I'm worried about.

Edit: This is a Miller type DVR dynamo, that is regulated by default by varying the voltage in the field winding. It has no permanent magnets and outputs a DC voltage.

  • \$\begingroup\$ First thing to do is to go set up a testbench with your "ultra capacitors" (whatever you want to try, first) and see how long, once charged (easy to do), they will run the LED lights. You seem to imagine they will last "for a while when the bike is stopped." I think you should test this and get it out of the way. (I think it's not a slam-dunk.) There are other important and yet quite simple tests to do in order to "clear the deck." One of them is also to test your dynamo with capacitor loads, under operation with the bike at various RPMs and measure the output with a voltmeter for example. \$\endgroup\$
    – jonk
    Nov 5, 2021 at 4:40
  • \$\begingroup\$ I have tried this and using some 300F capacitors I was able to run the Leds for over 5 mins on a single charge of the cap from a bench power supply. I can't try the second option until I get a buck converter, since the output of the dynamo exceeds the rated voltage of the cap. \$\endgroup\$ Nov 5, 2021 at 10:09

2 Answers 2


The way a conventional mechanical hybrid regulator on a dynamo, or generator, system works is by switching the field coil off and on changing the magnetic field produced by the same. This regulates the amount of voltage generated by the dynamo part of the generator. This is also done electronically by various solid state devices, including the DVR2 and the Boyer Bransden regulator for which Hacktastical posted a link. The standard connections on a DC generator regulator are:

F: Field winding

A: Ammeter (regulated output)

D: Dynamo

E: Earth

I will start a new thread / question about building one of these solid state versions myself, using power transistors to switch the field winding thus varying the output.

Thank you for the input about this, now aborted, starting idea.



This motorcycle has both a magneto for ignition and a generator (dynamo) for lighting (I know you know that, this is a note for other readers). The generator would use a field coil regulator, similar to that for an automobile of that era, that would regulate the voltage to the battery.

As for the unit you bought, a regulator for an alternator type would add a rectifier, and if it’s a larger alternator, a field coil driver. Voltage regulation and battery cut-out would also be dealt with there.

You could use large electrolytics in place of the battery. I'd be a bit concerned about the robustness of Supercaps to overvoltage, so select with care.

As for the regulator, perhaps a VW 6V regulator or similar could be adapted?

If your Velo is negative ground (or converted to it) maybe you could use something like this: https://vintagetriumphsupply.com/boyer-voltage-regulator-british-dynamo-gen-6v-neg.html

MORE: A possible solution: Building a solid state generator/dynamo voltage regulator, to replace an old mechanical regulator

  • \$\begingroup\$ You are right and this is exactly the plan that I was going to follow. I'm sorry that I don't have enough points to up vote your answer. I was trying to use a solid state solution instead of a pseudo mechanical regulator. \$\endgroup\$ Nov 5, 2021 at 10:16
  • \$\begingroup\$ Your solution that you have proposed at vintagetriumphsupply.com/… would work, as would matchlessclueless.com/electrical/lucas/regulator-replacement . The latter is a DVR2 regulator, which is commonly used on Vintage bikes. I was already aware of these options, but was hoping for a less expensive option (I know it's not sheep stations anyway). After a bit of thought, I realize that using a buck boost solution would require the field coil to be energized continually, which would both put unnecessary strain on the dynamo. \$\endgroup\$ Nov 5, 2021 at 21:07
  • \$\begingroup\$ ..... and increase the load on the motor. \$\endgroup\$ Nov 5, 2021 at 21:08
  • \$\begingroup\$ It would be an interesting project to make a PWM control for the field coil that models the way the old regulator works. Basically, sense the voltage, and if it gets above a threshold, start interrupting the field coil current. Maybe a comparator, a 555, and a good-sized FET? \$\endgroup\$ Nov 5, 2021 at 23:34
  • \$\begingroup\$ This site had some good diagrams: dunhackin.com/index.php/velocette-documents/mac-manuals/… \$\endgroup\$ Nov 5, 2021 at 23:35

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