Overview of the circuit

This is a shunt regulator which is wired up as shown below, in-between the pedal generator and the storage component (this could be a capacitor or a battery).

This design is based upon a hysteresis comparator. This means when the voltage goes above a set value the dump load will turn on and it will stay on until the voltage has dropped to another, slightly lower voltage. The difference between these values is called the hysteresis voltage.

This circuit has been designed to be inexpensive but also robust and reliable and configurable for different situations.

The final circuit diagram is shown here:

Download a large copy of the circuit diagram here.

The basic concept of the circuit is that the input voltage is measured and compared to a stable reference. If the voltage goes above the reference then the dump load will be switched on. When the voltage drops below the reference then the dump load will be switched off. Easy, eh?

The reference voltage is supplied from a 7805 5V regulator. The comparison is done using an LM393 dual comparator. We only need one comparator, so the other is grounded.

The input voltage is reduced using a potential divided from the potentiometer (variable resistor – RV1 on the circuit diagram), which allows control of the regulated output voltage.

The only thing to do is to add a capacitor to smooth the signal sightly to stop the MOSFET being switched on and off too quickly.

The feedback resistor around the comparator controls the amount of hysteresis. This can be adjusted using the variable resistor RV2 on the circuit diagram.

The output of the comparator controls a PNP transistor which supplies enough current to easily control a MOSFET (a voltage controlled switch) which switches on and off the dump load. The 1k resistor limits the current flowing into the MOSFET (which will only be very small) and the 100k resistor pulls the input down when the MOSFET is switched off.

A diode is used to stop the generator acting as a motor. The diode must be rated for the correct current, which for a 100W generator at 12V is around 10A. This design uses a ‘Schottky’ diode, which has a low voltage drop and hence does not dissipate as much waste power as other diodes.

This circuit is suitable for both 12V and 24V systems.

Dump Load

The dump load must be able to disperse any excess load, which could be up to 100W with my pedal generator design. I decided to use a high power resistor (50W) and an old computer processor heatsink. The resistors are available from Farnell and RS and are only a couple of pounds. Computer processor heatsinks are available new for around £10 (e.g. Maplin) and for virtually no cost at car boot sales and in junk piles. I paid 20p for two at a car boot sale, including the motherboards…

The only problem with this is that the fan must run to ensure the load is cooled. A future design will give a warning if the fan is not detected as running.

The resistor is screwed to the heatsink using drilled and tapped holes, along with some thermal transfer compound. (The photo here shows the prototype unit, hence the slightly inaccurate alignment)

Parts List

The full parts list:

Schematic Ref Component Value
C1 Electolytic cap 35V+ 100uf
C2 Poly cap 100nf
C3 Poly cap 10nf
D1 Diode 10A+ PBYR745
D2 LED 3mm LED Red
D3 Diode 1N4001
P1 Connector 25A screw terminal
P2 Connector external LED socket
P3 Connector 25A screw terminal
P4 Connector 25A screw terminal
P5 Connector 25A screw terminal
Q1 NPN general BC639
R1 Resistor 22k
R2 Resistor 2k7
R3 Resistor 2k7
R4 Resistor 1k
R5 Resistor 1k
R6 Resistor 100k
R7 Resistor 2k7
R8 Resistor 10k
R9 Resistor 100k
R10 Resistor 1k
RV1 Variable Resistor 10k
RV2 Variable Resistor 100k
U1 Voltage regulator 7805
U2 Dual comparator LM393
IC Holder 8pin
DUMP High power R 1 ohm 50W
Computer heatsink

These parts should be relatively easy to find. Other components with similar specifications can also be used.

PCB design and layout

The circuit schematic and PCB was drawn using KiCad, free, open source circuit and PCB design software. The PCB is given here, but must be printed out to an accurate scale if you plan on making your own. Please contact us if you would like a copy of the KiCad files.

The PCB layout is shown here:


Construction notes

Solder the circuit board first. There should be nothing too difficult with the soldering. Ensure that all the polarised components are correctly inserted. Take care to ensure the IC holder and IC are inserted correctly.

Kit of parts

Resistors added

Capacitor and IC holder

Add conenctors

Add variable resistors


Add transistor, voltage regulator and LED

Add MOSFET and diode – the finished board.

Attaching to a heasink

The MOSFET and high current diode should stick out at the side of the circuit board. They will also use the heatsink along with the dump load, but with insulating pads.

Set the dump load and the circuit board as shown here:

These are the items you will need: computer heatsink, resistor dump load (with soldered on wires), built pedal shunt board, 2 x TO220 insulators, 2 x plastic TO220 insulating rings, 4 x 3mm x 16mm machine screws, heatsink compound, 2.5mm drill bit, 3mm tap with tap holder.

These are the parts to insulate the MOSFET and diode from the heatsink (there would be problems with short circuits if you do not do this).

The computer heatsink with fan

Remove the fan and also the metal clamp (used to hold the heatsink to the computer chip)

Give the heatsink a good clean with some white spirit or meths.


Position the dump resistor to the top of the heatsink area and mark the two holes.

Position the MOSFET and diode at the bottom of the heatsink so that everything fits on easily. Mark the positions of the mounting holes.

Drill a 2.5mm hole for each of the 4 positions.

Use the 3mm tap to make a 3mm screw thread into the heatsink. Use some lubricating oil to help with this. Aluminium ‘sticks’ when doing this – I twist forwards a couple of turns and then back one turn or so to try and stop this.

Clean off any swarf.

To mount the diode and MOSFET use an insulating pad and a plastic cover for the machine screw (as shown here). You do not need any heat transfer compound with this type of insulating pad.

Fix both components and tighten well.

Apply a thin layer of heatsink compound to the back of the dump resistor. As expected – this helps transfer the heat from the dump resistor to the heatsink.

Screw down the dump load – this does not need to be isolated.

Re-attach the fan.

Connect up the dump load and the fan power supply wires.

You should now have a finished shunt-regulator for your pedal generator. You will need to calibrate the unit to ensure the output voltage is correct.

You can mount this unit into any enclosure you like. I have been using ‘Really Useful Boxes’ as they are readily available, easy to cut and inexpensive. You must ensure there is sufficient air-flow over the heatsink, so you must have holes in the box. I use a Dremel to cut out a large hole and then use metal guards to stop interested fingers.

Calibration and set up

Note: this procedure needs testing to check it is OK.

To calibrate the unit you will need a multimeter and either a variable bench power supply, or the pedal generator and a willing volunteer to pedal.

There are two potentiometers on the board. One adjusts the voltage cut-in point, the other adjusts the hysteresis (the difference between the turn on and turn off voltages). Change one can affect the operation of the other, hence this process is iterative (ie you might have to set it once and then test, then set it again and test again until it is working as you would like.

Ensure there is no load on the output. Do NOT connect the dump laod. Start with both the potentiometer in the middle position.

Connect the multimeter to show the output voltage.

Apply some input voltage, either with a variable bench power supply starting at a low voltage, or with a cyclist on the pedal generator pedaling very slowly.

Increase the power supply (or pedal slightly faster). You will see the voltage rise until the dump load LED switches on. Watch the voltage when the LED switches ON – this is the regulation voltage. If this is too high then you will have to adjust the potentiometer down and re-test. If the voltage is too low then adjust the potentiometer up and re-test. For a 12V system, I set the voltage to around 13.4V DC, which is similar to a charged 12V lead-acid battery. You will have to reduce the applied voltage to a lower level and then increase it to check the regulation voltage.

The check the hysteresis turn up the voltage until the dump load LED is ON. Then very slowly turn down the applied voltage. When the LED switches OFF, the difference between the switch ON voltage and the switch OFF voltage is the hysteresis. E.g. Regulation voltage has been set to 13.8V, you then reduce the voltage to 13.4V and the LED switches OFF, then the hysteresis is 13.8V – 13.4V = 0.4V. I would suggest having the hysteresis for dump load control to be around 0.4 to 0.2V.

With some form of voltage indication the cyclist can tell when the circuit is limiting and hence slow down slightly. Most of our pedal power monitor designs use a voltmeter to display the voltage. These are sometimes built into the ultra capacitors we use as an energy store.

****image of a set up generator****

We are also in the process of writing a more detailed guide to the electrical systems for pedal generators. Please check back soon or contact us.

Note: While every effort is taken to ensure this information is correct, no responsibility is taken for any damage to equipment or people that may occur.

To do list

  • Sort out fan connector
  • Test out low voltage disconnect idea
  • Double sided PCB – get prototype
  • Fit into metal box
  • PCB mounting spade connectors
  • Make calibration easier (use zener diodes?)
  • Check calibration procedure

One response to “Pedal shunt regulator design

  1. Hello!

    Thanks for this great tutorial. We are from a squat urban garden in Barcelona Spain and we are trying to make a bike generator with a 250W generator (MY1016).
    We want to construct your shunt regulator but we are not very sure about the changes to make to works with our 250W generator.

    Could it works only with the following changes?:

    D1 –> 20A diode
    Dump resistor —-> 150W

    Do you know the reference number of a diode with this capacity?

    Thank you very much and sorry for my tarzan english style.


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