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Motorcycle Mosfet Full Wave Shunt Regulator Circuit

Motorcycle Mosfet Full Wave Shunt Regulator Circuit

The following post of a full wave motorcycle shunt regulator circuit was requested by Mr.Michael. Let's learn  the circuit functioning in details.

How a Shunt Regulator Works

Shunt regulator is a device which is used for regulating voltage to some fixed levels by means of shunting. Normally the process of shunting is done by grounding the excess voltage, just as zener diodes do in electronic circuits.

However one bad aspect with such regulators is the generation of unnecessary heat. The reason for heat generation is the principle of its operation where the excess voltage is short circuited to ground.

The above practice may be implemented by simpler and cheaper means, but cannot be considered efficient and advanced. The system is based on destroying or killing energy instead of eliminating or inhibiting it.

The circuit of a motorcycle shunt regulator discussed in this article takes a completely different approach and restricts the in-flow of excess voltage instead of "killing" energy and thus stops the generation of unnecessary heat.

Circuit Operation

The circuit functioning may be understood as under:

When the mobike is started, voltage enters across the P-channel mosfet source/drain pins due to the gate trigger that becomes available via R1.

The moment the high voltage reaches R3, which happens to be the sensing input of the opamp, pin#3 of the IC senses an increased voltage.

As per the set reference at puin#2, the instantaneously reacts to the situation and the result puts the output of the IC to a high logic level.

The immediate high logic pulse restricts the negative base trigger of the mosfet, switching it OFF at that particular instant.

The moment T1 switches OFF, voltage at the junction of R3/R4 reverts to the original condition, that is the voltage here now drops below the reference level......this instantly activates the opamp output with a low logic signal which in turn switches ON T1 back into action.

The process repeats at a very rapid speed, keeping the output voltage marked with +/- at a constant level determined by the setting of R2/Z1 and R3/R4.

The above principle utilizes voltage inhibition technique of the excess voltage instead of shunting it to ground, thus saves precious power and also helps to control global warming in some way.

motorcycle mosfet shunt regulator

Parts List

R1, BR2 = 10Amp bridge rectifier

R1 = 1K
D1 = 1N4007
C1 = 100uF/25V
IC1 = IC741
T1 = mosfet J162

R2/Z1, R3/R4 = as explained in this article

Shunting Excess Power to Ground is Recommended in Alternators

When it comes to alternators, the best way to restrict or limit excess voltage is to short the excess power or shunt the excess power to ground. This eliminates the rising current in the armature and protects the winding from heating up.

A voltage regulator using this method can be witnessed in the following examples:

Video Clip below shows an opamp based shunt regulator circuit, and its testing procedure

Parts List

R1, R2, R3 = 10K
R4 = 10K preset
Z1, Z2 = 3V zener 1/4 watt
C1 = 10uF/25V
T1 = TIP142 (on large heatsink)
IC1 = 741
D1 = 6A4 diode
D2 = 1N4148
Bridge rectifier = standard motorcycle bridge rectifier

How to Set up the Circuit

For a 12V system, apply a 18V from a DC power supply from the T1 side, and adjust R4 to precisely set 14.4V across the output terminals.

An even simpler motorcycle shunt regulator using the shunt regulator IC TL431 can be witnessed below, the 3k3 resistor can eb tweaked to chnage the output voltage to the most favorable level.

motorcycle transistor shunt regulator using the shunt regulator IC TL431


About the Author

I am an electronic engineer (dipIETE ), hobbyist, inventor, schematic/PCB designer, manufacturer. I am also the founder of the website: https://www.homemade-circuits.com/, where I love sharing my innovative circuit ideas and tutorials. If you have any circuit related query, you may interact through comments, I'll be most happy to help!

49 thoughts on “Motorcycle Mosfet Full Wave Shunt Regulator Circuit”

  1. Hi Swag Once again.

    what to ask if there is a way to build this circuit with out the zener diodes
    since it is not available where i buy parts


  2. is the rectifier will experience extreme heating by the shunting process? passing high amount of current to laod also heats the rectifier thou

    do i have to change R1 and Z2 value if I want to use IRF540, also if i wanted to use more than one mosfet, do i have to clone R1 and Z2 also?

    • yes obviously the rectifier will heat up and that's why it must either rated 3 times more than the max alternator current or mounted over a suitable heatsink.

      adjust the preset such that mosfet gate receives around 9V at the desired trip point…for example if the regulation point is 14V, adjust the preset to produce 9V at the gate for the mentioned regulation limit, make sure to use a 8V zener in series with the mosfet gate, anode towards gate.

      No need to change anything except the above suggestion.

    • is this for the first last diagram or the second one? as i see there is already a zener in series with the transistor base (i'm going to use mosfet instead of tip transistor)

    • the suggestion for the both the designs if mosfet is being used. I am referring to R4 in the first diagram and 4K7 resistor for the second diagram.

      for the first design change the shown base zener to 9V zener if mosfet is used

    • Hello Swagatam,

      I need your advice again with this circuit, i'm having problem with the mosfet used in place of the bjt.
      Problem is the mosfet gets killed 2s from the time the input voltage reaching 18v dc, that it is drain and source now always conducting no matter there is a signal or not on it's gate.
      Tested the opamp circuit and it is working correctly, it's outputing 9v at the mosfet gate zener.

    • hello Ahmad, most Mosfets will not tolerate 18V at their gates and will get destroyed…you may have to modify the shown circuit and replace the shunt regulator IC with only resistors to form a potential divider at the gate of the mosfet.

      This potential divider values then could be adjusted such that it produces around 9V at the mosfet gate when the alternator output is around 18V.

    • now I get confused, it is working as expected in this page https://homemade-circuits.com/2014/12/3-phase-motorcycle-voltage-regulator.html with input voltage easily swing to more than 100V with full engine rpm.

      Why can't it work in this one with bench supply delivering not more than 20V 5A max. Is it because I connected the drain to input voltage by hand not making it fixed thus making some sparks which lead to damaged mosfet.

      I liked this circuit very much as it only requires few component and I can make the mosfet share their load to handle more current easily.

    • in the other design the supply is coming through the IC 555 and therefore it's restricted to 12V at the gates of your mosfets, and the output is determined through the PWMs….. whereas in the last diagram above, the supply to the gate needs to be equal to the LM431 voltage, which directly determines the regulation voltage for the output, so to get 18V regulated you are perhaps adjusting the LM431 voltage to 18V at the gate…which is harming the mosfet.

      if you have not adjusted the LM431 output at 18V then it could be strange…because the LM431 output at the gate will regulate the output at the same level….so if it's 9V at the gate, the output should be also approximately 9V regardless of the input.

      or may be your power supply current is higher than the mosfet rating which could be damaging it.

    • it's the one with 741 circuit not the lm431, at 18V the the output from the 741 is the same as the input but after the gate zener it is now become 9v at 18v input which i'm using 8v zener for the gate mosfet.

      according to the spec, gate voltage is +- 20V and maximum continuous drain current 30A. while my power supply is pretty much limited by the size of the transformer which is 5A and the voltage is set to 18V when the problem occures. or is it the 741 that subject to fail? could you point me to what to check to sort this problem out.

      you are very big help

    • mosfets are sensitive devices and sometimes unpredictable, unless these are precisely handled in the circuit, you can get more details here:


      I would recommend you to try with a BJT first, such as TIP142 and see how it responds, if it responds well then the problem could be specifically with the mosfet.

      and yes if the power device fails and stops shunting due to some reason then the IC 741 will be at risk and might get damaged due to unrestricted voltage..so I would recommend to provide some kind of protection to pin#7 of the IC, by using a regulator IC such as a 7812 or a 12V zener (through a limiting resistor)

    • I see, I will try with the suggested transistor then.

      what parameters and formula do you use for calculating R1 value, is it
      R = (Us – Vbe).Hfe / Load Current
      R = (100 – 3.5).1000/10 = 9650 Ohm

      I want to be able to calculate R1 value if I want to use another NPN transistor

    • the formula and the calculation is correct

      however here we would want to allow the transistor to shunt maximum amount of current so that the regulation is effectively maintained, therefore any smaller value resistor can eb selected, it does not need to be calculated….and moreover since the TIP142 is a Darlington it will have a huge gain and any resistor between 1K and 50K will have almost the same results, therefore precision can be ignored

  3. is the rectifier will experience extreme heating by the shunting process? passing high amount of current to laod also heats the rectifier thou

  4. Mosfet design may harm Stator and rectifier, as Hayden pointed out
    Second design may harm the rectifier
    Simple design will blow up.

  5. First design may harm your stator and rectifier. As pointed out by Hayden
    Second design puts unneccesary stress on rectifier.
    Simple design will blow up

    • Hi Chito, yes it can be easily done by disabling one of the main CDI wires through the wireless relay.

      If possible I'll try to present in my website soon.

  6. I like the circuit design above because the simplicity. At the above diagram I replace 4k7 resistor with 2k7 and than 3k3 with 47k potentiometer to get 14.8 volt dc. It's all okay except of heat dissipation.

    • Hi Julian, I am glad it's working,

      if the shunt regulator is controlling the voltage at the preferred level and also becoming hot, then that's a good sign, it means it's working as per its specifications.

      No mater which circuit you try all will show equal amount of dissipation, because the dissipation is the equivalent of the excess power that's being generated by your bike alternator.

      you can try adding a finned type heatsink to the transistor for enabling a better heat control, such as this:


      or you can also try replacing the TIP with a mosfet such as a IRF9540 and see the response.

  7. Thank you sir for your explanation. I have checked and yes tl431 ic was broken. When i replace it with a new one the circuit is working properly. But one thing that anoying me… the heat dissipation of transistor. When i touch the allumunium body It's too hot. I put the cooling fan too but the result is the same. Between of motorcycle regulator diagram that you have posted in this page and than at https://homemade-circuits.com/2014/12/3-phase-motorcycle-voltage-regulator.html?m=1 and also https://homemade-circuits.com/2016/02/motorcycle-shunt-regulator-circuit.html?m=0.. which one can give the best of reduce heat dissipation? Thank you so much

  8. I have made one using tl431 ic type. The problem is the output voltage of homemade regulator is increased also follow rpm rotation . Is the battery condition also affects the stability of the voltage at the output ? Need for help.

    • the regulator will regulate regardless of the battery condition, and not increase above the set value.

      First test the results on a work bench using a 5 amp variable voltage supply…and confirm the voltage across the collector emitter of the regulator transistor which should be constant regardless of the voltage increase from the input.

      make sure the transistor is mounted over a heatsink.

      you can tweak the 3k3 resistor value to achieve the correct output

  9. Thank you so much for the quick response sir. I have tip36 transistor and many 2SA1494 power transistor at home. Can i use them to replace tip142 or 147 transistor? Any solution to reduce heat dissipation of transistor to make it quiet cooler without a big heatsink. Cause when using a big heatsink will take more space on a bike.

    • Julian, you can try TIP36, although it's not a Darlington, it might work too since the base resistor is significantly low and the shunt IC is capable of providing sufficient current to its base.

      heat generation is the hallmark sign of shunt regulators and it's ths attribute which actually safeguards the alternator….so this cannot be and should not be changed.

      you can probably use an aluminum body and use it as the heatsink itself, but make sure it doesn't come in contact with the vehicle body

  10. Just information on my motorcycle: battery 12v 9Ah, and the load is head lamp 12v 55watt 2 noss, stop lamp 12v 21 watt and klaxon as usually use on a car and many more. The problem is my battery is always no powerfull and i have to charge it manually. When i measure from the alternator is very powerfull at idle is 20 volt and then when running in full power is more than 50 volt but when i measure the original regulator output I'm surprise the voltage just about 12 volt no matter at idle or running on full power. That's why i want to build homemade motorcycle regulator from the above article that you have post. I hope the circuit design above solve my motorcycle problem. Thank you so much

    • the output from the regulator must be set at 14.4V for charging a 12V battery, so if your existing regulator is giving only 12V then that's not correct…

      make sure to add a big heatsink with the shown TIP147 or the TIP142 transistors, otherwise the device could instantly get damaged

  11. Thanks a lot Mr. Swagatam for the simpler design.. but if the output after rectifier in idle condition more than 25 volt dc, I think when the bike running will produce more voltage and current. I have tested the output rectifier without regulator when running is about 80 volt dc. The question is.. can the 3 phase motorcycle shunt regulator in the above article is able to handling the high voltage at the input of regulator and then stabilize the voltage too?

    • Hi Julian, yes, the last two designs are perfectly capable of controlling all voltages below 100V, which can be further upgraded simply by changing the power transistor rating.

      Basically the designs are like higher power zener diodes, which will never allow the applied voltage to go higher than the specified rating.

      here R4 in the second last diagram and 3k3 in the last diagram can be appropriately adjusted for setting up the circuits to any desired stabilized output within 20V

  12. Sir, my motorcycle need 3 phase shunt motorcycle regulator. Could you help me to design that from the circuit above? Thank you.

  13. I thought back EMF doesn't happen in this case since there is no current generated. So your design was very worthed.
    Sorry for my bad English 🙁

  14. Hi Abu-Hafss, I think you did not get haydens point, according to him automobile alternator output needs to be loaded in order to regulate (bring down) the line voltage, other means of regulation that does not load the alternator winding will cause harm to the particular winding,
    The shunting of the excess voltage to ground satisfies the loading condition of the alternator and thus this method must be employed for regulating alternator outputs…. but I don't know whether this is true or not

  15. But since the voltage from the rectifiers will be higher than 12V, I doubt the 12V output from the op-amp will not switch off the p-mosfet. Same will be the situation if n-mosfet is used. You need to optimize the configuration such that p-mosfet is completely switched off against 12V output from the op-amp or such that n-mosfet is completely switched on against 0V output from the op-amp.

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