I am a hobbyst, and recently I've been working on a small project of my own: creating a rev limiter.

Following is my circuit which will explain a lot about what I am trying to do.


First of all, the following part of the circuit is original: +12 V, to vehicle's trigger, spark plug; transformer/ignition coil
The rest is DIY.

There have been a few problems I need help with:

  1. As soon as I connect the collector @ c and emitter to GND (vehicle's battery -ve terminal), the engine halts (or the bike won't start), even when the gate is pulled down.

--> Multimeter red probe on Ground and black on E (while DIY circuit is not grounded) showed a +0.18 V; ground is positive while E is negative. Why?

--> I figured out that when I disconnect the 1N5408, and the circuit when connected to GND and @ C to primary side of ignition coil, it won't halt the bike, but also then the gate won't open (applying 12 V from battery +ve).

So closed gate works fine, but open gate won't shut the bike off, i.e. the gate won't open.

  1. I've read about inductive loads, as soon as the vehicle's trigger switch opens there will be oppossing voltage ie: highly +ve @ C (hence 4.7 μF capacitor acts as IGBT protector while 0.1 μF cap prevents accidental gate opening since gate-collector absorb some charge on their own (same with gate-emitter)) so the current shall flow from C to E, but because of the diode I noticed current is flowing opposite, hence the bike is halting even when the gate is closed.

Observations :

"Ignition Coil Socket" when disconnected and (socket being measured) measured using voltmeter shows +ve and -ve (black wire) as shown in picture.

Only 2 wires @ ignition coil primary side socket, so I figured out the trigger is integrated with the ECU.

Also the ignition coil measured 2-2.5 Ω.

Ignition Cut DIY is a signal, for testing purposes I am using +12 V from the battery's +ve terminal.

IGBT is 25N120A.

Also, I've tried the above circuit with an inductive load (toy car motor) and it worked just fine.

  • \$\begingroup\$ You would use a series switch or a better shunt switch across coil for a magnetic driven open current. To coil. \$\endgroup\$ Commented Jan 23, 2021 at 17:40
  • \$\begingroup\$ There are many issues, so where did you get this circuit? Why IGBT, why not MOSFET? What's a Vehicle Trigger in your schematics? \$\endgroup\$ Commented Jan 23, 2021 at 18:28
  • \$\begingroup\$ IGBT have higher voltage rating so as to protect it from high back emf due to collapsing magnetic field, also last time I used MOSFET(IRFZ44N) it burnt...Vehicle must be having its own circuit to connect/dissconnect primary side of ignition coil to create spark on plugs.like ignition driver connected with Crank Position Sensor which according to timed signals fires-up the coil.. What is your solution to this. Can you provide your circuit diagram? How can I cut the ignition safely so as it doesn't in any possible way creates knock in engine due to bad timing or circuit failure @Marko Buršič \$\endgroup\$ Commented Jan 23, 2021 at 19:14
  • \$\begingroup\$ Could you provide the circuit ? @TonyStewartSunnyskyguyEE75 \$\endgroup\$ Commented Jan 23, 2021 at 19:16
  • \$\begingroup\$ i.sstatic.net/9zBeV.png \$\endgroup\$ Commented Jan 23, 2021 at 22:07

2 Answers 2


First you have to make sure that the coil can withstand a continuous current flow. For example you switch on the ignition key and then measure the voltage Vehicle Trigger VS GND. If this voltage is low, it means that the ECU is designed to emulate the old style mechanical distributor switch. In such case you do only need to connect in parallel the IGBT or MOSFET, without any other capacitors, snubbers,...etc because the ECU switching element already has those.

But it might be that ECU closes its switch only for a determined time with calculated advance time before spark is needed. In such scenario, the most probable result of your circuit is blowing the coil.

Note that adding the capacitor in parallel with ECU switch can damage the ECU.


enter image description here

As you can see, the example ingnition system all these snubbers, capacitors, resisors already exist in ECU. A tip: don't overcomplicate you life with MOSFETs, IGBTs use a BJT similar on this schematics. BU508A has Vceo max of 1200V.

Place your (whatever) transistor close to the ECU box to minimize inductance due to wire, so that internal snubbers of the ECU will be able to eliminate the spike.


Found BU941ZT, BU323Z Ignition coil driver NPN darlington BJT, made for the task.

  • \$\begingroup\$ Volatge between vehicle's trigger and ground (vehicle's body) is near 12. vehicle's trigger being positive. So does that mean that the coil can take upto that (high) current limit right !? \$\endgroup\$ Commented Jan 25, 2021 at 5:02
  • 1
    \$\begingroup\$ @PrashantRawat It could mean that the coil is not designed to be powered on, just short pulses. You would have to move the engine manually and see what happens, when near TDC position is reached. \$\endgroup\$ Commented Jan 25, 2021 at 12:18

There are multiple things wrong with that circuit it not only cannot work but it would probably burn out your ignition coil.

  1. The high-voltage that occurs when the switch opens is required - it is that high voltage that gets transformed to the high voltage needed to cause a spark. Putting the 4.7uF capacitor to ground will stop that high-voltage and so stop the spark.

  2. Turning on the IGBT will stop the spark but the current in the coil will then rise to a high level and most coils are not designed to be driven directly by 12V for any sustained period. It will overheat and probably burn-out.

Turning off the power from the +12V could work or another way might be to put the 4.7uF in series with the IGBT rather than across it. This will stop the spark but not cause a high DC current through the coil. Are you sure there was not an error in the original circuit?

  • \$\begingroup\$ 4.7uf Cap to protect the IGBT || AND || As far I have read that's how the spark is produced when coil is grounded it has high current & low volt...as soon as coil is dissconnected from the ground it collapses the magnetic field and creates v.v.high voltage in secondary side. || If you think its incorrect help me with your correct circuit. \$\endgroup\$ Commented Jan 23, 2021 at 19:04
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    \$\begingroup\$ @PrashantRawat - the usual value for the capacitor (used to be called 'condenser" for cars) across the ignition coil is ~0.1uF. 4.7uF is much too large. The capacitor resonates with the coil when the switch turns off. What value capacitor was present in the original vehicle? The IGBT has to have a sufficient voltage rating to tolerate the voltage when the coil is ringing. The voltage across the IGBT will go up to ~300V and go negative by nearly as much. \$\endgroup\$ Commented Jan 23, 2021 at 21:35
  • \$\begingroup\$ IGBT's Vce = 1200v, Vehicle Duke 250, I can't find ignition diagram of my bike online anywhere. @Kevin White \$\endgroup\$ Commented Jan 24, 2021 at 6:29

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