I am a hobbyst, and recently been working on a small project of my own, Creating a rev limiter of my own.

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

First of all following part of the ckt is original: +12v,To Vehicle's Trigger,spark plug;Transformer/Ignition Coil

Rest is DIY

There have been few problems I've encoutered and need help with :

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

--> Multimeter Probe Red on Ground and Black on E ( while DIY ckt is not grounded ) showed a +0.18 volts ie: ground is positive while E is negative !!! WHY ??

--> I figured out that when I dissconect 1N5408, and the ckt 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 volts from battery +ve)

ie: Closed gate works fine but open gate won't shut the bike off, ie: the gate won't open !?

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

Observations :

"Ignition Coil socket" when dissconnected 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 ECU.

Also the Ignition coil measured 2-2.5 ohms.

Ignition Cut diy is a signal, for testing purpose I am using +12 volts from battery's +ve terminal

IGBT is 25N120A


Also, I've tried the above ckt with inductive load (Toy Car Motor) works just fine.

Need Help..

  • \$\begingroup\$ You would use a series switch or a better shunt switch across coil for a magnetic driven open current. To coil. \$\endgroup\$ Jan 23 '21 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\$ Jan 23 '21 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\$ Jan 23 '21 at 19:14
  • \$\begingroup\$ Could you provide the circuit ? @TonyStewartSunnyskyguyEE75 \$\endgroup\$ Jan 23 '21 at 19:16
  • \$\begingroup\$ i.stack.imgur.com/9zBeV.png \$\endgroup\$ Jan 23 '21 at 22:07

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\$ Jan 23 '21 at 19:04
  • \$\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\$ Jan 23 '21 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\$ Jan 24 '21 at 6:29

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\$ Jan 25 '21 at 5:02
  • \$\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\$ Jan 25 '21 at 12:18

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