Revolution Limit circuit diagnosis

Thread Starter


Joined Dec 15, 2016
Thank you all for viewing my posting,
My intent is to create a revolution limiter for a high energy ignition to see if what I "thought" would work in theory actually works in a functional circuit. In testing my circuit (as shown in the provided schematic) on a distributor testing machine I found the following:

1) With no circuit used, I connected a wire to the coil -/ tach out connector on the cap and touched this to ground with the distributor spinning which shorted to ground and allowing the coil to not have output.
2) I next added the provided circuit which coming from my proven microcontroller circuit SHOULD ground the coil when I press a button/switch.

3) When I tried my circuit as described above I found that whether or not the distributor was spinning, I had no spark whether or not I was pressing the button which told me that the coil was being grounded by my circuit.(if the coil was not grounded the distributor machine would show a spark occurring) and the darlington transistor bu941zt became extremely hot.

4) I added different resistance values(100, 10k, 22k ohms) individually at point -A- to limit current to the darlington but found the circuit to NEVER unground the coil and the distributor worked as if the circuit wasn't even there.

1) Can someone point out to me why this circuit is not functioning as I had envisioned?
2) Is there an easier or better way of doing this circuit?

Thank you for helping me understand my errors and making this circuit work.



Joined Mar 14, 2008
You seem to be contradicting yourself when talking about the coil being grounded or ungrounded.
When the transistor is conducting is the coil supposed to work or not? o_O

It would help if would post a diagram of how the coil is connected to everything.

Thread Starter


Joined Dec 15, 2016

You are shorting the BE of the transistors:

View attachment 132622

Thank you for your reply. I actually made an error in my drawing of the circuit. As you pointed out the darlington transistor was indeed drawn to short out. My intent was to make the two right most transistors to be one BU941zt darlington transistor. The base of the darlington should see .143A correct?
That being said I still can not figure out why this circuit will not function.

Thank you for your help.

Thread Starter


Joined Dec 15, 2016
I have spent some time on this circuit I am working on the last few days and wanted to post my updated complete circuit as well as include the circuit that my circuit works (in parallel... I believe) with. I was able to make this (my) circuit interrupt the spark from the main ig. circuit by removing the voltage divider, (3) 2k resistors and Q3. I added a pull down resistor and a spst switch to control pin 5. I was able to cut the spark when pressing the button and when pressing the button a second time, return spark to normal operation. Again my testing takes place on a distributor testing machine I have access to. My coil is 8mH and .6 ohm primary.
My scope traces are labeled A and B as to where in the circuit I took the reading. The resultant traces can be seen as labeled near where the reading was taken. Trace "A" is a rough visual of what I saw on my scope. At rpm below 2000 this was very much the norm and I saw firings that were random but some always occurred the same way(if that makes sense), being a small firing after the normal firing of the coil. Trace B shows what I saw at this point in the circuit. This to me looks like some kind of decay, but I am not sure what It could be. If you watch the scope as rpm increases from 0 - 2000, you can see trace B increase the voltage where it is about 4v in my depiction, to 14.3v. from just over 2000 rpm to over 5000 and the trace looks normal (14.3v-0v-14.3v). If you have a constant rpm, let's say 4000, the left side of trace B looks like it is a flag flapping in the breeze(sorry for the bad analogy), as the voltage on the left side of the trace rises and falls from almost 14.3v to about 4v.
The circuit as it is drawn now is my attempt to have both circuits function together to normally fire the coil without any noise or unwanted triggers. Further evolution of this circuit will be for the PIC to have control over when the coil fires (delayed signal).
I hope I have supplied enough detail on what I have seen on my scope as well as my intentions with this circuit. Please guide me in solving these circuit issues so circuit function properly. I welcome your questions.


100_0280.JPG Master_circuit.jpg


Joined Jan 8, 2017
I think removing the 2K potential divider restored the function of the coil. When the switching transistor in the original ignition circuit turns off it's collector will normally go to several hundred volts positive. ( NOT JUST THE 12 VOLTS OR SO at the top of the coil.) The load of the top 2K resistor via the base emitter junction of Q3 i will prevent the voltage on the negative end of the coil going up to several hundred volts so the voltage on the coil secondary will not be high enough to produce a spark. I think increasing R3 to something like 220K will solve the problem and still give enough base current to Q3 for it to provide speed pulses to the PIC. Also your BU941ZT seems to be switched on when it should be off. As you say the coil resistance is 0.6 ohms there will be about 20 amps flowing through the transistor and coil which is likely to burn it out if it is left on for a significant time. (The coil will be disipating 240 watts !) This is also why the transistior is getting hot. You could put a zener diode of about 20 volts or a few hundred ohm resistor in series with the collector. This would still stop the spark without drawing a continious high current. The BU941ZT should only be switched on when the tacho pulses are above the threashold level.

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