Function Generator Controlled Ignition Coil

[Kming]

Hey,

I'm Kinda a newbie at electronics .. >.>" but i have been stuck on this circuit for sometime already..

Anyways , there were a few things i wanted to do with my circuit
1) Use a Function Generator instead of a 555 timer to run the Ignition coil/spark plug
2) Prevent the Fxn Generator from being damaged by isolating the function generator from the HV circuit.

This circuit was designed by someone else, and they built it and said it works, but when i follow their steps, it doesn't work. And i don't really understand the circuit that well.

It seems like he needs the op-amps because the current/voltage coming from the opto is too small, and he is using transistor as a switch to create the AC for the transformer.. but I don't know much about opto's so not really sure ..

Also, i'm actually using the 4N35 not 4N25 And there is +- 20V external supply to the op-amps. And the empty space is where the inductor coil goes..


Any help will be appreciated

 

[bychon]

This circuit looks more theoretical than practical because the DC voltage source at the opto is rated at one tenth of a volt. Why would anyone start with a tenth of a volt for a DC supply and them amplify it by 340?

This one tenth of a volt is amplified to 34 volts by the op-amps, except there is only 20 volts of positive supply for the op-amps, so they can't get to 34 volts as an output.

Then the alleged 34 volt pulses are delivered to a power transistor with no limiting resistor in its base to emitter circuit. Current tends toward infinity and the transistor smokes.

There is a first list of "what's wrong with this circuit".

 

[cobra1]

what is it you need to know??
i assume your using the function generator to allow for timing adjustments??

what will be the application

 

[Kming]

@bychon: Ok xD cause the circuit worked according to the person who built it but it didn't work no matter what i tried.. so i'm guessing i would need to redesign it XD

@cobra1: Well basically its part of an experiment setup and it would make the experiment much easier to just alter the frequency of the input signal to the ignition coil (or the empty gap) using a function generator instead of playing with the R/C values of a 555 timer circuit.

Anyways Since it seems that such a circuit doesn't really work im probably gonna have to try and redesign a circuit, and basically i just want to be able to make the ignition coil receive 12 volts time varying signal, where the frequency will depend on what we set in the function generator. And I want to be able to protect my function generator, hence the opto isolator.. So any ideas how i should start about designing such a circuit?

Btw Thanks for the help so far.. it has concluded that this circuit will indeed never work.. >.>" and that i just wasted time on it.. sighs ><"

 

[bychon]

Or you could change V3 to 10 volts, throw one op-amp away, make the other one have a gain of 2, put a resistor in series with the base of the output transistor, and make sure the output transistor can stand the inductive kick the coil will deliver when it shuts off by choosing a high voltage transistor...or change to a high voltage MOSFET for the power stage.

 

[Kming]

Ok So i kinda just redrew the diagram, but you were implying this right??

And The transistor im using is the 2n3055.

I'm gonna go try this circuit out to see if it works. But i think that this schematic is what you mean so i will test it out ..

 

[tcmtech]

The inductive kickback from a common ignition coil will blow right through the 2N3055's 60 volt limit.
I have built simple ignition coil drivers that ran right off of standard signal generators 50 ohm output just by using a single 600 volt 10 amp or greater Mosfet or IGBT with the gate driven through a 1K resistor.

Also to get the proper spark form a induction coil it needs a non polar HV capacitor in parallel to the switching device. For a 12 volt coil something like a common mylar of poly cap in the 1 -4 uf range will work well.
Depending on the ignition coils inductance you may need to play around with that value to find the best match. It could be as low as 1 uf or higher than 4 uf.
Or just use an old one off of a lawn mower ignition rebuild kit or old vehicle. Its that little metal can thing in the points and condenser kits you can buy at the parts store.

 

[Kming]

Also to get the proper spark form a induction coil it needs a non polar HV capacitor in parallel to the switching device. For a 12 volt coil something like a common mylar of poly cap in the 1 -4 uf range will work well.
Depending on the ignition coils inductance you may need to play around with that value to find the best match. It could be as low as 1 uf or higher than 4 uf.
Or just use an old one off of a lawn mower ignition rebuild kit or old vehicle. Its that little metal can thing in the points and condenser kits you can buy at the parts store.

When you say to put a capacitor in parallel to the switching device, what do you mean? like parallel to the inductor coil/ 12V DC loop in the second schematic?

 

[BrownOut]

A capacitor is not required if you're using a transistor as a switching device. A fast switching power transistor can cut the current pleanty fast enough by itself.

 

[tcmtech]

Parallel as in parallel to the switching device as I stated in the first sentence.

The old points and condenser ignition systems used this method to create a simple LC tank that rings the inductor and creates a much higher voltage and hotter spark.
Modern CD ignition systems still use a similar method as well but at a higher input voltage and or amperage.

Try it and see what you get with and without the capacitor. Its old school ignition tech but it still works with solid state devices.

 

[Kming]

Hm.. None of the circuits i tried seem to be working.. So i was wondering if anyone know a circuit that could be used for this circumstance?

Very simply put im trying to run an ignition driver with a function generator without frying the function generator from the HV involved with the transformer.

 

[BrownOut]

You'll probably fry your opamp with HV spikes that will be present on the rail. And 100 ohm resistor on your optical device seems low. Just use a CE trasistor to drive your 3055

The only trick to getting a good spark is adjusting the pulsewidth enough to have a couple amps flowing in your coil before quickly cutting the current off. For your setup, that should be about 60ms. Sorry, I don't have a ready made circuit. Mine makes about 30KV using a ford coil and a 555 to make the pusle.