Ignition Circuit help

You're going to need a storage scope to really see them. The biggest "offender" is the starter. Huge voltage drop while it's turning over, then a large spike when it stops.

Naw, just use any analog scope. You should be feeding a periodic signal, just as you would in normal operation, so set up your scope to use triggered sweep, and you'll see the signal being refreshed enough to get a good look at it. Just remember to turn up all voltages up s-l-o-w-l-y so you don't damage anything. If you see spikes that are getting out of hand, stop and figure out how to quash them.

BTW, you'll probably get spikes of up to 100's of volts out of the primary. That is normal. Be sure your equipment can handle that.

yeah, I just got a shiny high voltage probe brings back memories of high school...

I still haven't seen spikes over 30 volts or so, I may be reading it wrong though (guess I should have broken out my old textbooks first )
I'll do some more research and more measuring and let you guys know later tonight probably.

Thanks,
Xeno

The worst-case coil primary spikes will occur when the coil secondary is open (not connected to a spark plug).

xenocide702 said:

If i'm reading that datasheet right (and I'm probably not) It looks like to turn the transistor to the ON state, it requires 2-4v across the gate and the source, if this is correct, would that mean that the source is our ground and the gate is our positive voltage out of the arduino, and the drain is the ground for the primary winding on the coil?

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when the gate voltage is at the threshold voltage of only 2V to 4V then the Mosfet barely turns on with a current of only 0.25mA. That is its turn-on threshold voltage.
It is fully turned on when its gate voltage is 10V which an Arduino cannot provide.

Also, the gate of a Mosfet has a high capacitance that takes a fairly high current to charge quickly. An Arduino cannot provide the high current.

crutschow said:

The worst-case coil primary spikes will occur when the coil secondary is open (not connected to a spark plug).

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Gotcha, will include this in my next trial

when the gate voltage is at the threshold voltage of only 2V to 4V then the Mosfet barely turns on with a current of only 0.25mA. That is its turn-on threshold voltage.
It is fully turned on when its gate voltage is 10V which an Arduino cannot provide.

Also, the gate of a Mosfet has a high capacitance that takes a fairly high current to charge quickly. An Arduino cannot provide the high current.

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Ah, makes sense, I'll look around for a logic level mofset which looks good and run the spec sheet by you guys just to make sure

I've been digging deep into my old parts bin and pulling protection components from old computer power supplies and such. I found a schematic for an over voltage/over current protection circuit deep in the bowels of the internet.
It shows promise, i'll let you know how it goes (and I'll post a link as soon as i get home)

Ah, makes sense, I'll look around for a logic level mofset which looks good and run the spec sheet by you guys just to make sure

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Just drive the MOSFET with a BJT to boost the voltage. Use a 2n3904 set up for a gain of 2-5. I don't have access to my schematic software right now, but a common emitter configuration with a 300ohm emitter resistor and a 1K collector resistor will do it. Follow that with a secone 3904 in common collector with the emitter connectd to the MOSFET gate. There are websites devoted to using ignition coils that have drawings to show how to do that. If you don't have one after the weekend, I'll draw one up when I get back to the office.

Almost forgot; if you use the bjt as a voltage gain element, you'll have to reverse the sequence from the PLC. "Off" will then become "ON" and visa-versa, because the common emitter inverts the signal. If it in not possible to do that, you can use a emitter-coupled pair, or else we can figure out something else.

BrownOut said:

Just drive the MOSFET with a BJT to boost the voltage. Use a 2n3904 set up for a gain of 2-5. I don't have access to my schematic software right now, but a common emitter configuration with a 300ohm emitter resistor and a 1K collector resistor will do it. Follow that with a secone 3904 in common collector with the emitter connectd to the MOSFET gate. There are websites devoted to using ignition coils that have drawings to show how to do that. If you don't have one after the weekend, I'll draw one up when I get back to the office.

Almost forgot; if you use the bjt as a voltage gain element, you'll have to reverse the sequence from the PLC. "Off" will then become "ON" and visa-versa, because the common emitter inverts the signal. If it in not possible to do that, you can use a emitter-coupled pair, or else we can figure out something else.

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Wow, thanks man, I'll look around I'm sure I'll figure something out, but I really appreciate the offer (and may yet need it ). Switching on and off in the program would definitely not be a problem, these arduino boards are awesome, the programing environment is really user friendly and the code burns onto the board fast .

I'm not dead, just been extremely busy. I've been working a lot more and I started a megasquirt build on my 528e, so I've been living on little more than coffee for the last few weeks.

I've done some more research, and I think I'm on the right track, but I wanted to double check with your guys to be certain. I've found some high voltage coil driver IC's that look like they're pretty easy to use, **broken link removed** one in particular caught my eye. It appears to have a max operating frequency of 150 Hz, meaning one cycle every 6mSec which would allow plenty of breathing room as far as the redline is concerned (not enough for MSD but nobody's perfect).

From what I understand there will be a negligible delay (typically 1µSec, max of 5µSec) between triggering the coil driver IC (with +5v @ 100-150 mA) and the spark plug firing. And it appears that this only requires a ~5v DC power in, are these assumptions correct? I'm also unsure of what µP represents in their example circuit, any ideas?

Thanks,
Xeno

µP is the standard abbreviation for microcontroller.

That IC assumes that you have a microcontroller to work out when the coil should be turned on. The coil current takes the same time to build up to maximum whatever the engine speed, so the coil should be turned on a fixed time before the spark is needed.

If you turn on the coil early, the IC will be in current limit for a lot of the time so it will get very hot. If you turn on the coil late, the coil current will be low so the spark won't be very strong.

150 Hz seems a bit low for a multi-cylinder engine.
150 Hz is 9000 sparks per minute, so that would be 18,000 rpm for a single cylinder 4-stroke engine.
Or 9000 rpm for a 2 cylinder
or 4500 rpm for a 4 cylinder
or 3000 rpm for a 6 cylinder
or 2250 rpm for an 8 cylinder

I guess the IC is designed for cars where there is one coil for each cylinder or maybe each pair of cylinders.

Thanks for the reply and the clarification of microcontroller symbol.

I had planned on using 2 digital outputs from an arduino duemilanove (via protection circuitry of course) to drive 2 of these ICs, and then in turn 2 separate coils. The engine is a 4 stroke DOHC XS400. I already have both coils, and these chips are only ~$4.15 + shipping, is there a downside to running two of them?

xenocide702 said:

I've found some high voltage coil driver IC's that look like they're pretty easy to use, **broken link removed** one in particular caught my eye.

Thanks,
Xeno

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I have been following this thread, and checked out the High Voltage Ignition Driver IC . I think this chip could make a great replacement for the coil driver I presently use (an IGBT, with drivers).
Did you find any sources? All the sources I checked state it is a non-stock item - long lead times, and a huge minimum order - 400 or more.
I tried to get some samples from STMicroelectronics, but it looks like they do not sample it.

Ken

I was able to find a source, I'm at my parent's house right now and the link is bookmarked on my desktop computer, I'll see if I can find it here, if not I'll post it tonight.

UPDATE: that source I had found actually did NOT have them in stock, must have mis-read... I did find one source though: https://www.mymectronic.com It seems like a rip-off, they charge you monthly to use their service, but they do have a 30 day free trial, which I opted into.

UPDATE (again):Well, their site must still be in beta, or they're just really bad at what they do, their search turned up one distributor, when I contacted the distributor, they don't even carry it...

GOOD NEWS UPDATE: https://avnetexpress.avnet.com/stor...art&catalogId=500201&langId=-1&storeId=500201 they carry them, it appears they're out of stock, but they have no minimum order, it'll be 12 weeks while they're on backorder, but it appears they CAN get them