Using a simple PWM generator to drive a MOSFET for ignition coil / injector test

[Chris Wilson]

I have built a simple PWM circuit, that seems to work OK. It's schematic is hopefully attached. I also attach a screen shot of its output with just over 13V supply voltage on my scope. I would like to take it to the next stage and use it drive perhaps a big MOSFET and use it for testing vehicle ignition coils and fuel injectors, and also to use a similar circuit to drive a home built fuel injector tester / cleaner. How would I take its output and drive a big MOSFET from it? What MOSFET might I look to using?  Most coil on plug direct fire ignition coils draw a few amps, as do injectors.  Perhaps a maximum of 10 to 12 amps for a coil. I think I would need some back emf protection too? Thanks.

[T3sl4co1l]

LM358 is pretty slow to be using for this.  Though considering ignition isn't very fast to begin with, it might actually be only marginal.  But I'd like to see that done with something a bit faster, like a proper comparator.  (LM393 is popular and similar, but has open-collector output, so you have to do some handiwork with the output to get the same voltage levels).

If you don't care about speed, you can hook the output right to a MOSFET gate (via 100 ohm series resistor, say), which I should think would be fine for injector drivers, if maybe not ignition coils.

Tim

[Chris Wilson]

Thanks for your advice yet again Tim, very good of you. I think I may get away with the current speed as the coils i will want to test are coil on plug type, and only fire sequentially, so at the same speed as sequential injectors. if it were a single coil firing a distributor on a V12 i totally agree, speed would be very marginal. I am thinking more a "does it work" test, rather than a test at full RPM signal levels. I'll give it a shot as is, with a big MOSFET and gate resistor. Can i put a current limiting resistor in the output, to save the MOSFET should a short be applied? How would I calculate a suitable value for a given MOSFET please? Thanks again

[T3sl4co1l]

Sure.  For a MOSFET of some current rating:

- Disregard the current rating.
- Note how much power dissipation you can tolerate for your setup (no heatsink, <= 1W; small heatsink, up to maybe 5W; large heatsink, 25W+)
- Find Imax = sqrt(Pd / Rds(on)max).  That's Rds(on) max at rated temperature by the way, which is usually about double the headline rating (they won't say this in the data tables, you have to infer from the Rds(on) vs. TJ curve)
- Knowing these, we can find Rmin = Vin(max) / Imax.  Subtract coil resistance and whatnot to find how much is needed externally.

Note that Vds(max) needs to be pretty high to properly switch an ignition coil, and you don't want to exceed this rating, so a TVS of 70% that rating is a good idea.  Something like P1.5KExxxA would be fine.

Tim