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Registered Member #4426
Joined: Thu Feb 16 2012, 09:03AM
Location:
Posts: 2
I'm trying to get an ignition coil to create sparks running it with a Mosfet powered by a 555 timer. The Mosfet is definitely switching. I checked it with resistor, ampmeter, and o-scope. Everything is behaving predictably but when I use it to power a car Ignition Coil, I don't get any sparks. I can tap the terminals of the Ignition coil by hand -- Even with it connected up to the mosfet circuit -- and get sparks. But when I try to let the mosfet drive it, there are no sparks. There is definitely current flowing. And if I change the frequency to super low (like a pulse every second), the ampmeter is registering the pulses. If I change the frequency to super high (like 100khz), I get averaged current flow on the ampmeter. But I don't get sparks!!!!! I also switched out mosfet with other kinds (different ratings). Didn't fix it.
Registered Member #2099
Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1716
Since you have an o'scope, how about comparing the waveforms on primary winding, with your two different switches. Maybe your driver doesn't turn the MOSFET off fast enough. Are you sure the MOSFET is on long enough (some milliseconds) to charge the coil?
[edit]what happens when mosfet gate is pulled up with a resistor and turned off by manipulating a jumper wire to ground?
The grounding topology could matter. Avoid having coil current flow in the wire between your 555 circuit ground and the mosfet source.
Registered Member #195
Joined: Fri Feb 17 2006, 08:27PM
Location: Berkeley, ca.
Posts: 1111
you may need a one shot to adjust the pulse width to 200us or so. you may need a scope to see this. you could make a one shot and slow down the 555 and increase the pulse width a larger timeing cap and use a LED to observe it then just decrease the timming with a smaller cap while it is connected to your fet coil setup. as you get to the right pulse width you should see a spark. make shure you have a resistor in series with your coil and fet I would strongly invest in a 100$ scope. I found a old tektronix on ebay. just make shure it works and make shure it isn't big and clunky
Registered Member #4426
Joined: Thu Feb 16 2012, 09:03AM
Location:
Posts: 2
The coil was bad. It was working just enough that I could get sparks by hand... which is strange. But they were inconsistent and rather weak. I got another coil and it produced very consistent strong sparks by hand and with circuit.
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Ignition Coil Won't Spark from Mosfet but DOES spark By Hand??? HELP!?
