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Problem with Dash module in a Mosfet

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losullivan59

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Hi, I am working on the electrical system for a small race car, and have a problem. In my dash, I have a Polaris/Arcticat snowmobile/atv kill switch (Polaris part no 4012269). The kill switch is to turn on/off the master relay (tyco 1904000-1 with internal voltage suppression resistor) in the fusebox which in turn feeds the switched +12 to the fuses and components. The kill switch action is reversed, however - it is open when in the up (running) position, and closed in the down (killed) position. I believe this is used to short the ignition system to ground in some way in the snowmobile/atv. Anyway I want to reverse the action by having the kill switch ground the gate of a mosfet (IRFZ44ZSPBF) through a 10k pullup. The schematic for my setup is attached. This setup doesn't work correctly, however. When I first set this up it appeared to work, but then every once in a while the car would stay on when the kill switch was pushed. After a lot of testing, I figured out it the battery voltage was causing this in some way - below ~12.8V, the mosfet would function correctly, but above that, the mosfet would stay partially closed when the kill switch was depressed. The kill switch is working correctly - the gate sees 12-14 V when the kill switch is up, and 0V when the switch is closed. The last time the problem occurred, I took down the voltages: the gate was at ~10mV above the source, the battery voltage was 13.27V, and the drain / bottom side of the relay coil was at 10.39V. I've heard of mosfets latching when subject to voltage spikes, so even though there was an internal resistor in the relay, I added a flyback diode. That didn't fix it. I also tried wiring in a voltage divider at the gate to reduce the gate voltage to ~6V. That didn't work either. I am kind of new to mosfets, so maybe the problem is obvious to someone a little more experienced.

Thanks!
 

Re: Mosfet Problem

Try to add a couple of protective diodes, as shown on the attached picture ..

Rgds,
IanP
 

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