If there was any kind of electrical surge/transient, it's possible the gate-source was damaged, though we'll never know for sure. MOSFETs can fail in the shorted (on) state, and that would also fry the coil. Automotive electrical environments are not well-behaved, and prone to transients when cranking the engine, or when the alternator kicks in/out.
The first thing I noticed is the 2 resistors really dont protect the gate-source from overstress (most-sensitive part of MOSFET). You should put a zener diode between gate-and-source (anode to gate, cathode to source). I think extra capacitance across gate-source will help because the MOSFET has about 1500pF, which gives an RC time constant around 1.5usec; that should be enough for the zener diode to conduct, but if you want to play it safe, add 0.1uF across gate/source. Next, what does the pulse look-like ? In order for the MOSFET to be off, the input signal needs to close to the VDD supply value. Is that 12 volts since this is a vehicle ? Per the datasheet, the MOSFET will start turning on if Vgs is -4 volts. So, if your supply voltage is +12V, the input must be at least +8V to keep the relay off. -- You received this message because you are subscribed to the Google Groups "neonixie-l" group. To unsubscribe from this group and stop receiving emails from it, send an email to [email protected]. To view this discussion on the web, visit https://groups.google.com/d/msgid/neonixie-l/b7f2196c-f34f-4ef0-b661-3dd71132567f%40googlegroups.com.
