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Old 04-14-2009, 03:41 PM   #922 (permalink)
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Originally Posted by MPaulHolmes View Post
I also am adding a zener diode (I never knew what those really were until today. Don't tell anyone! It's a secret. I'm the "expert" hahahaha!) to not allow voltage spikes from gate to source. It will clamp the voltage from gate to source to no more than 20v. The voltage from gate to source can now freely rise to 20v, and then all of a sudden, BAM! NO MORE! HAHA! If even a small voltage spike of 30v hits the gate, it can destroy the mosfet. That could have been what happened last time. Now, that potential problem is eliminated!

Two quick notes on the zeners:

1) Make sure that the zener is tied after the gate resistor. Zeners need to be current limited, or they release their magic smoke.

2) Do a quick power rating check to make sure that the zener is beefy enough, If you have a 20 ohm gate resistor and are clipping the voltage to 20V, then a 30V spike will draw a half amp's worth of current (30V-20V)/20 ohms and would need 5W to dissipate.

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