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Old 22nd February 2012, 02:43 PM   #1
Rixsta is offline Rixsta  United Kingdom
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Default Mosfet Protection ?

Hey, Ive built a couple of Pass amps, the Zen 4 and the Zen (light bulb)
The Zen 4 id had working for a year then accidently messed with the interconnects while it was on and i guess it must have sent a buzz to the input buffer then somehow blew 3 of the mosfets too!, it was lovely while it worked tho!
Then just recently i built the Zenlightenment and had 500w bulbs on it worked fine with the 250w, i expect i had the bias too high or something and fried one of the mosfets (should have had a multimeter to check voltages at the time!) other channel biased up and is fine!, anyway i know those IRFP240 and 140's are a bit senstive so i ask here is there any way of protecting those fets sensitivty while experimenting with the amps ? i know ive heard of putting in Zener Diodes on the gates of the Mosfet but not sure how to do that.

If anyone has any good ideas let us know
Those Mosfets are pretty expensive realy so it would be nice to not blow any more up!

Last edited by Rixsta; 22nd February 2012 at 02:46 PM. Reason: spelling mistakes!
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Old 22nd February 2012, 03:16 PM   #2
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The zener at the gate only protects gate. Normally, you can put a zener cathode to the gate (For N channel) and anode to source (not ground), also you can add a second diode in reversal series with it, to protect in case of -VG. But although zener is in the circuit, short circuit at the load or VDD >> VDSS the fet also will be destroyed.
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Old 22nd February 2012, 03:32 PM   #3
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Download GX400 rev.1 project here:
GX400 rev.1 - 400W Audio Amplifier - Comunitatea Tehnium Azi
In that project you will find in amplifier diagram a MOSFET transistor protection circuit. It is one of the best short-circuit protection circuit on the output audio amplifier.
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Old 23rd February 2012, 12:01 AM   #4
eyoung is offline eyoung  United States
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use an in series ~60 w light bulb in supply line as a current limiter, a variac would be nice to bring it up slowly to help you find the real culprit. o/p trannies when they go usually take a driver with them....hafler dh200 thread has a great section on starting a bad amp up to troubleshoot.


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