Re: Failed IRF510 in BJ

Jim, N5IB

Very likely not a "dead short" failure. The gate structure isn't suposed to pass current at all, so even if it's compromised it may end up being a "lower" resistance path. To make a non-heatsinked TO-220 really hot doesn't take much current. The IRF510 junction-to-ambient thermal resistance is spec'ed at 62 deg C per W. So a couple watts gets you better than a 100 dec C rise. And 2 W at 12 V is just 170 mA or so. That would be some 4 amp-hr per day drained from your battery.

Now, that doesn't take into account heat that is transferred away via the soldered pins, but even if that reduced the temperature rise by a factor of 10, you could get a 50 deg C rise above ambient (90 F - pretty hot to touch) with 8 W. That would be less than 700 mA and wouldn't blow the 1 A fuse.

It would be an interesting and useful interesting data point if you could do a quick measurement of that current before you make the repairs, just to test the above theory.

In normal operation the voltage drop, Source to Drain, is so small with the device fully turned on that the power dissipation is very small as well. With an on-resistance of less than half an ohm, the 750 mA Tx current will cause less than 0.4V drop, and dissipate less than 300 mW. And, in theory, there's no current at all Gate to Source.

Proverb ---  "In theory, there is no difference between theory and practice, but in practice ....."

Jim, N5IB

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