Re: IRF510 amplifier failures

Jerry Gaffke

A suitable fast-blow fuse or polyswitch in the PA-PWR line into the board should be sufficient
to protect the IRF510's from too much current.
The polyswitch is like a fuse, except it resets once it cools down after you remove power.
A thermal sensor on the IRF510 heatsink that at least turns on a front panel LED might be a good idea.

This old post here from Allison is informative, well worth re-reading every few months:
As she says, lots of things can kill an IRF510, but the most common is heat if you have a good amp design.
Powering at 28v for 55W from a push-pull pair, she didn't see trouble when driving improper antenna impedances.

I'd think that if powered at 12v, and if they have a reasonable heatsink, and if you don't screw up 
when fiddling with the gate bias pots at rv2 and rv3, or drop a screwdriver into the wrong spot,
the IRF510's should not blow.  Even when driving a non-existent antenna, the RF voltages at the drains
go up some without a load, but not drastically, T11 is just a 1:2 voltage transformer.
Driving a 0 ohm antenna might be a bit worse since the currents will go up, but probably survivable.

Jerry, KE7ER

On Thu, Jan 25, 2018 at 09:11 pm, M Garza wrote:

There is a schematic for the current limiting circuit that is used.  This might be something to incorporate into the design.  It is only 7 more parts.


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