Re: IRF510 amplifier failures

Arv Evans
 

Hello

Still thinking about IRF510 failures and ways to avoid making smoke.....

It seems that there may be two ways to limit current in BITX RF PA stages that use
the IRF510 device.  First would be to incorporate a current limiter in the positive
voltage supply to the RF PA stage.  Second might be to use current sensing in the
RF PA supply line to cause a voltage drop in the MOSFET bias line(s). 

In the first instance it should be possible to use another IRF510 or preferably a higher
power, MOSFET as a control that limits voltage to the RF PA section if the current exceeds
a preset value (usually this would be 2.0 to 3.0 amperes). 

This circuit has not been built or tested by me (too cold in Idaho hamshack in winter).
The MOSFET could be an IRF840, or similar.  Option-B  potentially solves another
problem, that of low level noise from powered RF PA stage during receive mode. 

In the second instance we could get by with smaller control devices if we used
current sensing in the RF PA power line to turn off bias to the IRF510 device(s). 

This bias control circuit used by AC2CZ in his 50 watt IRF510 linear.  You could
probably do this using 2N3906 transistors.  I like the fact that it includes a warning
light/LED.


Arv  K7HKL
_._

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