Re: IRF510 amplifier failures

Arv Evans


No modulation involved.  The operation is as a switch between ON and almost
OFF.  Gain of the two BJTs makes it operate quickly and drop the output voltage
to the point where the current sensing resistor has less than the 0.65 threshold
voltage across its base-emitter junction.

I see where it does look like some of the CB-era AM modulators because it
causes a drop in voltage when current exceeds the base-emitter threshold.   

I'm still looking at the current limiter with an eye toward possibly adding another
MOSFET to get both polarity protection and current limiting.  Problem with fuses
for protection is that most do not operate fast enough.  There are specialty
Fast-Acting fuses  and Slow-Blow fuses but most hams do not have a ready stock
of these.


On Sun, Jan 28, 2018 at 9:59 PM, Tim Gorman <tgorman2@...> wrote:
It would seem to me that this type of operation is actually AM
modulation of final. When you limit the current flow in an FET that is
very similar to plate modulation in a vacuum tube. It certainly can't
be good for the linearity of the amplifier since the operational curve
would no longer be linear but, instead, flat-topped at some value.

Why not just fuse the power lead? It would tell you immediately that
you have a problem.

tim ab0wr

On Sat, 27 Jan 2018 12:13:49 -0800
"Arv Evans" <arvid.evans@...> wrote:

> 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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