Re: ?- How high SWR before smoke?

ajparent1/KB1GMX <kb1gmx@...>

Yes< RD16s are speced for 12V nominal nd16V max.  Its not wimpy but specifically
designed for 12V RF systems.   

IRF510 is speced for 100V, max 4A @100C and is a very robust device given half a chance.
So its seemingly hard to fry.

BOTH devices will disappear if the voltages to the gate (RF or DC) exceed a safe value
like 12V for the RD16 and 20V for the IRF510.  THis can happen if the amp is unstable
and oscillates, due to Miller feedback (gate to drain capacitance) if there is a large
RF voltage on the drain and the circuit is not RF stable the coupling can be sufficient
bring the gate over the breakdown rating.  All mosfets have this limit, I've fried
MRF137s that way and exotic GaN mosfets too.

The fact that the RD16 tab is ground is handy... BUT at RF where you ground things
can be precarious as you can create incidental current loops that incite instability.
Please remember there is no such thing as ground only the circuit common
return point for currents.  If you ground the board and the tabs of the RD16HHF
you created a set of current paths that Farhan has no design participation in and 
may or may not be stable, likely the latter.

I run IRF510s a pair push pull in an amp at 28V and they do about 55W at 40 or 20M
and have done so for more than a decade.  24-30V range is a sweet spot for these.
Its suffered shorted coax, open coax, no coax and wrong TX filter switched in
without failure.  I will point out the power supply used inside that amp has a current
foldback at 4A, hint 4A at 28V is 112W input to the amp and its trying to put out
60+ watts at 40M.

I've also paralleled 8 IRF510s as 4x4 push pull to make a 200W+ 6M amp.  Runs best
at 28-30V for SWR resistance and max power at 36V was near 275W.  They are
decently good IMD at 28V not exceptional but in the range of as good as commercially
made VHF radios..  Seems to tolerate most abuses well so far.  At 28V it wants a 16A
(500W switch mode power supply) supply minimum and the devices are peaking
about 4A each!

I've used RF16HHF at 20W max because the are 12V use where the IRF510 is not quite as good
for IMD.    I have not found them to be fragile but not quite as rugged as the 510.

Both parts will self destruct if the amplifier is not stable.  This means either part.
It happens faster than a fuse.

With all that said....

Just worked on someone else's uBITX that had a problem on 40M, it produces half the
power as 80M (15W) or 20m!(10W)  Reason was the output PI filter has coils that were
mis-wound (wrong inductance).  Easy fix once discovered....  Why relevant  here? 
The amp was putting 11W into the filter and getting 5W out.... That means the reflected
power the amp was seeing was 6W  That is an SWR well over 6:1 and the finals were
happy and never failed.

High "SWR" rarely kills finals its often instability with reactive loads.  Overheating, over
voltage and over current can.    Believe it or not those are the same things that can kill
the venerable 6146! 


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