------------ ORIGINAL MESSAGE ------------(may be snipped)
On Tue, 13 Oct 2015 17:31:31 +0000, you wrote:
>
>So, is the best solution adding another reflectometer after the output
>transformer and prior to the filter to protect against that? With an expected
>third harmonic being 10-12 dB down at this point, the threshold might need to
>be high, like 5:1 ...
>
>Chris Scott
>W4NEQ
REPLY:
It isn't SWR that kills a transistor, it's the EFFECT of SWR - excess
voltage or current at the drain.
Instead of monitoring SWR, why not just monitor for over voltage and
over current? Those transistors are rated for Vds max of 135 V, so
just use a simple diode rectifier at the drain to produce a DC output
and set it to trip just below that voltage. In normal operation the
idling Vds will be 50 VDC and with max signal it will swing between
zero and 100 VDC, approximately. Make your circuit trip at something
between 100 and 135 VDC and you're all set.
You may need a resistive voltage divider to keep the PIV rating of the
diode within limits. RF-rated diodes like the 1N914 only have a PIV
rating of 100 volts. That's a very old diode so there may be something
with higher ratings these days that doesn't cost a fortune. Maybe.
Likewise monitor whatever the normal current is at 1200 watts output
and trip accordingly.
73, Bill W6WRT
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