Hi John

See the references in the theoretical section of my 50W PA manual to the document by W6JL. And his QRZ page https://www.qrz.com/db/w6jl 

Don W6JL uses a single 1N5408 diode for switching 600W RF, forward biased with 100mA. 

The requirements on Don's switch are not the same as mine in the 50W PA. And it's different again in QSX, and in QCX. Often T/R switches need to be considered at the system level. A T/R switch design can't necessarily just be transplanted from one place to everywhere else. 

Note that sufficient forward bias current is very important, as I found out in another project. All the more so at lower frequencies, where eventually the 1N4007 loses its PIN characteristics and starts to return to rectifier behaviour; then you suddenly end up with a whole bunch of unwelcome harmonics starting with the 2nd. 

Solid state T/R switching is not, in my opinion, when done properly, a reasonable way to reduce current consumption of the switch, compared to relays. But solid state does have a lot of benefits compared to relays including smaller physical size, lower cost, silent operation, very fast switching and longer lifetime. 

73 Hans G0UPL 

http://qrp-labs.com 

On Mon, Jan 27, 2020, 16:39 John Seboldt K0JD <k0jd-l@...> wrote:

Glad to see the 1N4007-based diode TR switching in the 50W amp kit. I
also have a 120W linear amp kit, long ago completed but has been waiting
for final packaging with TR switching and filtering. Do you think this
switch will work at the 100W or so level? If not, who can guide me to a
nice polished design for a switch at the 100-200W level?

John K0JD
Milwaukee, WI

Hint there is none.
Did you consider a fuse?

Even the QCX, I use a 1A fuse, because wires cross.

FYI 30w at likley less than 17V is doing good.  Oh, why less than 17V, at 4a you
get voltage drop in your wires to the battery and unless you measured the battery
while transmitting you do not know your getting the same voltage as open circuit.

High current reverse polarity protection is not a 1$ as a series diode is cheap but costs 
about .7V (sometimes more at high currents) voltage loss and would have to be a
heavier 5A part.  Works at QRP power levels, but not at all at 80W input.

Power out for a fixed amount of drive will vary based on voltage applied so .7V
(minimum some diodes are more) so loosing that voltage to the diode would
be costly in terms of power out and energy wasted.

The people put in diodes across the power leads.  A parallel diode to
"blow the fuse" on reverse polarity.  About some percentage  of the time
the transistors go first.  Or the user puts in a bigger fuse to "correct that"
weak fuse resulting in a more disastrous  failure.

If you have room about 1sq inch of board and more than 1$ ts possible to do
relay based protection.  Note the relay can't be some cheap thing as reasonable
contact rating is 5A or more.  Since in a kit the user is also testing if it was done
right there is again a percentage of time when smoke is the only course.

Which brings the point.  Kits are assembled and final tested by the builder.

As a pilot you make sure there are wheels with air in them and the oil in the
engine, adequate quantity and correct fuel, control surfaces that operate properly,
plus a long list of other checks before climbing into the flier as once in the air it
can be terribly unforgiving of error or inattention.     Kit built airplanes have a
very rigorous test program that does not even allow for builder saying its right,
he has to prove it with  third party checks during assembly and beyond.

Electronics is the same way.  At high currents parts will evaporate if 
subjected to current well past reasonable limits, and same for excess
voltage.  There are few if any  commercial amps with reverse polarity
protection as well.   Most errors are preventable by not proceeding
without double checking.  

I've built more than a few amps, some using devices with 200$ price tags,
you just don't power up and pray.  There are a list of things to do before
attaching a 50V at 30A power supply.   So when that stage is reached 
there is no surprise, or smoke.   Why?  Not only would the expensive
LD-MOSFET vaporize but likely half the traces on the board because
1500W of energy will do that.  That's a lot of electronic and mechanical
work to "fix" it.

Allison
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