I hadn't thought of that but, yes, the relay does need an external 12V
supply. There absolutely must be a diode across the relay coil to
prevent the voltage spike at turn-off from exceeding the 200 V limit and
shorting the FET. For the 40 mA relay, a 3.3K resistor could be used in
series with the diode to speed up the turn-off.
73,
Scott K9MA
On 10/13/2017 11:13, James Wilson wrote:
Pete, I think you have misunderstood the purpose of the Key Out jack
on the K3. It does not provide 12 volts out at 40 ma. It is a keying
circuit that can withstand a external keying circuit up to +200 volts
DC and can handle a keying load of up to 5 amps. (See the K3 manual
regarding the Rear Panel) If you want to key your external relay, you
will need to provide a source of power for the relay - and use the Key
Out connection as the "switch" for it.
Jim - W4RKS
--------------------------
I am trying to use the Key Out jack on my K3 to key a small relay to
protect my SDR from RF. The 12V at 40
<http://mailman.qth.net/mailman/listinfo/elecraft> ma is well within the stated
rating
of the jack, but to my surprise it does not work. Measuring with an
analog multimeter, the jack appears to be shorted all the time, and also
to have a very small DC voltage on it. I don't have a schematic for the
radio, so I'm stumped. Is this a function of the way I'm measuring, or...?
--
Scott K9MA
[email protected]
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