Key Out Jack

[Jim Wilson]

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...?

--

>73, Pete N4ZR
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[K9MA]

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

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