[Elecraft] Amplifier Relay Keying Circuit

[Iain MacDonnell - N6ML]

Patrick,

Move the K3 Key Out to the other side of the relay - think of it as a
sink to ground. i.e. connect +12V directly to one end of the relay
coil, and connect the other end of the relay coil to the center of the
Key Out jack. When the K3 transmits, the path to ground will be
connected, and the coil will be energised.

Of course it's always good practice to put a diode across the relay coil
to prevent the kick-back from potentially damaging the solid-state
switch that's driving it.

     ~Iain / N6ML


Patrick Skerrett wrote:
> Hello folks,
> 
> I'm trying to build a simple relay circuit to key a Yaesu FL2100B 
> amplifier. The amplifier uses negative keying voltage, so I need to have 
> some sort of interface in between the K3 and the amp (Per the manual, 
> the K3 supports +200v but not negative voltage).
> 
> If I put an ohm meter on the K3's key out jack, and hook the positive 
> lead to the center conductor, and the negative lead on the shield, I 
> measure an open when the K3 is not keyed up, and measure a short when 
> the K3 is transmitting. So far so good..
> 
> I built a simple circuit that looks something like this:
> 
> 
>                    Center conductor on K3 Key line
>                            |
>                            |
>                            |
> +12V from power supply ------             ----------------Relay 
> coil                                               
>                                          |
>                                          |
>                                   Shield on K3 Key line
> 
> 
> -12v from power supply ------------------------------------Relay coil
> 
> 
> 
> When I DO NOT have the K3's key out plugged into this circuit, and short 
> the RCA cable, the relay closes & the circuit works. Again, so far so 
> good.....
> 
> However, when I try this plugged into the K3 and hit the transmit, the 
> relay does not close.
> 
> Why would I be able to measure a short on the K3's key out when 
> transmitting, however it will not bridge my 12v & close the relay circuit?
> 
> I hope I'm not doing something very stupid here :)
> 
> Thanks,
> 
> W9PDS
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