[Elecraft] Fwd: KX3 Relay Control For Desktop Amplifiers

[Rich Heineck]

Jim,

The FET shouldn't have had any problem with 50 mA.  It's a 2N7002 and is 
rated for 115 mA continuous and 800 mA pulsed, with 60V max voltage when 
off.  One possibility is that the load being switched was inductive 
(like a relay), and the FET's voltage rating was exceeded when the relay 
was released due to inductive kickback.  The simplest solution to snub 
the kickback spike is to put a reverse diode, like a 1N4002, across the 
relay coil (cathode side to the relay's '+' supply).

It's also possible the FET was defective or damaged to begin with, which 
led to an early failure.

73,
Rich  AC7MA


On 03/12/2013 01:28 PM, Jimk8mr at aol.com wrote:
> I blew the switching FET with about 50 ma of current (12 volts) from a
> DownEast Microwave transverter. Elecraft says it should have handled it, but
> anyway, FWIW.
>   
> The FET has been replaced, and an intermediate switching transistor
> installed in the switchbox for the several transverters. Now switching 1 or 2  ma,
> hope that holds!
>   
>   
> 73  -  Jim   K8MR
>   
>   
>    
> ____________________________________
>   From: robertwschumacher at yahoo.com
> To: elecraft at mailman.qth.net
> Sent:  3/12/2013 12:07:26 P.M. Eastern Daylight Time
> Subj: [Elecraft] KX3 Relay  Control For Desktop Amplifiers
>
>
> Most common desktop amps have a relay circuit that requires  a transceiver
> to short 12 VDC at 100 ma to key the amplifier on.
> In the KX3  specs, the maximum amp relay current is stated to be 100 ma.
> Has anyone keyed  a desktop amp directly with a KX3 or is it recommended to
> use an intermediate  external relay  circuit?
>
> Thanks,
>
> W9DBR
>
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