[Elecraft] Fwd: KX3 Relay Control For Desktop Amplifiers

[hawley, charles j jr]

The 2N7002 has an internal diode for any inductive collapse.

Sent from my iPad
Chuck, KE9UW 
(Jack for BMW motorcycles)

On Mar 12, 2013, at 6:58 PM, "Rich Heineck" <richei at frontier.com> wrote:

> 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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> 
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