[Elecraft] Fwd: KX3 Relay Control For Desktop Amplifiers

[Rich Heineck]

There's a drain-source diode, but it doesn't protect the 2N7002 in this 
configuration.  Typical sink-to-ground relay driver circuits put a diode 
across the coil so it keeps the switched side of the relay from going 
way above the supply voltage when the relay is turned off.

Rich  AC7MA


On 03/12/2013 05:01 PM, hawley, charles j jr wrote:
> 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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