[Elecraft] Fwd: KX3 Relay Control For Desktop Amplifiers

[Jack Smith]

Isn't the 2N7002's internal diode arranged to conduct if the drain is 
negative with respect to the source, but the inductive spike from low 
side switching will be positive with respect to the drain?

What you really want for this situation where the relay is presumably 
inaccessible and hence cannot be fitted with a clamping diode or RC 
snubber is an avalanche rated FET, and the 2N7002 is not avalanche 
rated. It may well be that the 2N7002 will handle some number of 
breakdown events but the breakdowns degrade the device and will cause 
failure sooner or later.  Unless a particular device is explicitly rated 
for avalanche operation, it must be assumed it is not to be used in 
breakdown mode.

I like the Vishay/Siliconix SI2328DS-T1-E3 for FET switching of relays 
where you cannot be assured access to the relay coil. It's rated at 
100V, 1.5A and is avalanche capable to 6A. It's an SOT-23 surface mount 
package.

Or, if an avalanche rated device is not to be used, a zener with a 
breakdown voltage less than the maximum rated drain voltage of the FET 
can be used from the drain to ground. It also has the advantage of being 
more easily replaced in the future as bog standard parts are involved.

Jack K8ZOA

On 3/12/2013 8:01 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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