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"<[email protected]> 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,[email protected] 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:[email protected]
To:[email protected]
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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