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" <[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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