Norm, yes, there are dragons here!
Relay isolation has to be sufficient keep the rf voltage from destroying
the preamp. Often times a single relay doesn't give enough isolation and
tandem relays are required. This applies to both the input and output of the
preamp.
Another issue is the relays will generate an inductive voltage kick at turn
off. You can add filtering and clamping diodes but the spike can happen so
quickly that the preamp still gets a hit before the protection circuit
does it's job.
Timing is critical. The relays need enough time for the contacts to settle
before rf is present.
VHF/UHF operators typically use a sequencer to turn things on/off in the
right order and time to protect the preamp. The requirements for complete
protection are not trivial!
73,
Gerald K5GW
In a message dated 1/21/2014 8:14:48 A.M. Pacific Standard Time,
normanlizeth@gmail.com writes:
Hi all!
I have a preamp that I switch in and out with a pair of spdt coaxial
relays. The NC positions pass through and the NO positions hold the preamp.
The relays are +24Vdc and the preamp needs ideally 12Vdc.
Presently, I have the preamps hooked up to +24Vdc and they are frying.
What I'd like to do is when I key the relay, also feed the preamp through
20 diodes to drop the Voltage to something respectable. Possibly using a
100uF cap across the preamp.
Am I asking for trouble? Do dragons live here?
Norm n3ykf
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