[TowerTalk] Protection of a preamp

TexasRF at aol.com TexasRF at aol.com
Tue Jan 21 11:31:03 EST 2014

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 
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!
Gerald K5GW
In a message dated 1/21/2014 8:14:48 A.M. Pacific Standard Time,  
normanlizeth at 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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