[TowerTalk] Fwd: Protection of a preamp

Hans Hammarquist hanslg at aol.com
Tue Jan 21 14:21:29 EST 2014


Use a three terminal regulator such as MC7812. 


Select the TO-220 version (MC7812T) as they can be mounted with one screw and can handle 1 A, I guess more than enough for your application. They work with an input voltage of 15 - 35 volts. You should put a protective diode in front of the input to avoid any negative input voltage, as that would destroy the regulator in a split second. You should also put some diode/resistor network across the relay to reduce the switching transients as a voltage more than 40 volts would severely damage the regulator.

One spin-off with this is that you will have a nice, clean voltage to your pre-amp, improving the noise level in the amplifier.

You can write me directly for any question.

Hans - N2JFS

-----Original Message-----
From: Lizeth Norman <normanlizeth at gmail.com>
To: towertalk <towertalk at contesting.com>
Sent: Tue, Jan 21, 2014 11:14 am
Subject: [TowerTalk] Protection of a preamp

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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