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[TowerTalk] Fwd: Protection of a preamp

To: towertalk@contesting.com
Subject: [TowerTalk] Fwd: Protection of a preamp
From: Hans Hammarquist <hanslg@aol.com>
Date: Tue, 21 Jan 2014 14:21:29 -0500 (EST)
List-post: <towertalk@contesting.com">mailto:towertalk@contesting.com>
Norm, 


Use a three terminal regulator such as MC7812. 


http://www.onsemi.com/pub_link/Collateral/MC7800-D.PDF 


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@gmail.com>
To: towertalk <towertalk@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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