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Author Topic: To what extent are modern rigs protected against o  (Read 647 times)
KF6IIU
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Posts: 293




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« on: May 06, 2003, 02:17:11 AM »

I am just curious - I am rehabilitating an old Calrad 12V allegedly 20A power supply and thinking of ways to replace the crowbar circuit in it. The supply puts out about 25V when the transistors open up. (It has 3 TO3 NPN transistors on the back, spray-painted black but they are probably 2N3055 equivalents - 20A with 3 pass transistors?)

Anyway, I recall connecting my Icon T2H handheld to a power supply that was miscalibrated to 14.4 V and it would not run and displayed "oVeR vOlT" or something to that effect. (I fixed that power supply simply by putting a largish silicon diode in series with the output.)

Are most 12V rigs designed to shut down on moderate overvoltage? Can they withstand 14.5V 25V? I *seriously* doubt 25V - but maybe not, it would not be hard to build a crowbar or some other kind of protective circuit into the rig. Only problem would be getting it to work fast enough to shut things off before damage occurs.

At the very least I will replace the current "classic" crowbar, which puts a dead short across the supply and blows the fuse, with one that can be tested with a circuit breaker.
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WA9SVD
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Posts: 2201




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« Reply #1 on: May 06, 2003, 11:20:31 AM »

Most "mobile" rigs will easily withstand 14.5 Volts, but that IS a bit high.  There should be an (internal) adjustment in the power supply to get the voltage down to about 13.8 Volts.
    Handheld radios are another matter; some may or may not operate properly even on 13.8 Volts.  If they have a power cable option for mobile use, and offers a direct connection (or simply some filtering and fusing) then they should operate well on 13.8 or even 14 Volts.     On the other hand, if the H-T cord has some sort of  circuit to drop the typical auto battery/electrical system voltage, then don't try to use the H-T directly from the Power Supply.  The overvoltage circuit in the supply should be set at approximately 15 volts, and you CAN replace the fuse with a similarly rated circuit breaker.  The ARRL Handbook (1988 and others) has overvoltage circuits shown if you need to add that to a power supply.
 
    But don't expect or rely upon the rigs to have overvoltage protection!  That's NOT usually included in most rigs; at BEST, polarity protection is all you can expect.
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