From: rohre (
Date: Sun Jan 22 1995 - 21:06:40 EST

If fuses were ideal circuit interrupters, then putting the diode either way
would be OK. But---fuses take a finite time to blow; and in this circuit the
designer said it would operate all the way to 9V and using 12 V you did not
need to regulate down, so I would put a diode rated to cary the full normal
current in series with the external battery input say on the plus line, with
the cathode away from the battery, so you can get only current from a proper
polarity external battery. For the CB transceivers, you used to see a 1N400x
where x was one of several, like the 1N4007, 1 amp power rectifier diodes used
as protection devices. You want the thing to be able to block the wrong
polarity voltage, and not have much leakage, so many silicon power diodes would
work in the series method. They will drop a little voltage, about 0.6 volts DC
in the normal correct polarity direction, but since you were told the circuit
works from 9-12 Volts, this is not a concern. Because many silicon rectifiers
work pretty well at RF, I think I would put an RF bypass cap on the cathode to
ground, but this may already exist in the circuit. I am not familiar with the
specific schematic and am trying to speak genearically here. 73,
Stuart K5KVH
To: QRP Group
>From: Michael Marmor on Sat, Jan 21, 1995 2:36
Subject: OHR WM-1
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Date: Fri, 20 Jan 1995 22:55:04 -0500 (EST)
>From: Michael Marmor <>
To: QRP Group <>
Subject: OHR WM-1
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My WM-1 meter kit arrived a few days ago from OHR. I talked to Dick
on the phone for a few minutes. I asked him how the meter would work
on 12V (it is normally run off a 9V battery) and was told that it would
work fine. Dick suggested that I install a switch that would let me
switch between the 9V battery and an external jack for 12V. Apparently
the meter does not need to be aligned differently for use at 9V or 12V
and switching between them does not effect alignment. He also
told me that future versions of the WM-1 might include the external
jack and switch. I was planning on using an LM317 to provide 9V to
the meter from my 13.8V supply, but was told this was unnecessary.
I would like to use a diode to protect the meter if the 12V polarity
is reversed. How should I connect the diode and what type should I
use? (Is it better to use the diode to shunt the current to ground
and blow the fuse -- or use the diode to block the voltage going in
the wrong direction?)
The weekend is here!! Time to start building!
Michael, AA2UJ

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