ORIGINAL MESSAGE:
On Mon, 16 Jul 2012 09:44:00 -0400, AB9GO wrote:
> and protect your analog meter with 2 diodes across the terminals.
REPLY:
Two diodes alone will not protect the meter. You need a resistor in series with
the meter and the 2 diodes back-to-back across both meter and resistor.
Here's why: Silicon diodes do not conduct until the voltage across them reaches
about .6 VDC. Most meters have a resistance of a fraction of an ohm, so with
diodes alone, the current must reach several amps before the diodes even begin
to conduct. By the time they do, the meter is long since destroyed.
You need to choose a value for the resistor so that the voltage across the
combination of meter + resistor will reach .6 volts just as the meter is
reaching full scale. At that point, the diodes will conduct any current in
excess of that and the meter is protected.
73, Bill W6WRT
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