[AMRadio] protect that panel meter

[wwd at netheaven.com]

> Speaking of meters, I hope the *first* thing every AMer does with a 'new'
> rig is drop a diode or two across the meter posts. Happiness is never
> having to ask: "do you have any spare meters?"

> * Even before replacing that dual-fuse linecord.

Someone asked for details, forcing my pen to paper, as it were.  ;)

The common, everyday silicon diode is in fact a 0.707v zener diode. Thus
if placed across those fragile and expensive panel meters on AM-CW rigs,
any input voltage will be clamped at that value, and no damage to the
meter movement will result. Since we do not know the polarity of any
insulting voltage, place a second diode the other way across the meter.
Use nice terminal connectors and solder so the protector will remain with
the meter forever.

Why should I care, you ask? If the meter switch gets dirty and your
precious meter is exposed to normal voltage, but without its proper shunt,
poof! That meter is now history and the begging and whining for a
replacement begins.

Schottky diodes provide even more protection, somewhere around 0.4 volts
(?). Sometimes this is a bit too much limiting as the meter does not do a
full swing. But you may need it to get better protection as a trade-off.

Bill    K2AME
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wwd at netheaven.com  To the warriors go the spoils of war; to the hindmost .  .  the surplus.
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