[Amps] Westinghouse BC Ammeter QX-37

[Ian White, GM3SEK]

> 
> Subject: [Amps] Westinghouse BC Ammeter QX-37 Dear Jim, Dick and all
> the others who responded,
> 
> Thank you for your replies.
> 
> Yes, the meter will go in the negative return line to the power
> supply, and will have back to back 200 peak amp diodes plus disk
> caps.
> 
> The "peace-of-mind" request was specifically written because I didn't
> wish my ignorance of the Westinghouse panel meter design to enable me
> to connect them and possibly burn them out after fourteen hours of
> metalwork fashioning 3.625" holes in a 1/8" thick aluminum front
> panel that costs $35.00 each to replace.
> 

Hal, the best way to protect your investment is to download another copy
of your Triode Board manual  :-)

The ideal situation for diode protection is when the meter has a voltage
drop of just 0.5V at full scale. At this voltage, a typical large
silicon diode is right on the threshold of conducting. It isn't
conducting enough to affect the normal meter reading significantly - but
with any overload, the diode and would conduct quite heavily.

Unfortunately your 0-2.0A meter has a resistance of 0.3 ohms, so the
voltage drop at full-scale is 0.6V. A shunt diode would be conducting
quite heavily at this voltage, so the meter would read significantly
low. One solution would be to use two diodes in series, but then you
wouldn't be protecting the meter as effectively.

But you can still achieve the "ideal situation". Remove the internal
shunt in the meter, and follow the instructions in the Triode Board
manual to reconfigure it as a 0-0.5V voltmeter, measuring the voltage 
drop across a 0.25 ohm resistor. Then shunt the *resistor* with a large 
diode and a disk cap. The full-scale reading of this setup is still 
2.0A, same as before, but the meter movement will be much better protected.

You don't need precision resistors. As you know, you can calibrate the
meter very accurately as you wish using a set-and-forget trimpot.

For other Amps readers, all the details (including schematics) are in 
the Triode Board manual which is a free download from 
www.ifwtech.co.uk/g3sek/boards



-- 

73 from
Ian GM3SEK