------------ ORIGINAL MESSAGE ------------(may be snipped)
On Sat, 11 Jun 2016 16:55:55 +0000 (UTC), Jack wrote:
>Also wondering about design additions that you may have included in amps to
>protect the circuitry from catastrophic failures.
REPLY:
Proper meter protection is easy if one understands the basics. Too
often someone just puts a diode or pair of diodes across the meter and
thinks it is protected. Not so.
Typical silicon diodes do not conduct until the voltage across them
reaches about .7 VDC. By the time the voltage across the meter
movement reaches that level, the meter will be toast.
The answer is to put a small value resistor in series with the meter
and then put the diode(s) across the combination of meter plus
resistor. Simple Ohm's law will tell you how much resistance is needed
to reach .7 VDC at the meter's full scale current. Add about 10-20% to
keep it linear at full scale and you're done. Don't forget to include
the meter's resistance in the calculation.
This is normally only needed for current meters. Voltage meters rarely
need this kind of protection since voltages do not spike like currents
do.
73, Bill W6WRT
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