Rich was talking about putting diodes across the current measuring
resistor, not directly across the meter itself. Typically the
resistor generates something in the region of 0.5-2V which the
meter reads via a series resistor.
Most moving coil movements need less than 10mA and less than 0.2V
to go to full scale. A meter which reads higher current without
external resistors will almost certainly have an internal shunt.
If you're trying to protect a meter with an internal shunt you
probably need to look at the biggest Shottky diodes you can afford
as they conduct at lower voltages than silicon ones.
At 20+A glitch current, the voltage across a 1N5400 type diode
will be in the order of 1.5-2V.
Steve G8GSQ
The diodes(s) should be direct across the meter, and enough in series as needed
to excedd the full scale deflection
voltage required before forward bias is achieved in the diodes. It's all very
simple, as stated here a few times now. re ;
I would rather follow Rich's advice on the subject as per my previous link.
contained withjin ;
"It may take more than one diode to protect a meter shunt resistor. A silicon
diode begins to conduct at a forward voltage of about 0.5V. To avoid affecting meter
accuracy, the operating voltage per glitch protection diode should not exceed 0.5V.
For example, a 1 ohm shunt, at a reading of 1A full-scale, has 1V across it. Thus,
two protection diodes in series would be needed to preserve meter accuracy.
Similarly, if the shunt resistor for a 1A full-scale meter is 1.5 ohm, the maximum
shunt voltage is 1.5V--so three diodes are needed.
Glitch protection diodes should not be petite. Big, ugly diodes with a peak
current rating of 200a or more are best. Smaller diodes--and the meter they
were supposed to be protecting--can be destroyed during a glitch. Suitable
glitch protection diodes are 1N5400 (50PIV) to 1N5408 (1000PIV). In this
application, PIV is not important. The 1N5400 family of diodes is rated at 200a
for 8.3mS.
During an extremely high current surge, a glitch protection diode may short out--and
by so doing protect the precious parts. Replacing a shorted protection diode instead
of a kaput meter is almost fun."
ref; http://www.somis.org/D-amplifiers2.html
Once a petite signal diode blows apart it is no longer protecting the meter.
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