[Amps] SB220 Meter blown

[Steve Thompson]

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
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> voltage required before forward bias is achieved in the diodes. It's all very simple, as stated here a few times now. re ;
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> I would rather follow Rich's advice on the subject as per my previous link. contained withjin ;
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> "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.
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> 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.
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> 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."
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> ref; http://www.somis.org/D-amplifiers2.html
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> Once a petite signal diode blows apart it is no longer protecting the meter.