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Re: [Amps] SB220 Meter blown

Subject: Re: [Amps] SB220 Meter blown
From: Steve Thompson <>
Date: Mon, 18 May 2020 09:59:52 +0100
List-post: <>
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."


Once a petite signal diode blows apart it is no longer protecting the meter.

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