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Re: [Amps] SB220 Meter blown

To: Steve Thompson <>,
Subject: Re: [Amps] SB220 Meter blown
From: Adrian <>
Date: Mon, 18 May 2020 19:41:38 +1000
List-post: <>
The shunt resistor is across the meter to set the working range of application for the FSD corresponding to the full range of measurement.

, that's why it is called a shunt resistor. The protection diodes are in parallel, and so is the meter.

Your statement is technically incorrect.

Maximum Forward Voltage Drop per element at 1.0A DC -*1.1*V Once it shorts VD falls closer to zero.

On 18/5/20 6:59 pm, Steve Thompson wrote:
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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