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Re: [Amps] SB220 Meter blown

To: Adrian <>,
Subject: Re: [Amps] SB220 Meter blown
From: Steve Thompson <>
Date: Mon, 18 May 2020 21:26:57 +0100
List-post: <>
OK, I should have said the diode voltage could be 1.5-2V rather than 'will be', but I'm happy with the numbers. When adding something as protection and/or safety, I reckon it's good to work with worst case values so the circuit is most likely to do what you want if you need it.

Data for 1N5400 family diodes includes a 'typical' voltage vs current chart, which shows just over 1.2V drop at 30A. It also shows 0.9V at 3A whereas the spec. says the drop at 3A can be up to 1.2V - a third higher. Apply that scaling to the 30A value and you get 1.6-1.7V drop. Not knowing if the scaling is linear at different currents I widen the window pessimistically.

Metering - the thing with meter shunts is that they don't change the voltage needed for FSD. Take a typical 1mA 100ohm meter which reads FSD with 100mV across its terminals. Want it to read 1A - add a 0.1ohm resistor in parallel as a shunt (actually 0.1001 ohms but let's use round figures). The meter reads FSD with 1A flowing and 100mV across the terminals.

Add the 1N540* in parallel too and apply a glitch. At 30A the diode typically clamps the voltage at 1.2V - 12x the voltage needed for FSD and 144x the max. power the shunt has to deal with in normal operation. My worst case 17x and 300x respectively.

The meter and resistor might survive, but it doesn't feel enough protection to me. In that situation, you would do better with MBRB430 or similar, which will clamp at about 0.5V for much lower overload.

In practice, the vast majority of meters indicating current are actually reading voltage across a current sense resistor. Take our 1mA meter and put 400ohms in series with it. It then reads FSD with 0.5V across the combination. Now put that across a 0.5ohm resistor and the whole kaboodle reads FSD with 1A flowing and 0.5V voltage drop.

Now when you add the 1N540* clamping at 1.2V during a 30A glitch, the meter and resistors are subject to 2.4x overload, or 3.4x in my pessimistic worst case. I believe it's this configuration Rich Measures was describing.

Steve G8GSQ

On 18/05/2020 10:41, Adrian wrote:
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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