[Amps] SB220 Meter blown

Adrian vk4tux at gmail.com
Mon May 18 05:41:38 EDT 2020

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."
>> 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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