[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.
https://www.youtube.com/watch?v=Q-CE-ZbBuTQ
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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