[Amps] SB220 Meter blown

[Adrian]

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.


No need to yell Rob.



On 16/5/20 9:41 pm, Rob Atkinson wrote:
> W8ZR is 100% CORRECT.  You DON'T JUST WILLY NILLY SLAP ANY OLD DIODE
> ACROSS A METER!
>
>
> 73
>
> Rob
> K5UJ
>
> ------------------------------------------------------------------------------------------------------------------
> For meter protection I also use back-to-back diodes across meter
> terminals. However, rather than using rectifier diodes, I use fast
> small signal diodes, like the 1N914a. Aside from responding more
> quickly to possible damaging spikes than rectifier diodes, they also
> short circuit when overloaded, which is a kind of last ditch
> protection against meter damage  Given that the diodes cost only about
> $0.05, replacing a shorted diode is cheap insurance.
>
> One other comment: it is important to compute the maximum expected
> voltage across the meter terminals in normal operation. This voltage
> needs to be smaller than the “turn-on” voltage of the diodes, which is
> about 0.7V. Otherwise the meter will read low at large values because
> current is shared with the protection diode. This is also a
> consideration with rectifier diodes like the 1N4005.
> 73,
> Jim w8zr
> Sent from my iPhone
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