[Amps] SB220 Meter blown

[MU 4CX250B]

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

> On May 15, 2020, at 3:08 PM, Adrian <vk4tux at gmail.com> wrote:
>
> from ; http://www.somis.org/sb-220ci.html
>
> "6. Meter damage can be avoided by connecting two, ordinary = or >1A, any PIV, silicon rectifiers across the terminals on each meter. The two diode arrows should point in opposite directions."