John Lyles wrote:
>you do meter the negative lead, insulate the thing for HV on the power
>supply side, and install some sort of MOV/transorb/spark gap to snub the
>spike. It should go to earth/chassis/ground. Then the potential rise is
>clipped before it breaks things. Otherwise it may do damage to your meter,
>your wiring, or your body.
If you let the voltage on the B-minus rail rise to "MOV/transorb/spark
gap" levels, you can probably say goodbye to the meter (yet again?).
The only sensible protection component for this position is a gurt big
rectifier diode, connected with anode to chassis and cathode to the B-
minus rail. The diode is forward-biased under fault conditions and
ensures that the voltage on the B-minus rail will never go beyond about
The diode has to be used with a suitable current limiting resistor in
the B+ rail (or an electronic crowbar circuit). The current rating of
the diode should be such that it WILL NOT fail before the fuses or
contact breakers open. A current rating of (V+/R_lim) seems sensible, ie
typically 40-50A. The cheapest source of these diodes is as packaged
bridge rectifiers, which offer various combinations when wired in
If the metering circuit is designed to have a normal voltage drop of
0.5V or less, then the protection diode will have no effect on accuracy.
In the fault condition, the meter will only see about 2-3x normal FSD,
which will cause no damage whatever.
73 from Ian G3SEK Editor, 'The VHF/UHF DX Book'
'In Practice' columnist for RadCom (RSGB)
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