Steve Katz wrote:
>
> Most "4 kV" (or any high voltage) meters are actually 1 mA or similar
> small-current meters that rely on an external, rather than internal,
> multiplier resistor in order to achieve the scale indicated.
>
> Have you actually measured the resistance across the meter terminals?
> Unless it's in the Megohm region, you've got a low-current (typ 1 mA) meter
> that needs an outboard multiplier resistor. If so, just make that resistor
> large enough to create any scale you wish.
I'd be careful if you do intend scaling any meters for use with high
voltages by external multiplier resistors. Should the meter go open
circuit for any reason, it puts 4 kV on one of the terminals.
It's better to have a resistor in parallel with the meter, in addition
to the series resistor. It will keep the voltage across the meter to
sensible limits in the event of the meter becoming damaged. A parallel
resistor of say 9x the meter's internal resistance will have only a
small (10%) effect on the meter sensitivity and only a small (10%)
increase in power dissipation of the series resistor(s). However, it
will limit the voltage across the meter terminals in the even the meter
goes O/C.
Also be aware of the voltage rating of resistors. Most high value
resistors are not limited by their power rating, but their voltage
rating. A 1 W 10 Meg Ohm resistor is unlikely to be able to take the 3.1
kV the power dissipation would suggest it can. It is highly likely,
unless you get special resistors for high voltage, that the voltage
rating of such a resistor is more likely to be 300 V.
Dr. David Kirkby,
Senior Research Fellow,
Department of Medical Physics,
University College London,
11-20 Capper St, London, WC1E 6JA.
Tel: 020 7679 6408 Fax: 020 7679 6269
Internal telephone: ext 46408
e-mail davek@medphys.ucl.ac.uk
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