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reading 30kV dc in a 30 V dc voltmeter


rsilva.pt
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Using a commercial HV probe like the Fluke suggested by Ken is definitely the best way to go.  However, such an accessory must be used with a DVM having the proper input resistance.  This is automatically taken care of if you use the probe with most Fluke DVMs.  It will definitely NOT work with any arbitrary 30 volt analog panel meter.

The reason is that the probe contains a high impedance voltage divider consisting of an approximately 1000 Megohm series HIGH VOLTAGE resistor (one that resists flashover across the resistor) and a nominally 1 Megohm shunt resistor to ground.  The actual values of the resistors form a 1000:1 voltage divider when the shunt resistor is loaded with the 10 Megohm input resistance of the DVM.  Loading the probe with an analog d'Arsonval (moving coil) panel meter will seriously degrade the voltage division ratio to the point that you will have no idea of what you are measuring (in the absence of careful experimentation or calculations to determine the actual voltage division ratio).

Look at the spec sheet for the probe here:
<http://assets.fluke.com/manuals/80k40___iseng0900.pdf>

Note that the probe meets specs only if used with a DVM having an input resistance of 10 Megohms +/- 1%.

You can fabricate you own HV probe using an analog meter but you will be messing with potentially lethal voltages.  In fact, you can buy relatively low cost HV probes with built-in analog meters.  To make your own voltage divider for use with a meter you have on hand you must determine the exact resistance of the meter so that the required resistor values can be calculated.  Do not try this with a VOM Ohmeter as you will probably burn out the meter under test.

Rather than me trying to explain all the details and the cautions of working with high voltages, check out the very useful and detailed discussion of high voltage probe construction here:  <http://www.repairfaq.org/sam/hvprobe.htm>  Incidentally, Sam Goldwasser has collected and created an enormous amount of useful information on the web site linked to above.

Note especially the safety considerations.

awright

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