Megger Insulation Tester Megohm meter MOD question

[Electro Detective]

Sometimes you just don't need 50/100/250/500 or 1000 volts to test insulation properties for thin wires, fiddly connectors or other stuff that may need a bit more zap (or less) than a plain continuity/beeper etc.

It would be handy to vary the DC output from 0 volts to whatever the minimum first setting is on an insulation tester (which might be anything from 50v to 500v depending on make and model)

A separate interface/box with a potentiometer (and chicken head knob!) with  + - input sockets would be ideal, with 2 breakout leads/male plugs going to the host meter   

The short circuit current is usually anything from 1ma up to 10ma max on most of these zap testers

Can it be done simply/lazy with just a standard pot or a higher wattage wirewound type, or do I need a circuit/bug orgy ?

Or just get a pile of resistors and rotary switch, and settle for some fixed value attenuated output voltages ?


Any suggestions, thoughts, 'NO-o-o don't do it!'    or 'please explain again in English'       requests are welcome   

[Ian.M]

I'm virtually certain it cant be done as an external box.  How are you going to drop the voltage without drawing any current from the host meter?  If you did draw current it will read as a low insulation resistance.

However if you build a Megger equivalent from scratch, you could regulate the boost converter generating the test voltage as you like.   Unless you add some smarts to calculate the result allowing for the variable test voltage, readout would have to be as leakage current  rather than insulation resistance.

[Electro Detective]

My reasoning is/was that the megger is feeding out high DC voltage and very small current, all I need is to tone down the voltage output somehow. 

I can scribble up a compensation list if the readings on the megger are off track, or ignore it 

It's not for precision or vital measurement, just small scale go/no go checking stuff

[Gregg]

I think what you are looking for is called a Hi-Pot tester.  Most Hi-pot testers allow you to vary the voltage and have a meter to display the amperage; from there it is pretty simple math to determine the resistance

[Neilm]

I doubt you would be able to get the output voltage all the way down to 0V. Ones I have worked on would either have the supply voltage appear on the terminals when it was not operating, or would have an output that was unstable at such low values. Also, the control circuits might have offsets in them (either deliberatly to bias op-amps or as a result of component tolerances) that would give a definate minimum voltage. The last I designed had a minimum of 15V out to ensure the op-amps in the feedback loop didn't go too close to the lower rail.