How to make a cheap VOM more sensitive than a VTVM or DMM

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How to make a cheap VOM more sensitive than a VTVM or DMM

Post by Ragwire on Sat Oct 19, 2013 1:22 pm

Just for experimental and general edification purposes, I thought I would share my little potentiometric DC voltage measurement with everyone for the sake of interest. Of course this circuit is only as accurate as the meter selected, and it could certainly be built into a permanent enclosure with a selection of reference voltage sources and resistances for the voltage divider network.
The important thing is that the reference voltage is best chosen to be a little higher than the expected DC voltage to be measured, and that the potentiometer and resistor are chosen to be easily selective for sensitivity, and not be overloaded by the reference voltage source on the right side of the diagram.

This circuit is an improvement of one that I found on the internet. I have added a .22uF capacitor here on the bridge between the reference voltage source and the test voltage source. This allows the capacitor to charge to any voltage difference between the two sides, and release its stored charge quickly into the headphone transformer. This makes it vastly more sensitive than just hooking up the transformer primary in series with the bridge.

S1 can be a momentary switch, or one that is simply opened and closed manually. The transformer here is just a 1.2A 120V to 6-0-6 power transformer, but a suitable audio output transformer could be found as well.

The headphones click when the switch is operated. The click is quieter the closer you get to the test voltage across the potentiometer. Higher or lower = more volume. Adjust the potentiometer for null--no click--and read the meter. This is the voltage under test, or very close to it.

I am able to easily distinguish +/- about 25mV or less on this cheap, 20K ohm/volt VOM when reading an approximate 750 mVDC test voltage from this high impedance source. Even a 10 megohm VTVM or DMM will load this circuit, and require the voltage to be calculated based on meter and source impedance...and that can only be done if the source impedance is known. With this circuit, it reads the voltage directly without needing to know the impedances or calculate anything.


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