The Mann Voltmeter Bridge

As far as I can tell this is a novel voltmeter design, and nobody has designed this before. Has anyone? Given the best way to get the answer to something on the Internet is to post the wrong information, I’m assuming that someone will correct me if it’s not original (see contact page).

Introducing the Mann Voltmeter Bridge Circuit. This is a voltmeter circuit that allows a voltmeter to distinguish when a zero volt reading means “floating probes” and when it means “the probes are on the same net.”

This was developed as part of my broader custom DMM / voltmeter project.

It works by putting a MOSFET in a differential voltmeter bridge. Normally the MOSFET is closed, and it doesn’t affect voltmeter operation. When the ADC measures ~0v differential between sides the MOSFET is turned off, breaking the bridge, and the ADC measures the far side of the bridge. If the voltage is still about where is was before (2.5vdc in my circuit) that means the probes forming a closed circuit. If the ADC measures close to zero that means the probe is floating.

This design allows the voltmeter to measure voltage (I’ve tested my circuit up to standard North American 120VAC) and automatically report to me voltage, open leads, or shorted leads on the fly.

There appears to be a small accuracy penalty (on the order of mV), I have not yet quantified it, nor built a version with good noise shielding.

Here is the circuit under different conditions (also see top image):