DVM measuring volts and amps

I have not, and have the strong impression that switching from amps to volts under load will end badly. At the very least, the V and I measurements will interfere with one another - this is not a use case discussed in the manuals.

Joe Gwinn

We do this a lot. The shunt is always very low resistance.

On the order of 1 milliohm at least for measuring hundreds of amps. Bidirectional current too.

A few millivolts of drop can offset your output load V measurement but looks like you are measuring the input source V ?

boB

Works fine.

It probably is, especially if the meter can measure power.

If it cannot measure power there is probably a single ADC and there may be insufficient isolation between the current shunt and the voltage divider such that signals on one could peteturb readings of the other.

I tried on a cheap hand-held and it didn't work well, current showed on the voltage range.

Do you have the schematic for your instrument?

In a typical VOM, the 'I' sense resistor is switched. So, doing an I measurement ony happens when V measurement is antiselected. A few high-current-range meters have an I terminal with a herky resistor outside the switch connection, but if that's the circuit you want, just use two V meters and a shunt, no need to trust the DVM box's innards.

My Fluke 87 chirps a complaint if V and I probe plugs are both inserted...

There's no end of optimistic idiots.

That's not the problem. He needs to be able to work out when it would be useful.

Understanding the potential problem with what you are doing isn't always necessary, but it can be vital.

It will be in an automated test set, with a Python program running everything. No need to push buttons.

The alternative would be a little box with a current shunt or two, and some SSRs to switch between voltage and current measurements. That would need logic signals to do the switching, but I can get that from somewhere.

I assume the folks at Fluke and Keithley do too.

The DVM will be rackmount and interfaced to a computer running automated tests.

Some DVMs have extra terminals on their front panels to make it easy for people who know what they are doing to exploit the technique.

That doesn't mean that the connections will be set up to avoid putting lead resistance where it is unhelpful.

for current and voltage??

Can't remember the details, but if memory serves it was it would have been fours sets of banana plug sockets to which you could also clamp spade terminals. You'd clamp the shunt resistor between the current terminals, clamp a pair of flat metal links up to the voltage terminals, and feed the current to be measured into the lower (current) pair of banana plug sockets, using cables terminated with banana plugs . Nothing all that exciting, but handy.

makes no sense

Didn't stop the meter being produced in large enough volume for one of my employers to buy one. It did come off a production line.

but what would be the point?

Reality is it depends on the meter. So you can design a universally compatible setup or restrict it to ones that work ok with your setup.

Quite possibly just to let purchasers who knew about Kelvin connections make a fuss about the technique in front of their less sophisticated colleagues.

I had a least one boss whose sales technique did depend on making the people he was selling to happy to buy the gear, rather than concentrating on making the gear easier to use for the people who ended up using it.