Hi,
I want to use mini digital voltmeter in my project and I find voltmeters with three wires ( for example this ).
What is the difference between 2-wire voltmeters? For which is used the third wire?
Thanks.
(sorry for my english)
Power, Ground and one to stick it to something you want to measure. The measured voltage should share the same ground with the supply for the meter.
With two wires, the meter measures the voltage of its own supply.
1 Like
Shpaget:
Power, Ground and one to stick it to something you want to measure. The measured voltage should share the same ground with the supply for the meter.With two wires, the meter measures the voltage of its own supply.
So the only difference is, that it could measure different then power voltage? And if I connect power wire and third wire to the same power source, is it OK?
Sure.
parmanik:
So the only difference is, that it could measure different then power voltage? And if I connect power wire and third wire to the same power source, is it OK?
I have bought a number of these. Quite often you will find on the two-wire version that there is actually the connection on the PCB for the third wire, and it has simply been jumpered to the power wire.
Which means by the way, that there should be no difference in cost between the two.
And incidentally, you will note that the power supply rating is 3.0 to 30V while the sense wire can be used to indicate anything between zero and 100V. So you need the three wire version to be able to read low and high voltages, and to measure voltages without drawing significant current from what you are testing.
Thank you guys.
Just one question: I founded this one and on 11th foto is the volt meter's back side, and except gnd, vcc and measure pins, there is even gnd, rx, tx. Does it mean, that it has serial port? If yes, will it be possible to use arduino to get the value?
OK, I could not figure which the 11th photo was, but ferreting through my bag of these voltmeters, I found two of those and connected them to the USB to TTL converter and activated the serial monitor. Powered at 5V, reading the 3.3V line.
I am unable to get any data - indeed any response at all from these connections. Both simply float at some 2.7V and the only activity I can see on the CRO is a slight ringing which would be in time with the digit multiplexing.
At this point, cannot find any useful functionality here. The module would need a resonator or crystal to generate serial data in any case.
And - to amplify on a previous point, of those modules where the terminations to the wires are visible (as some have the wires being anchored by the segment display and terminate on the PCB underneath it), the "sense" wire labelled "I" is jumpered to the "+" wire by a "0" resistor so of course, you could easily convert it to a three-wire.
Paul__B:
I am unable to get any data - indeed any response at all from these connections. Both simply float at some 2.7V and the only activity I can see on the CRO is a slight ringing which would be in time with the digit multiplexing.
You might want to try pulling up (or down) on that line and then prod it with your scope and see if you see any activity.
Some 3-wire digital volt meters, because of their circuit design, are incapable of "measuring" their own power supply voltage. Not a very helpful comment but one possible problem to be aware of.
Some 3-wire digital volt meters, because of their circuit design, are incapable of "measuring" their own power supply voltage. Not a very helpful comment but one possible problem to be aware of. For the application I was working on, I had to use an isolated inverter to produce the meter power supply so's I could measure the system voltage - that also powered the isolated inverter. 
Well, I realise some of the ICL (IIRC) designs required a "floating" supply where the "negative" input was about halfway, but if it genuinely has three wires and only three wires, the power supply rating is +3.0 to +30V and the sense wire can be used to indicate anything between zero and +100V, then there really is no way it could not be able to measure its own supply.
{It would be absurd to have an "isolated" power converter whose negative input and output were connected together. I think you need to explain this one a lot better. 
}