Author Topic: Why can't a multimeter measure current and voltage simultaniously?  (Read 10379 times)

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Offline iloveelectronics

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Thanks, after seeing the board & reading the impressions made there, not very convincing.  ::)

Thats why I don't talk about that meter much   ;D It is nonetheless a very unique meter.
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Offline Wytnucls

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Thanks, after seeing the board & reading the impressions made there, not very convincing.  ::)

Yes, not impressed either. Kudos for innovation, but the implementation is dodgy at best. The better option is to buy 2 meters, I think.
 

Offline retiredcaps

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Thats why I don't talk about that meter much   ;D It is nonetheless a very unique meter.
Franky, I think it is time for you to post a photo of all your multimeters in this thread ...

https://www.eevblog.com/forum/chat/show-your-multimeter!/
 

Offline jpb

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My bench multimeter will take two measurements - the ADCMT 75451A, it has two A2Ds for 4 wire resistance measurements and allows you to also use them separately for V and I with a B input though the voltage is limited to 10V.

I've not used this facility and the manual/meter is at home so I'm not sure what it displays in terms of the second reading, it might be more geared for computer controlled measurements via the USB.
 

Offline Neilm

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I'm surprised that that many meters do measure both at the same time - the potential for safety issues are quite high. Imagine accidentally probing your voltage source with your ammeter.

Neil
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Offline stazeTopic starter

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I'm surprised that that many meters do measure both at the same time - the potential for safety issues are quite high. Imagine accidentally probing your voltage source with your ammeter.

Neil

Means you blow a fuse, if the meter is "safe". I did this before I realized what I was doing... used an old Radioshack meter, set it to amps, and measured across a circuit. And what do you know, it no longer measured current (until I replaced the fuse). With a crappy meter, yes, you could have things go BOOM.
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alm

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My bench multimeter will take two measurements - the ADCMT 75451A, it has two A2Ds for 4 wire resistance measurements and allows you to also use them separately for V and I with a B input though the voltage is limited to 10V.
Why would it need two ADCs for four terminal resistance measurements? Four terminal measurements use the same constant current source + voltmeter approach that regular two terminal measurements use.
 

Offline robrenz

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My Fluke 8846A measures DC current and DC voltage simultaneously.

From the manual:
 
"The resistance of the common lead combines with a
small amount of internal resistance in the meter to cause an IR drop
that affects the accuracy of the voltage measurement. Depending on
the circumstance, this effect can be substantial. As an example, 20
m? of lead resistance can cause more than 20 mV of additional error
at 1 A."

I would suspect that this error probably applies to most of the meters that can do this.

alm

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Yes, I don't see how any meter that shares an input for current and voltage measurement would avoid this. 20 mV may very well be negligible, however, depending on the required accuracy and voltage. For mains for example, you can safely ignore it, unless you have one of those rare meters that do 0.01% accuracy on AC (Agilent 3458A maybe?).
 

Offline robrenz

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The 8846A only does DC V and I simultaneously.  Even if it did AC it is only 0.06% rdg +0.03% rng 1year spec.

Offline jpb

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My bench multimeter will take two measurements - the ADCMT 75451A, it has two A2Ds for 4 wire resistance measurements and allows you to also use them separately for V and I with a B input though the voltage is limited to 10V.
Why would it need two ADCs for four terminal resistance measurements? Four terminal measurements use the same constant current source + voltmeter approach that regular two terminal measurements use.
I don't know in detail but I'd guess it was to get an accurate measurement of the nominally constant current. The B channel is used for the sensing (at least the terminals are labelled that way) so presumably the main channel is used to determine the current. The test current ranges from 50nA to 10mA. I would also surmise that it is difficult to make a perfect constant current source and it is easier to make one that is close to constant and measure it to get accurate results.
 

alm

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So how would this be any different for a two terminal resistance measurement?
 

Offline jpb

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So how would this be any different for a two terminal resistance measurement?
The current would be measured on different wires to that for which the voltage is measured so there would be negligible voltage drop on the wires used to measure the voltage across the device.

A high impedance voltmeter attached to the DUT by one pair of probes. A second pair of probes supplies current via a known internal resistance the voltage across which is used to measure the current.

The current is not measured by measuring voltage across the DUT as the resistance of this isn't known anyway.
 

alm

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I'm aware of the difference between two and four terminal resistance measurements, but why would connecting the source and sense wires at the DUT require more ADCs than connecting them at the DMM terminals, as is the case for two terminal measurements?
 

Offline jpb

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I'm aware of the difference between two and four terminal resistance measurements, but why would connecting the source and sense wires at the DUT require more ADCs than connecting them at the DMM terminals, as is the case for two terminal measurements?
OK I get your point. I don't know the inner workings of the meter as though I took it apart to replace the electrolytic caps after buying it on e-bay I wasn't trying to reverse engineer it.

But what I guess is that when measuring resistance it applies a fixed voltage with a known resistance in series. Measuring the voltage across the known resistance gets an accurate measurement of the current and the voltage across the terminals (being the reference voltage minus the voltage across the resistor). This is all you need for a 2 wire measurement. (It probably measures the voltage across the terminals and subtracts this from the reference voltage to get the voltage across the resistor.)

For a 4 wire measurement it has a second meter (which can be limited to 10V) which measures the sense wires to get a more accurate measurement of the voltage across the DUT instead of at the terminals.
 


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