34405A Multimeter Strange Malfunction

Thread Starter

ErnieM

Joined Apr 24, 2011
8,150
I was setting up some test circuitry to calibrate an inline current shunt chip. My scheme was to use a test resistor as a load, measure the current thru it then measure my shunts output like so:

Untitled.jpg
This way I am not changing anything so both measurements capture the same same thing to compute a cal constant.

BUT... it did not work. somehow the voltage measurement was off if I had the current going thru the meter at the same time. My work around was to use a spare relay to break this up into 2 different circuits, one for the current thru the meter, then the current direct to ground to read the shunt voltage.

Anyone else ever see this, or have a reason a very fine meter would do this?
 

LowQCab

Joined Nov 6, 2012
517
Operator Error.
It's extremely unlikely that the Meter can display Voltage and Current at the same time.
Get a second Meter.

Your description, and Schematic, does not explain what you are trying to create or measure.
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Thread Starter

ErnieM

Joined Apr 24, 2011
8,150
Operator Error.
It's extremely unlikely that the Meter can display Voltage and Current at the same time.
Get a second Meter.

Your description, and Schematic, does not explain what you are trying to create or measure.
The intent is to leave the circuit intact while taking two separate measurement on one device.

Perhaps one should read a post before responding as measurement simultaneity was never discussed as the issue or an issue at all.

... My scheme was to use a test resistor as a load, measure the current thru it then measure my shunts output...
 

du00000001

Joined Nov 10, 2020
62
From my experience, it is a bad idea to have a multimeter connected in more than 1 configuration: I've already measured fancy leakage currents on the current input (shunt) of a Fluke multimeter that was configured to measure voltage. Generally, such a "lazy setup" is not considered by the designers and might even destroy the instrument.
(Consider a multimeter with single terminals for all current ranges: which shunt would you expect to be connected if you're not in some current measurement mode?)
 

Thread Starter

ErnieM

Joined Apr 24, 2011
8,150
DU00000001: The calibration issue was solved (actually before I posted) by separating the measurements into two similar circuits that should be adequate so the "how to fix" part is over, still I'm curious about the "why" part.

I did wonder about what shunt was connected to the current terminals. Having little time and nothing handy nearby but my trusty Fluke handheld I stuck the ohmmeter of the Fluke into the terminals of the HP and noted the current terminals had a similar resistance in either the current or voltage measurement ranges. However, as I was reading 6 ohms I didn't put much stock in the observation, just noting the current terminals do not go open circuit when measuring voltages.

Interestingly enough the voltage terminals DO go open circuit when measuring current, as I recall 20M to (open).

Leakage currents do make sense as you suggest are probably what was interfering with my measurements. Thank you for an honest thoughtful answer.
 

Thread Starter

ErnieM

Joined Apr 24, 2011
8,150
6 Ohms? For which measurement range?
(Usually you have a 1 Ohm shunt for the 200 (400) mA range, less for higher currents.)
I put little stock in the reading value as it was simple a 2-wire hand held meter inserted into the other meter's current inputs. What surprised me was the reading didn't seem to change when I switched between a voltage and current scale. 6 ohms for a short is higher then this meter typically does (0.6 to 1 ohm is typical) but I did not see if the handheld was just having a bad day.

Some can and dual measurements are displayed although each measurement is taken alternately.
Simultaneous measurements were only mentioned by a thread hijacker, please do not feed the troll.
 

LowQCab

Joined Nov 6, 2012
517
""Perhaps one should read a post before responding as measurement simultaneity was
never discussed as the issue or an issue at all"".


One Picture (Schematic) is worth a thousand words.

I see 3-wires attached to a Meter,
and you claim it was "never discussed",
I agree that it was never discussed,
that's my entire point,
it is not clear what you are attempting to do.

Field specific Acronyms quite often do not communicate well.

I would be perfectly willing to help if your communication was more comprehensive.
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Thread Starter

ErnieM

Joined Apr 24, 2011
8,150
Why would it change?
Well something was changing when I switched between a voltage and current scale. I'm into the realm of reasonable guessing now but it is possible for a meter to completely disconnect its current shunt when in voltage mode. However, my crude measurement indicated that is not happening.
 

anniel747

Joined Oct 18, 2020
1,040
Well something was changing when I switched between a voltage and current scale. I'm into the realm of reasonable guessing now but it is possible for a meter to completely disconnect its current shunt when in voltage mode. However, my crude measurement indicated that is not happening.
Maybe if it was an uncommon meter for some sort of special application. Regular meters have no way to disconnect the high current shunt short of pulling the banana out.
 
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