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Agilent 34401a multimeter limited to 0.4a?

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I posted this in beginners because I think I am missing some stupid menu setting or something. I am still getting used to my 34401a multimeter as well.

I purchased a 34401a multimeter about a week ago and I put it through it's paces with a few different test setups and it seems to be working fine. It also passes all internal tests and has no errors. However, there is one weird quirk: There seems to be a hard limit of about ~0.4a on DC amp range on the multimeter.
It is rather strange.
DUT: 34401a
Setup: Charging an A123 AHR32113M1 battery with an E3632a power supply, measuring the current through the battery. Battery can handle high current fast charging.
The power supply is set at ~4.1v in CV mode and the PSU measures the charge current at 1.2amps, and it starts dropping. This is normal.

I plug in my fluke 115 in line with the charging battery and it measures about 1.2 amps. This is consistent with the power supply. I disconnect my fluke 115 and use my 34401a multimeter. The charge current reduces to ~0.4amps. I know the multimeter is limited to 3a RMA, but 0.4a is well below that. I dont know why there is such a significant difference between the current readings. There was a service note for the 34401a about replacing a few internal resistors on the multimeter to improve dc current measurement, but I don't think the issue would be this significant. This also seems to be too significant of a difference for burden voltage to be the reasoning.

Am I missing something? I checked the multimeter menu settings and it the only "limits" I found were in the math modes. High and low limit were set to 0.0000.
I checked the terminal fuse and it looks correct at 250v and 3a. The overall multimeter fuse was 250v at 325ma, as stated on the back.

I have no idea what else can be causing this.
If anyone has any input, that would be fantastic. Thanks!

When you connect something in series there is a voltage drop on it.
Use a voltmeter on the battery and re-adjust the output of the power supply to have the same result.

A multimeter does not limit the current, regardless of the settings you choose, the only that can happen is to burn its fuse

Also using the sense terminals on the E3632a would be nice... It will compensate automagically the voltage drop caused by the 34401a.


I prefer you use your brain for educational purpose. Anyway it's not that hard.   8)

PS: This make sense only if the current sucked by the sense line is much smaller than the one you want to measure.

The Soulman:
google "burden voltage".
The 34401 uses a resistor to measure the current (voltage drop across resistor), and there is more resistance added by the fuse, cable and connections.
All that resistance causes a reasonable voltage drop (related to the current) and reduce's the voltage that is "seen"
by the battery, reducing charge current.

Hey guys. I just updated the post to clarify my question. The 34401a is the device under test, and all I am doing is measuring the inline charge current to the battery. Thanks for the input about burden voltage, but I do not think that is the main cause of this issue. I substituted my fluke 115 as the external current meter and it matches the power supply. The 34401a should have measured similarly to the fluke and the power supply, but it clearly isn't. I still trying to find the root cause of this and will keep messing with the DMM. Thanks.

edit: I looked at the fluke 115 datasheet and the 34401a datasheet and holy balls, the burden voltage specifications are quite different. The 34401a states that for a 1a range the burden voltage is close to 1v DC. The Fluke 115 mentions "Amps input burden voltage (typical): 6 A input 2 mV/A, 10 A input 37 mV/A" which is a few orders of magnitude smaller!
I have used portable multimeters for almost all my previous measurements and have not had the luxury of using a dmm until now. Is it normal for DMMs to have a higher burden voltage than portable multimeters? Is that the trade off for higher accuracy, that a more complex and accurate meter will take more current when performing a current measurement?
Thanks   :-+


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