Agilent 34401a multimeter limited to 0.4a? - Page 1
Forum section: electronics / beginners

author: salvagedcircuitry
author_rank: Regular Contributor
timestamp: February 22, 2019, 06:06:08 am
content: 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: 34401aSetup: 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!

author: Damianos
author_rank: Frequent Contributor
timestamp: February 22, 2019, 07:37:00 am
content: 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

author: Zucca
author_rank: Unknown
timestamp: February 22, 2019, 08:29:29 am
content: Also using the sense terminals on the E3632a would be nice... It will compensate automagically the voltage drop caused by the 34401a.How?I prefer you use your brain for educational purpose. Anyway it's not that hard.PS: This make sense only if the current sucked by the sense line is much smaller than the one you want to measure.

author: The Soulman
author_rank: Super Contributor
timestamp: February 22, 2019, 11:04:48 am
content: 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.

author: salvagedcircuitry
author_rank: Regular Contributor
timestamp: February 22, 2019, 05:04:47 pm
content: 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

author: JDubU
author_rank: Frequent Contributor
timestamp: February 22, 2019, 05:17:29 pm
content: Set the 34401A to "DC I" and measure the resistance between the "3A" and "Lo" inputs using your Fluke DMM.

author: HighVoltage
author_rank: Super Contributor
timestamp: February 22, 2019, 05:23:23 pm
content: Hook back up your 34401A to measure the DC current.1. Measure the voltage drop (V DC) across the input to the 34401A with the Fluke 115How much do you read?2. Install a shortage wire across the input to the 34401A.Now the 34401A should read almost 0 A in current.Does the power supply now increases to almost 1A again?

author: salvagedcircuitry
author_rank: Regular Contributor
timestamp: February 22, 2019, 05:48:57 pm
content: @JDubU resistance between the "3A" and "Lo" inputs is 5.6ohmsQuote from: HighVoltage on February 22, 2019, 05:23:23 pm1) The voltage drop is about 0.2v DC2) Yes, that is correct.So the end result is that burden voltage has a lot more impact on current measurements on my 34401a than my fluke 115. Ok that makes sense now. Thanks guys

author: JDubU
author_rank: Frequent Contributor
timestamp: February 22, 2019, 06:01:30 pm
content: FYI:Here is an excerpt from the 34401A service manual showing shunt resistor values for each current range setting.

author: HighVoltage
author_rank: Super Contributor
timestamp: February 22, 2019, 06:10:36 pm
content: Quote from: salvagedcircuitry on February 22, 2019, 05:48:57 pmYes, this is a easy trap to fall in to and a good lesson to learn about the 34401A current measurements.