E3634A 4 wire connected to electronic load displays incorrect monitor current - Page 1
Forum section: electronics / testgear

author: Helix70
author_rank: Unknown
timestamp: November 13, 2024, 04:51:08 am
content: Hi all,I have an Agilent E3634A that I have repaired, but there is one fault that eludes me. Everything works correctly with static loads, and also when the sense terminals are linked at the terminals.When I run separate sense leads and power leads to an electronic load (Tenma/Korad) set to constant current 4A, and set the E3634A power supply to 2V at 5A limit (CV), the monitor current reads 1.0-2.0A despite the load drawing 4A as expected. Changing the current limit on the 3634A down to 3.9A forces into CC mode and then the monitor reads accurately as expected.When I probe the sense terminals when I see the fault, there is a 60 kHz ripple of about 0.5V visible, which disappears when CC is forced or the voltage is higher in CV mode.Anyone else able to replicate? Is it the PS, the load, the idiot operator? Any other ideas?Regards,Josh

author: Ugur
author_rank: Regular Contributor
timestamp: November 13, 2024, 11:08:12 am
content: Have you tested it by connecting a resistor to the output of the power supply?

author: Helix70
author_rank: Unknown
timestamp: November 14, 2024, 01:01:14 am
content: Yes, as I said in the initial post, works correctly with static loads.

author: Ugur
author_rank: Regular Contributor
timestamp: November 14, 2024, 08:42:28 am
content: Sorry I missed it. In this case I think there is a problem with the electronic load. Have you tried connecting the electronic load to another power supply to be sure?

author: Helix70
author_rank: Unknown
timestamp: November 15, 2024, 03:12:44 am
content: I am probably hoping for someone who can test it on their power supply. The electronic load works fine, and it is a very specific problem, but thanks for your advice.

author: Kean
author_rank: Unknown
timestamp: November 15, 2024, 01:31:49 pm
content: This oscillation sounds like it could be due to inductance in your wiring.Can you try making the load carrying leads between the PSU and E-Load as short as possible. Also twist the sense wires together.

author: gburdzin
author_rank: Contributor
timestamp: November 15, 2024, 01:45:15 pm
content: For your reference...I tried a similar test with my E3633A power supply set to output 2V with a current limit of 6A, and a Maynuo M9712 electronic load set to CC mode of 5A. In both cases, with the sense terminals connected at the power supply terminals (via the shorting bars), or with the sense terminals connected via wires to the terminals of the electronic load (shorting bars removed), I am seeing the correct current reading of 5A on the display of the power supply.

author: Helix70
author_rank: Unknown
timestamp: November 15, 2024, 11:43:56 pm
content: Quote from: Kean on November 15, 2024, 01:31:49 pmI can certainly influence the readings by moving the leafs. I tried twisting them together for each load and sense pair, didn't really help. There leads are 1.5m, I will try shorter when I can. I can work around it, just trying to determine if there is another fault lurking, I have replaced 2 precision resistors so far, and wanted to know if this is normal or strange. Thanks

author: Helix70
author_rank: Unknown
timestamp: November 15, 2024, 11:45:25 pm
content: Quote from: gburdzin on November 15, 2024, 01:45:15 pmAny ripple on the sense terminals with a four wire setup?

author: gburdzin
author_rank: Contributor
timestamp: November 18, 2024, 01:19:12 pm
content: There is no noticeable ripple to me.

author: nctnico
author_rank: Super Contributor
timestamp: November 18, 2024, 02:45:00 pm
content: Quote from: Helix70 on November 13, 2024, 04:51:08 amThis can happen when the control loops of the PSU and DC load start to interact. Connecting a big capacitor (> 1000uf) across the terminals can help to stabilise the control loop. You have to keep in mind that a DC load in constant current mode has a high impedance (lookup the definition of a current source) for which the PSU isn't designed.

author: Kean
author_rank: Unknown
timestamp: November 18, 2024, 02:53:38 pm
content: And a similar issue was just reported by another user with different equipment.https://www.eevblog.com/forum/testgear/siglent-sdl1000xsdl1000x-e-electronic-load/msg5718259/#msg5718259It is a reasonably common occurrence, and I believe often goes unnoticed.