R&S HMC8043 unreliable behaviour at very low current limits - Page 1
Forum section: electronics / testgear

author: glx
author_rank: Newbie
timestamp: October 15, 2023, 12:37:14 am
content: Hello,I have an HMC8043 power supply here, on which I noticed some weird behaviour when using very low current limits.I was playing around with some solar-powered things that I wanted to power from the HMC8043 to take some measurements. I did set the current limit to the lowest possible value of 0.5mA, but it somehow doesn't respect that (reliably). I can pull up to around in 4.5mA in CV until the OCP triggers and it switches to CC mode. I could understand if the supply couldn't measure such small currents, but it shows the value, so it knows but doesn't care.I couldn't find anything specific in the datasheet besides "0.05% + 2 mA" readback accuracy, also it is pretty accurate and once the OCP is triggered it regulates pretty accurately to the 0.5mA but I have to go well over the set limit to make that happen.Is this a normal behaviour? Probably a supply like the NGL/NGM/NGU series would be rather made for that job (at least they are specified to be more accurate at low currents) but those are quite expensive and I don't use these low limits (especially THAT low) often, also the HMC8043 pretends it can do it, but somehow doesn't.It is the most recent firmware (01.400) installed on the supply.I noticed that sometimes there are R&S-people around here, maybe someone there has some insights.I did shoot a short video to show the behaviour, for simplicity I did just measure two LEDs there (sorry for the messy desk):

author: Darkover
author_rank: Regular Contributor
timestamp: October 15, 2023, 02:36:09 am
content: > HMC8043 pretends it can do it, but somehow doesn't.My guess, there is noise on the line and on the ADC. You expect a very high dynamik range from your power supply.Perhaps it is better to develop your own powersupply with only an opamp that can deliver 0-10mA, but withmore accuracy?Olaf

author: EEngo
author_rank: Unknown
timestamp: October 15, 2023, 09:08:50 am
content: Hey,I can confirm that with my unit (SW V1.303 (2016))At very low currents below 5 mA, the current limitation (set to 0.5 mA) only becomes active when the current briefly increases above 3-5 mA or when the threshold is changed from 0.5 mA to 0.7 mA or 0.8 mA.* Then suddenly the current limitation kicks in.EEngo.*Edit: That seems to be not always true. Sometimes current is limited, sometimes not.

author: alm
author_rank: Super Contributor
timestamp: October 15, 2023, 09:31:49 am
content: The programming accuracy is listed as 0.05% + 2 mA, so if you program it to 0.5 mA, it should technically source between - 1.5 mA and 2.5 mA. But I think it's unreasonable to expect that kind of dynamic range from a general purpose bench supply.

author: glx
author_rank: Newbie
timestamp: October 15, 2023, 10:21:15 am
content: Quote from: EEngo on October 15, 2023, 09:08:50 amOh yeah you are right, I haven't tried that before. It also seems to struggle a bit there to dial in a corresponding voltage since it sometimes shuts the channel down to 0V for a little moment before it settles to something.Quote from: EEngo on October 15, 2023, 09:08:50 amI did also observe that it sometimes triggers like the first event but doesn't trigger again for the same event after recovery, but I cannot reproduce that in a controlled manner, it seems like it randomly happens.Quote from: alm on October 15, 2023, 09:31:49 amYeah I kind of agree, I didn't even expect it to be able to set it to such low limits, but I somehow expect when there's a setting it acts accordingly and not just apparently ignore it. But I agree, it's pretty much pushing it to the limits of a general purpose supply.Quote from: Darkover on October 15, 2023, 02:36:09 amThat's a nice idea for (another) side project actually.

author: Wade2019
author_rank: Contributor
timestamp: October 16, 2023, 03:05:50 pm
content: Does your device require calibration?