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R&S HMC8043 unreliable behaviour at very low current limits
glx:
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):
Darkover:
> 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 with
more accuracy?
Olaf
EEngo:
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.
alm:
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.
glx:
--- Quote from: EEngo on October 15, 2023, 09:08:50 am ---[...] 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.
--- End quote ---
Oh 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 am ---*Edit: That seems to be not always true. Sometimes current is limited, sometimes not.
--- End quote ---
I 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 am ---But I think it's unreasonable to expect that kind of dynamic range from a general purpose bench supply.
--- End quote ---
Yeah 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 :-/O. 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 am ---develop your own powersupply with only an opamp that can deliver 0-10mA, but with
more accuracy?
--- End quote ---
That's a nice idea for (another) side project actually ^-^.
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