Array 3721A electronic load oscillations - Page 1
Forum section: electronics / testgear

author: Zbig
author_rank: Frequent Contributor
timestamp: September 19, 2016, 10:01:15 am
content: HiI've connected my Array 3721A to my DC lab supply with a Keysight U1273A meter in series. When testing using small, less than 30 mA loads on Keysight's 30 mA range, I noticed my Keysight meter (in current mode) reading a bit high and its bargraph jumping up and down like crazy, some ~5 mA peak to peak, with a lowish frequency of few Hertz. Keysight's peak detection mode reported around 10 mA p-p. I confirmed these load pulsations by scoping across the multimeter (measuring across the meter's shunt resistor/burden voltage, basically). I've then switched from lab supply to the battery as a source to rule out the instability on the source side; no change. Now, I'm aware that the control loop on the DC load couldn't possibly be 100% stable and Array themselves allow up to 30 mA p-p ripple in the specifications. I'm also aware that every piece of cable, shunt resistor in the meter, etc. has its parasitics. Nonetheless, I'm a bit surprised by the low frequency nature of this. Is this normal?Regards,Zbig

author: MarkL
author_rank: Unknown
timestamp: September 19, 2016, 03:21:39 pm
content: I wouldn't expect it to oscillate, especially with a battery as the source, but I can't say I'm totally surprised.I was unable to recreate any kind of instability with a 3723A from 0-40mA and different voltages. I tried it with and without a Fluke 79 in the middle.Does your 3721A show any instability in the displayed measured current? Can you give an example setting on the power supply and load where it's unstable? Is there a range where it becomes stable? How about if you increase the burden voltage or remove your U1273A from the circuit?If you set up for constant ohms, is that unstable too?Although I haven't done it, the firmware on these units is upgradable. Maybe that's worth a try. The latest is 1.43:http://www.array.sh/yqxzcnE.htmYou can see your current version by holding down "Tran" when powering on or querying it with "*idn?" .

author: Zbig
author_rank: Frequent Contributor
timestamp: September 20, 2016, 09:52:02 am
content: Hi Mark,thank you for responding. My Array is already at firmware version 1.43. If I recall correctly, the current reading on 3721A is stable.An example setup resulting in instability:- 5V on the lab supply- 300 mA current limit on the lab supply (just to protect the meter's fuse in case of any mistake on my side; I don't actually hit it)- Array 3721A in CCL mode, sinking 1 to 30 mA- Keysight U1273A on 30 mA range, either high- or low-sideIn this case, the bargraph on U1273A does its happy dance. Raising the compliance voltage on the supply doesn't seem to affect anything, neither does substituting it with a 11.1V 3S Li-Po battery. I'll test with different shunt resistor values and other 3721A's modes when I manage to get to this topic again. Also, I'll try to get some reliable scope graphs as the ones I got were swamped in noise due to U1273A's low burden voltage (it only gets close to 1V on its 300 mA range). I could also test with an LED (I think low frequency, ~10 mA current swings should be fairly visible).Regards,Zbig

author: MarkL
author_rank: Unknown
timestamp: September 20, 2016, 03:13:38 pm
content: Hi Zbig,I set up 30.0mA on the 3723A and 5.0V @300mA on the supply and I get the waveform below. This was measured with a current probe. The waveform looks pretty much the same from 5mA to 100mA.The power supply alone set to 30mA is perfectly flat with a tiny bit of AC hum (it's a linear supply), so it's definitely the load.Strictly speaking the load is within the spec of:Ripple and NoiseCurrent(rms/p-p) 3mA/30mAThat spec is presumably for the entire range. It would have been nice for them to break it down to CCL vs. CCH if there was any difference.Perhaps your U1273A does not like the ripple? Maybe a nice large value capacitor across the input can help smooth out the reading.It would be interesting to see what you get on the scope. I wouldn't have expected your ripple to be so much lower in frequency.

author: 2N3055
author_rank: Super Contributor
timestamp: September 20, 2016, 05:13:32 pm
content: Put a snubber network across the load (2-3uF block capacitor , not elco, in series with 4,7 Ohm resistor)..Also try to increase current limit on PSU, and see if it changes..Power supplies are inductive in their output characteristics, and active load can be made to oscillate...

author: Zbig
author_rank: Frequent Contributor
timestamp: September 20, 2016, 05:17:59 pm
content: Thank you for your effort, Mark. I think I know what's going on with my meter's bargraph. As both the ripple frequency on your screenshot and as measured by my meter is around 200 Hz, it interferes with the bargraph's refresh frequency of some 60 Hz which results in this weird low frequency beating. I just have to confirm this by some proper scoping as soon as I get some shunt resistors.

author: Zbig
author_rank: Frequent Contributor
timestamp: September 20, 2016, 05:19:35 pm
content: Quote from: 2N3055 on September 20, 2016, 05:13:32 pmWill try that too, thank you.