what will hp 6181c do if you try to sink current with it? (misusing lab supplies - Page 1
Forum section: electronics / projects

author: coppercone2
author_rank: Super Contributor
timestamp: November 03, 2024, 05:38:59 am
content: I think some DC power supplies can sink a little bit of current (undefined operation) and be OK. At least a tiny bit. ?How about a current source?I thought someone might know the answerActually I am also curious what lab voltage supplies do if you try to use them as sinks for small current limited sources. Like a few mA. I mean, you can adjust them when they are connected to loads with filter caps, and they don't usually break, i.e. when you have a low current and a big filter capacitor on a circuit. I think that it must be 'sinking' current a little when you turn it down? For instance I have a device with like 1000uF capacitors on a PCB, and it draws like 2 miliamps when doing nothing. I can turn the knob right down to 0V when it connected and nothing breaks.

author: ArdWar
author_rank: Frequent Contributor
timestamp: November 03, 2024, 06:25:00 am
content: Quote from: coppercone2 on November 03, 2024, 05:38:59 amPretty much all lab PSU using series regulating pass element will have some sort of output sink if they're specified to regulate down to zero current zero voltage output.Of course there may difference in implementation. Cheap one may use simple permanent load resistor, some use weak constant current load just enough to discharge the pass element leakage (plus some headroom to satisfy ripple spec etc), some have fairly beefy current sink. My 2281 can sink up to 1A in quadrant 2 operation, and that's actually specified in datasheet.However beware that most of them do not regulate, or even protect, during Q2 operation. It's up to you to limit the current going into the PSU.

author: RoGeorge
author_rank: Super Contributor
timestamp: November 03, 2024, 07:18:47 am
content: Quote from: coppercone2 on November 03, 2024, 05:38:59 amIn certain situations there is a trick to turn any power supply into a controlled load, where you can adjust the current. For example, if you want to do use the power supply to discharge a battery at constant current (to measure the mAh of the battery):https://www.eevblog.com/forum/projects/cc-cv-power-supply-as-a-contolled-load-for-batteries-(i-e-rigol-dp832)/So, as long as you put the PS and the load in series, and add an element to shift the negative voltage to a positive level, any power supply should work as a current "sink". "Sink" is between quotes because that is what the load "sees", while the PS is in fact sourcing current (from the PS perspective) and can regulate voltage and current full range, so not just a little bit of current, but the full amps the PS is capable of.Say, you have a 0-12V/0-3A adjustable PS. With the trick from the link it is possible to discharge with that a Li-Po battery at any current you wish between 0-3A. That discharging current will be regulated by the the power supply.If some other conditions make that trick not applicable, then instead of a normal (first quadrant) power supply, you'll have to use a two quadrants or a four quadrants (often called a SMU, Source Measure Unit,https://en.wikipedia.org/wiki/Source_measure_unit) capable power supply.

author: coppercone2
author_rank: Super Contributor
timestamp: November 03, 2024, 07:26:26 am
content: I have a number of multi quadrant power supplies designed to do this but I am interested generally what happens if you try this with a regular (older) voltage supplyand also specifically this current source.Like the ones with ten turn dials to let you really carefully adjust it

author: RoGeorge
author_rank: Super Contributor
timestamp: November 03, 2024, 09:58:04 am
content: Can't say for sure, it depends of the exact schematic. Many schematics have a series regulator transistor, like the one you attached. I imagine if you push too much current, the BE of transistor(s) might become reversed polarized. Usually Vbe in reverse is very small, only 5-10V. So if the output of the PS becomes -10V instead of +10V, then it might damage the transistors, though that would probably need a few hundreds of Ma to happen, but I'm not very sure, I didn't try.A small current from the outside should be safe, because it happens all the time when, for example you set the voltage 10V, the capacitors of your circuit charges at 10V, then you set the voltage to 5V. After this, the output capacitors will push their accumulated energy into the power supply, until their voltage drop from 10 to 5V. That wouldn't damage any power supply.The problem is when you connect a powerful battery, for example you charge a car battery and an unexpected mains power surge occurs. In this case, the car battery will start to discharge through teh PS, which might damage the reversed polarized transistors. Because of this, in the user manual it might be mandated to always charge batteries through a series diode.

author: ArdWar
author_rank: Frequent Contributor
timestamp: November 03, 2024, 10:18:41 am
content: Quote from: coppercone2 on November 03, 2024, 07:26:26 amLook at the (appropriately named) Q2, it should clamp output voltage by sinking current if you operate your supply in quadrant 2.

author: factory
author_rank: Super Contributor
timestamp: November 03, 2024, 01:49:53 pm
content: Quote from: ArdWar on November 03, 2024, 10:18:41 amQ2 is a small TO-39 part, Q6 (TO-3) is used for the internal voltage limit in that circuit, but there are diodes in both the positive & negative output rails, not sure how it would work, see description in 4-39 of the manual.Also see 3-40 for reverse current info, loss of regulation will result.David