Test procedure for cheap lab supplies - Page 1
Forum section: electronics / beginners

author: Siwastaja
author_rank: Super Contributor
timestamp: February 19, 2021, 05:58:44 pm
content: Cheap CC/CV lab supplies are discussed time-to-time. Many are broken-by-design, or crappy beyond belief and should not be marketed as CC/CV lab supplies. And I don't mean a cheap unit must be accurate, no; but it must not blow up or destroy something elsewhen used as expected for a lab CC/CV supply. The car analogy: it's not acceptable if a new cheap car blows up in flames when you try to use the parking brake for the first time.Anyways,As a reference for beginners, here's a test procedure I would suggest that verifies if they blow up instantly doing completely normal CC/CV lab use cases. Note this doesn't prove they last long, or that they are good at all, but at least you will be able to prove a completely broken-by-design supply, or make a supply blow up instantly (for money-back return), before committing any time doing actual work with it (and possibly blowing up your projects with it).This test assumes a 30V, 3A unit. Scale numbers for other ratings.1) Current limit test 1Set the output voltage at maximum (30V or more). Connect a 10 ohm power resistor. Adjust the current limit knob. Verify you can adjust current from 0A to 3A (or whatever will be the maximum your load resistor takes at max output voltage).Success criteria: current adjustment is fairly smooth and current doesn't randomly jump all over place. Current can be adjusted down to zero.Bonus points: use two external multimeters to verify current and voltage readings are within the specs. (A few %fs of error is acceptable for a cheap unit, 30% for example is not.)2) Current limit test 2Still using the 10 ohm resistor, adjust the current limit to, say 0.1A. Short the output. Verify current is still limited to 0.1A.Success criteria: A few % of full-scale error, or whatever the spec sheet says, is again acceptable; current jumping to like 1A is not.3) Current limit test 3With output shorted, adjust the current limit.Success criteria: current adjustment is fairly smooth and current doesn't randomly jump all over place. Current can be adjusted down to zero, and up to the rated current.4) Voltage adjustment testA) with no load, B) with the 10 ohm load resistor, adjust the voltage knob. Measure output voltage with a multimeter.Success criteria: Output voltage adjustment is fairly smooth and doesn't suddenly jump all over the place. A few %fs of inaccuracy is to be expected with a cheap unit. Suddenly outputting 30V when you asked for 10V is not.5) Voltage-at-output testFor this, you need another power supply. Any voltage source like a wall-wart or a few 9V batteries does ok.Turn the unit off. Connect voltage close but below the maximum voltage rating to the output terminals. Keep for a few minutes. For better test coverage, while holding the voltage, turn the unit on, and adjust the unit's voltage while still keeping the auxiliary supply connected to the output. Don't go over the output voltage of your auxiliary supply to avoid damaging it (if it is not a proper lab supply).Fuse is recommended in series with the auxiliary supply, especially if it is a battery, for safety.Bonus points: For interest: measure current that leaks into the unit.Success criteria: the unit still works after the test (re-check other tests)6) Thermal testA) With the 10ohm load (make sure it handles the dissipated power!!), B) with short circuit, set the voltage and current limit to maximum values. Let run for half an hour to an hour.Success criteria: the unit doesn't emit visible smoke (some smell is OK), and is functional after the test (recheck other tests).Note: A proper supply could be tested this way for a year. But we are talking about cheap devices, give it a break.In case of failure:Ask for money back. It is not a CC/CV lab supply, it's a scam or a faulty unit. Be happy you tested it sooner than later.Any other ideas to add?

author: golden_labels
author_rank: Super Contributor
timestamp: February 20, 2021, 05:41:33 pm
content: Quote from: Siwastaja on February 19, 2021, 05:58:44 pmMaximum voltage is the easiest part for linear power supplies. In this mode they need to drop close to no voltage at the regulating transistor and therefore power is also very low. Setting it just above each point at which it switches taps is what stresses them most.

author: Rick Law
author_rank: Super Contributor
timestamp: February 21, 2021, 06:27:10 am
content: I use a low-cost CC/CV buck converter (B3603) as my "lab" power supply. These two failures are actual failures I encountered, and two more not exactly failures but not very nice stuff.(1) On power up, before the buck converter IC kicks in, the B3603 outputs basically pre-buck voltage for a very brief moment. Not nice. This was brought up in a reply by someone on this "mini-review" thread I wrote eons ago, I don't remember who. With 10 pages of replies to the thread, I don't want to page through 10x25 replies just to hunt for that particularly reply.https://www.eevblog.com/forum/reviews/b3603-dcdc-buck-converter-mini-review-and-how-the-set-key-could-be-fatal/I have not actually had anything killed by it, but I know it could happen. My scope confirmed that yeah it output the 24V I put into it -- for a blinking of an eye moment.(2) I have a few of the B3603 for a few years. With my main one which had been in service for a few years -- I got very confused one day. The B3603 displayed voltage/current isn't making sense. Took me a bit of investigation to discover that the custom-calibration is no longer in effect and was operating at default calibration. The default calibration was about 10% under (it's output was about 10% over). I did re-calibration but it would not store. If I use external DMM to read the V&I and set it according to external DMM reading, it will hold CC/CV to those settings. I think the EEPROM or flash that store the calibration data failed. No matter what I calibrate it to, it felt back to the default.(3) This one is not a failure one needs to test for. This one yells out at you. With one of my B3603, the shift-register IC failed rather prematurely when the thing was but a couple of months old. The LED reading was garbage. I swap out the IC and it came back into service. That was 5+ years ago by now and is still in service running right now trickle-charging an SLA.(4) In the thread I referred to above, I pointed out the poor design in the UI that could kill anything connected to it if one is careless. I suppose that is not a failure in the converter itself, just poor UI and user carelessness - one can mistakenly took it to calibration mode which makes the converter output 30V (or whatever max it could up to 30V) -- and woops - gone is your project connected to it.

author: Jan Audio
author_rank: Frequent Contributor
timestamp: February 21, 2021, 04:41:22 pm
content: Are you trying to kill those cheap PSUs within 1 day ?Make sure to get your money back, dont let them repair your warranty away, china shipping is very slow.