Why did I buy a crappy switching bench supply? - Page 1
Forum section: electronics / testgear

author: rdenney
author_rank: Regular Contributor
timestamp: January 18, 2025, 08:49:47 pm
content: This isn’t a new topic but digging around in the archive didn’t show any actual scope traces, so maybe this adds value to the archive. I’m newly registered but have been reading this forum for several years when researching bench equipment, and tend to research stuff pretty deeply.My main bench supply is a beautifully made and operating Power Designs 2015A, obtained a few years ago from a ham radio buddy. But one is not enough and I succumbed to the temptation to get one of the not-cheap-enough Chinese supplies, this one made by “Nankadf”. Yeah, it works, but I threw it on the cheapie DSO to check how well it worked. Yup, transient switching pulses galore. If I’m looking at the scope display correctly, the frequency of the pulses is 80-90 KHz, and the amplitude is significant.I think I’ve screwed this up, so advice will be welcomed. I’m thinking these pulses might be minimized by being fed into a load impedance a lot lower than the input impedance of the DSO.And, of course, there are many needed attributes of a bench supply beyond just having a clean steady-state signal.First photo is the switching bench supply, set to 1.00VDC output. The voltage it claims is close enough, as shown in the second photo. The signal is in the third photo.To compare it to my usual bench supply, I set my PD2015A for 1.000VDC output and the signal is as clean as the scope’s inputs.So, I bought another PD supply—they are not expensive—but I need a linear supply that will do higher voltage and positive and negative around a central common, so I also bought a triple-output GW Instek GPC-3020, which won’t be as good but at least shouldn’t have switching artifacts.The switcher will not be on the bench but it will work fine powering my electroplating stuff.Rick “never enough power sources” Denney

author: radiolistener
author_rank: Super Contributor
timestamp: January 18, 2025, 09:09:28 pm
content: switching psu are not a lab bench psu. It is intended to do something like charge batteries in garage, where using lab bench psu is too expensive and inefficient. From that point of view these pulses and other issues of SMPS are acceptable.If you're looking for a good lab bench psu, then look for linear psu.

author: TheoB
author_rank: Regular Contributor
timestamp: January 18, 2025, 10:15:23 pm
content: You did not show your probes. Did you measure with the long ground lead of the probe? Common mode switching noise can flow through the ground of your probe. Unless you made sure this is not the problem, you can start to blame the PSU.Noise will remain higher, but it should be limited to a few mV triangular shaped noise. Improper decoupling and too high ESR can lead to high peaks. That can be fixed in the PSU by adding a small cap. directly over the terminals. Common mode noise on the other hand could flow out via the ground lead through the ground of your scope. Try adding chokes over both wires.And the ground leads of your probe will pick up any magnetic field from the switcher.

author: rdenney
author_rank: Regular Contributor
timestamp: January 18, 2025, 10:23:32 pm
content: Don’t have to do any of that with the PD supply.Rick “it’ll be fine powering the electroplating rig” Denney

author: DaneLaw
author_rank: Frequent Contributor
timestamp: January 19, 2025, 12:08:52 am
content: QuoteLikely because its cheap as, and will do most of what people use a "crappy" switching PSU for.As mentioned above, you need to take your precautions when trying to isolate sm-noise from a given unit from your surroundings.An adj. linear bench PSUs are often heavy & bulky and tend to be quite current/power limited, but both "linear or switch mode" PSUs have their places, it depends on what you wanna use it for.

author: OLderDan
author_rank: Regular Contributor
timestamp: January 19, 2025, 12:37:48 am
content: Quote from: radiolistener on January 18, 2025, 09:09:28 pmI feel your judgment, but my fleet of helicopters and warbirds thumb their lipo batteries in your face when, thanks to my foray into cheap chinese switching supplies, they finally get a decent constant current followed by constant voltage full capacity charge, instead of the stock charger criminal acts that were perpetrated on them!