Checking specs of a 48V PSU - Page 1
Forum section: electronics / beginners

author: tatel
author_rank: Frequent Contributor
timestamp: March 04, 2023, 09:13:03 pm
content: I recently got a 48V/400W PSU. I was wondering about checking if specs are more or less true. But, thinking about it, when it comes to measure riple/noise, overshoot, etc, I can see some problems coming:a) Oscilloscope manual says to put no more than 300 V RMS on the inputs, and I assume this is with a 10x probe. So, with an 1x probe, I would surely toast my scope by putting 48V DC on its inputs.b) Even if the scope could handle that 48V input, then I would need something like a 50W terminator, or perhaps a dummy load attached to a T adapter. Neither easy to find nor cheap. Perhaps a suitably-sized cheap resistor could do the trick? But they all seem to be wirebound, and ripple/noise would be AC, perhaps at high enough frequency to be worried about inductance effects in the measurement?c) I could simply use a 10x probe, then my scope could cope with the task. No terminator needed. Ripple/noise in this family of PSU is said to be "less than 240 mV". So I could see anything significant ripple/noise, even if attenuated, by setting AC coupling and low mV/div on the scope. Unless it's an exceptinally good PSU, which I don't expect it to be. That way I could get rid of a) and b). Looking at overshoot would be more difficult, however. Is that right?d) It's said that ripple/noise has to be measured at near full load. Cheap way to do that would be to put, say, half a dozen 100W/40R resistors in parallel. Again, these would be wirebound, so perhaps some inductance would get again into the business?e) Last, I have seen recommendations about having a couple capacitors in parallel at PSU output. Say, 10 uF and 100 nF. I can understand how these capacitors would filter the output, but isn't that cheating?Well, after some readings and videos watched, I now understand why it's said that looking at ripple/noise is quite difficult. So I could easily let this idea pass away. However I'm quite interested in any comments you could do.

author: Benta
author_rank: Super Contributor
timestamp: March 04, 2023, 10:20:55 pm
content: a: don't make assumptions. If the 'scope front panel/manual says 300 V, then that's it. Don't try to be smarter than the manufacturer.b: you don't need anything. Just connect your probe to the PSU output.c: total overthinking. I'll ignore this point.d: yes, this is the difficult point. You need to find a way to dissipate 400 W.e: rubbish. You can be certain that all this has been taken care of inside the PSU.When you've found a suitable 400 W DC load, AC couple the 'scope input and set it to 500 mV and check what you see.

author: tatel
author_rank: Frequent Contributor
timestamp: March 05, 2023, 04:17:00 pm
content: As I understand it, your advice is don't worry and take the measurements with your o'scope probe(s). OK, will do. Thanks for your answer. Getting me out of my wrong understanding of input voltage has been helpful. Also, I'm sure just using my probes and accounting for it will be good enough and that way I'll keep my device safe.However, I see Dave advising to use coax cable to make a differential measurement on 31:26 mark here:https://www.youtube.com/watch?v=Edel3eduRj4.Any comments?And here he speaks about terminators power rating (5:06):Again, I understand your answer provides a safe and easy way to have a good enough measurement. But, since I'm here looking for knowledge, could you please coment about these other ways?

author: Benta
author_rank: Super Contributor
timestamp: March 05, 2023, 08:25:12 pm
content: All I see is someone's left hand shaking a black cable for several minutes, which is a waste of my time.I don't care much for those kind of videos anyway, they're not very helpful compared to own solid understanding and knowledge.

author: pdenisowski
author_rank: Super Contributor
timestamp: March 06, 2023, 08:40:47 am
content: Quote from: Benta on March 04, 2023, 10:20:55 pmWhen I had to prove to someone that one of our benchtop DC power supplies could source ~800 watts (4 x 20V channels, each producing 10A), I took the cheap and easy way out and used an automotive battery tester as a load. Can only use it for a few seconds at a time without it getting hot / smoking, but might work if you're quick about it.(Actually, the cheap banana to alligator leads I was using melted first ....)

author: tatel
author_rank: Frequent Contributor
timestamp: March 07, 2023, 07:53:41 am
content: Wow, that PSU looks like a really big pepino... it must be quite expensive, it isn't?I have some aluminum extruded heatsink lying around. Im thinking about cutting some 18" from it and use with 6 100W 40 Ohm resistors. This should give some 350W - 85% load. I wonder what temperature it could reach. Anyway, I also have a bunch of 3D printer fans if needed.https://www.digikey.com/en/products/detail/wakefield-vette/125308/9957658

author: pdenisowski
author_rank: Super Contributor
timestamp: March 07, 2023, 10:52:45 am
content: Quote from: tatel on March 07, 2023, 07:53:41 amDepends on what you mean by "expensive", but it's not exactly priced for the hobbyist markethttps://www.rohde-schwarz.com/us/products/test-and-measurement/dc-power-supplies/rs-ngp800-power-supply-series_63493-670592.htmlQuote from: tatel on March 07, 2023, 07:53:41 amBefore I took the lazy way out, I did consider building my own load and had planned to suspend the resistors in mineral oil to help dissipate the heat. I've done this for RF dummy loads in the past and this helped reduce heating issues. Mineral oil is both cheap and non-toxic (you can even drink it!).

author: tatel
author_rank: Frequent Contributor
timestamp: March 07, 2023, 04:05:24 pm
content: Actually I was wondering about using distilled/deionized water. That would give a nice evaporative cooling. I knew that oil is used in transformers, etc, but didn't knew what kind of oil exactly. Mineral oil is easy to get, but I would love to know what the cooling process is. I guess it's not evaporation. Will have to google some more time yet.BTW, first thing that came to my eyes while reading the page from Wikipedia was "laxative"Thank you very much for the tips

author: David Hess
author_rank: Super Contributor
timestamp: March 08, 2023, 02:44:14 am
content: Quote from: tatel on March 04, 2023, 09:13:03 pmThe oscilloscope input overload specifications have nothing to do with the probe attenuation. You should be able to safely apply 300 volts RMS directly to the oscilloscope input, and do so at any vertical sensitivity setting.