AC/DC converters stable output - Page 1
Forum section: electronics / projects

author: arduic
author_rank: Contributor
timestamp: March 18, 2025, 01:11:15 am
content: I am working with some cheap AC->DC converters for high current applications and ran into an, obvious in retrospect issue, power stability. When I first plug the supply in I get massive swings from +20V to -5V for a +5V power supply. Once I realized this I put a switch in circuit so output is not provided to my electronics until I hit go but this will not work for my final design. I was initially thinking of getting a massive 50A relay to put in line but that feels overkill. I noticed that some cheap power supplies label a "remote on/off" feature but I am unsure does the on/off happen at the AC or the DC end? If the DC end I would assume using the remote on/off will ensure a relatively (in rush excluded) stable output but if the on/off is at the AC side I am honestly confused what the purpose is.If anyone has experience with these converters I would really appreciate the insight I am much more used to benchtop where power is just safe enough to ignore all this headache.

author: Konkedout
author_rank: Frequent Contributor
timestamp: March 18, 2025, 03:40:13 am
content: I have to admit that your post does not make a lot of sense to me. In a lifetime of designing power supplies, I cannot remember ever seeing any DC output power supply with such a radical + to - swing on startup.Can you provide any details?1) Links and/or part numbers for the power supplies in question.2) Specifications and any other information for the power supplies in question.3) An oscilloscope photo showing this voltage swing.4) What is your load when you observe this?5) What is your measurement method? I can imagine an underdamped movement of an analog voltmeter first overshooting and then bouncing back to briefly indicate negative voltage.

author: arduic
author_rank: Contributor
timestamp: March 18, 2025, 09:21:26 am
content: I can certainly sympathize with the confusion as the behavior I'm describing is way out of spec. I'll do my best to answer your questions below.https://www.digikey.com/en/products/detail/mean-well-usa-inc/RSP-320-5/7706335The spec claims a max ripple of 150mV but I'm suspecting that is only once power is stable.see attachment.load in this picture was I think a couple amps. Certainly no where near the maximum of the supply. Once I put a switch in line with the output I do not see this startup ripple so I do not think it's a rush issue.A digital oscilloscope (Rigoal) attached to output. I put about 1000uF on the output after this picture and while it damped the output slightly the spikes were still massive.From what I can tell this spike has killed multiple components I've been working with. Simple components were failing constantly but once the switch was added deaths stopped.

author: moffy
author_rank: Super Contributor
timestamp: March 18, 2025, 10:32:18 am
content: That looks terrible, if you put a relay coil across the output which also switched the output that might provide the necessary logic/delay to ensure the output is stabilised before it is switched. If you do that you might want a clamp diode across the relay coil, with the coil and diode with a low value series resistor to limit current.

author: arduic
author_rank: Contributor
timestamp: March 18, 2025, 11:10:04 am
content: I agree with the terrible.If I am understanding your circuit correctly for my device that relay would need to be rated for 60A (PSU goes to that).I was trying to avoid needing to spec such a large relay in my final design hence asking about supplies that list a "remote on/off" feature. This PSU sadly does not have the ability so I can't test and I don't want to buy another supply just to learn it won't fill my need.

author: moffy
author_rank: Super Contributor
timestamp: March 18, 2025, 11:22:11 am
content: Quote from: arduic on March 18, 2025, 11:10:04 amProbably more like a mini contactor then, Meanwell from experience are pretty good supplies, nicely made and well rated. Are there no environmental factors interacting with the supply like very long input or output connections? And are the measurements made across the PSU output terminals or at a remote point?

author: arduic
author_rank: Contributor
timestamp: March 18, 2025, 11:38:19 am
content: The cabling on the DC side is not ideal. I only have some 22awg solid core wire so it's 4 wires on positive and 4 on negative twisted together at the supply side. As an additional not great the output is plugged into a breadboard. The cable length is less then 5 ft.

author: moffy
author_rank: Super Contributor
timestamp: March 18, 2025, 11:47:41 am
content: If me, I would make the measurements at the PSU terminals with and without the cabling to see if that is reacting with the PSU. By twisting I assume you mean that the positive and negative cables are twisted the entire length to the load.

author: arduic
author_rank: Contributor
timestamp: March 18, 2025, 12:48:27 pm
content: No I mean the 4x wires are twisted together at the end so they are all shorted. All of the wire is just kind of free floating. I've never heard of twisted pairing your power supplies. That shouldn't matter should it?When I get home today I'll test the supply with loads removed and loads connected measuring at the supply. Is the thought that my crappy cabling could cause that much noise?

author: Haenk
author_rank: Super Contributor
timestamp: March 18, 2025, 12:52:22 pm
content: 1.5s or 3s (under full load) setup time, according to datasheet. That's probably the reason you see this crazy initial voltage swing.

author: temperance
author_rank: Super Contributor
timestamp: March 18, 2025, 01:32:37 pm
content: What you are looking at is common mode noise generated when the power supply is being switched on. You can verify this by connecting a shorted probe into the GND of the power supply.

author: mtwieg
author_rank: Frequent Contributor
timestamp: March 18, 2025, 02:18:58 pm
content: Yeah what you're seeing in that scope capture is certainly not the "real" output voltage of the supply. Even if unstable, there's no way for the PSU to actually produce such high slew rates on its output voltage.What you're seeing is certainly the result of a large common mode transient (due to you closing the switch) and your scope/probes limited common mode rejection ratio (basically what temperance said above).