Sagging voltage under load PSU - Page 1
Forum section: electronics / repair

author: megaChungus
author_rank: Contributor
timestamp: June 03, 2021, 09:15:18 pm
content: Hi:I'm 14 and I got into the hobby a few months ago. I had bought a cheapo china psu for 60-70 bucks (model Eventek kps3010d). The voltage reading had started to sag when loaded. For example, if I loaded it at 12V 2A it would go down to 6V and if I set it to 10A and short it the voltage goes down to 0.02V (this is a reading on the psu not my multymeter). For my 12V example, setting the voltage higher only brings it up to 7 or so volts under load.I believe it might have been caused by me shorting the supply out to much (cutting through aluminium foil). I didn't know that the supply would get damaged.I've taken it apart and diode checked both mosfets (k3569) for shorts.Any help would be appreciated.

author: bob91343
author_rank: Super Contributor
timestamp: June 04, 2021, 03:09:51 am
content: Before you can troubleshoot this thing you need to have a diagram of it. If the voltage sags, it may be due to overload or perhaps it's poorly regulated.(If you short the output, how can there be any output voltage?)

author: MarkF
author_rank: Super Contributor
timestamp: June 04, 2021, 03:42:09 am
content: This might be a dumb question.But, might the supply be going into over-current mode?If so, that would be the expected behavior.Try adjusting the max. current setting.

author: Unica_Research
author_rank: Contributor
timestamp: June 04, 2021, 03:44:49 am
content: you need set parameter of psu in normal.if some damages here, check current feedback loop / dual schottky diode,

author: bdunham7
author_rank: Super Contributor
timestamp: June 04, 2021, 03:48:49 am
content: https://www.eevblog.com/forum/blog/eevblog-1397-dc-voltage-current-source-theory/

author: Alex_Baker
author_rank: Frequent Contributor
timestamp: June 04, 2021, 02:52:07 pm
content: I have that exact PSU! I have never had any problems with it. It is a great beginner bench PSU but its build quality and regulation is a little bit lackluster.If there is an electrical problem with your unit I could take mine apart and help you out,

author: BrokenYugo
author_rank: Super Contributor
timestamp: June 04, 2021, 05:28:04 pm
content: That's sort of describing how a CV/CC supply is supposed to work. If the current drawn exceeds the set current it will drop the voltage below the set number until the set current is reached. Voltage should drop to near zero when output is shorted.

author: megaChungus
author_rank: Contributor
timestamp: June 04, 2021, 05:37:06 pm
content: Quote from: bob91343 on June 04, 2021, 03:09:51 amThanks for the suggestionsThe output voltage is being read from the psu screen.-On another note, is there a way to comment under a reply?

author: Alex_Baker
author_rank: Frequent Contributor
timestamp: June 04, 2021, 06:14:27 pm
content: I think you just post a simple reply, If you want to address someone specifically I would use the quote function like you did.Just to get it out of the way, when you put a load on your power supply does the red C.C LED light up?Me being a kid like you when I got my PSU (except now I am in college) I did plenty of stupid stuff with it, including shorting it out a lot and I haven't had issues. That being said, were you shorting it out for extended periods of time? given the build quality of my unit I would bet that it wouldn't last too long being shorted out continuously.One funny thing about my unit is that it uses rejected Nichicon caps for the input DC filtering, my particular ones were dented. So basically Eventek got ahold of some cheap Nicicons because they were factory rejects...

author: megaChungus
author_rank: Contributor
timestamp: June 04, 2021, 06:29:42 pm
content: Quote from: BrokenYugo on June 04, 2021, 05:28:04 pmhmmmmmmmmThis makes me very confused. My problem started when I was running a flyback that I had salvaged. To my understanding you have to connect the primary from positive to negative though a mosfet to switch it (i followed a schematic). I had set the current to 2A 12V and ran the circuit. it worked at first (badly-only about 2mm but I think that was just my bad primary coil not the PSU) but then I swapped out a mosfets and tried it again it would still draw 2A but the voltage would go down to 6V on the screen. I tried measuring it just now with my multimeter by placing the probes on the positive and negative busses but it still shows 6V when powered on.Is it supposed to do that?

author: Alex_Baker
author_rank: Frequent Contributor
timestamp: June 04, 2021, 06:31:28 pm
content: while you are doing that is the red C.C LED lit up next to the amp readout?

author: megaChungus
author_rank: Contributor
timestamp: June 04, 2021, 06:37:53 pm
content: Quote from: Alex_Baker on June 04, 2021, 06:14:27 pmWhen I connect it to a load such as an LED the C.C light does not come on and the led lights up like it should. But, for example if I try to run the primary of the flyback I just wrote about the red C.C light does turn on and the voltage drops.On the inside mine doesn't have Nichicon caps and there is no visual damage to the components.

author: Alex_Baker
author_rank: Frequent Contributor
timestamp: June 04, 2021, 06:43:22 pm
content: Quote from: BrokenYugo on June 04, 2021, 05:28:04 pmBrokenYugo described what is happening. The voltage is dropping because you limited the current to 2A, the power supply limits the current by dropping the voltage. Essentially this is ohms law: I=V/R (or V=IR). for a given load(ohms, R) the power supply has to adjust the voltage(V) to keep the current(I) at a constant set value (in this case 2A).

author: megaChungus
author_rank: Contributor
timestamp: June 04, 2021, 06:48:57 pm
content: So in other worlds what your saying is that my current demand (2A) is to high so it has to drop the voltage to compensate.But isn't the supply supposed to pump out a constant 30V at 10A without having to compensate??

author: bdunham7
author_rank: Super Contributor
timestamp: June 04, 2021, 07:04:23 pm
content: Quote from: megaChungus on June 04, 2021, 06:48:57 pmEither your PSU is set for 2A of current output or it is broken. Is the upper left knob (coarse AMPS) turned all the way up?

author: megaChungus
author_rank: Contributor
timestamp: June 04, 2021, 07:14:03 pm
content: Quote from: bdunham7 on June 04, 2021, 07:04:23 pmThis might have been a dumb mistake but the PSU is set to 2A. But why is this bad? If I turn the current knob all the way up wont the primary draw 10 AMPS because it basically a short when the mosfet turns on. Right?My set up flyback driver can't handle 10 amps, also I read that it was best to keep a flyback between 2 and 3A.How would you do it?

author: bdunham7
author_rank: Super Contributor
timestamp: June 04, 2021, 07:53:25 pm
content: Take the time to watch the video that I linked. What is happening is simply a necessary result of circuit loading. You can't control both the voltage and current, you can control one and the circuit (load) decides the other for you.

author: BrokenYugo
author_rank: Super Contributor
timestamp: June 04, 2021, 09:07:07 pm
content: Quote from: megaChungus on June 04, 2021, 06:29:42 pmYes, normal operation. Your circuit was trying to pull more than 2 amps, so the supply did what it's supposed to and drop the voltage down until it did draw 2A.As others have mentioned, there should be a light that comes on when the supply is operating in this constant current mode, typically marked "CC".

author: megaChungus
author_rank: Contributor
timestamp: June 04, 2021, 09:19:38 pm
content: Yup, you are completely right bdunham7. I watched the video of that crazy aussie bloke and it just seems that my PSU is just to weak to do 12V 2A at the same time (although I still don't get how that can be since it's advertized as a 10A 30V unit).I still gotta figure out a different way to power my flyback but it's good to know it's not brocken it's just weak.Thanks a lot ot everyone that helped me out.

author: bdunham7
author_rank: Super Contributor
timestamp: June 04, 2021, 09:25:14 pm
content: Better watch it again....If you set the current to 2A, the load will determine how high the voltage can be. Both your voltage and current settings arelimits, not conditions the PSU can force if the load isn't right for it. What happens if you turn the current up? Some circuits will draw a lot of current on startup, then draw less once they are operating. You may just need some more current to get over that hump. Or it may burst into flames.