Newbee working on a 24vdc power supply for a Lite Panel - Page 1
Forum section: electronics / beginners

author: gica69
author_rank: Contributor
timestamp: June 13, 2025, 06:29:35 am
content: The power supply specs are 24vdc, 5A, 120W. It stopped showing any output power. I started reading testing and found R1 and R2 resistors to read .3 even though they are 12ohms and 16 ohms. I changed them and I show output voltage, but it is oscillating at23.2 to 24. something volts. The panel of course goes in and out. So I took out one of the output capacitors and it output stops oscillating at 24.01VDC. Taking all 3 caps and it stays at 24.01. I don't know if the caps are the cause of the oscillation. Am slowly learning how to test power supplies have watched everything on YouTube and have all the testing means one could get without going broke hahaha. I am really looking for help so please go easy on me.Including pictures of the board. Thanks in advance to all.

author: tunk
author_rank: Super Contributor
timestamp: June 13, 2025, 09:52:11 am
content: No expert, but I think without those output capacitorsyou will have a (too?) high ripple. I think you shouldreplace them with low ESR type capacitors with thesame voltage and capacity. If it doesn't work, you mostlikely have another problem.

author: fourtytwo42
author_rank: Super Contributor
timestamp: June 13, 2025, 09:57:39 am
content: When you say it is oscillating are yo using an oscilloscope to measure that, can you post the waveform ?It is not a good idea to remove the output capacitors, you will likely blow something!

author: D2236
author_rank: Regular Contributor
timestamp: June 13, 2025, 02:46:49 pm
content: If you remove all output capacitors the control loop gain will shoot up high, and mess with stability. This is most probably why you see oscillations.

author: MariuszD
author_rank: Regular Contributor
timestamp: June 13, 2025, 03:01:23 pm
content: Output capacitors are essential for the system to be stable. Since their removal helped, it means that another component in feedback loop is damaged. Without capacitors, voltage surges will occur that could damage transistor or something else.R1/R2 look like film resistor. It is almost impossible for such damage to occur that the resistance of a film resistor decreases. Did you measure it after desoldering?Is the marking of resistors unambiguous or are there any doubts? Do you have old ones? Don't throw it away.Check if the resistors you changed are working in the current sense circuit on the primary side. Wrong value here can lead to instability.I would check the optocoupler and capacitors close to TL431 (or other IC responsible for voltage regulation).Pic1.jpg is blurry. Make better one.

author: ahsrabrifat
author_rank: Regular Contributor
timestamp: June 13, 2025, 04:34:34 pm
content: Quote from: gica69 on June 13, 2025, 06:29:35 amThe caps are highly suspect here. Even if they look okay, they could be Leaky (internal ESR too high), shorted (pulling down output or causing oscillations), or just out of spec due to age or heat.

author: Buriedcode
author_rank: Super Contributor
timestamp: June 13, 2025, 10:15:31 pm
content: Since this is beginners section, and you admit you're a newbie, I just wanted to post a word of caution. I know people don't want to clog up a thread with safety concerns, but it really is very easy to get a wack/burn from the charge on the input cap, and it has to be stated explicitly -do not probe anything on the mains side whilst active.With supplies like these, despite the complexity, there are only so many failure modes, so I have found there is often no need to scope the gate/drain waveforms - but randomly replacing parts is tedious and unless you're lucky, probably won't solve the problem quickly. Also it doesn't really tell you whats going on.If it outputs a voltage - whilst attached to a moderate load (say 1A?) - thats roughly what its rated for then its clearly oscillating, so most likely everthing on the primary side is ok. The secondary side is generally quite simple - diodes, output caps, the opto feedback network and perhaps an aux supply from a linear regulator. Diodes generally fail short, caps can go either way and aren't always a yes/no failure.

author: gica69
author_rank: Contributor
timestamp: June 15, 2025, 06:22:29 am
content: Sorry for the late reply so it does oscillate between 23.7V to 24.02 V. I did change caps already and same thing. As soon as I change them it stays at 24.02 the very first time I turn it on the second time it goes into the oscillating mode automatically.

author: gica69
author_rank: Contributor
timestamp: June 15, 2025, 06:23:45 am
content: I have not used the oscilloscope yet even though I have 2 of them. I will have to set it up and do it.

author: gica69
author_rank: Contributor
timestamp: June 15, 2025, 06:25:33 am
content: Ok the removal of the capacitor(1) did make the output stay on 14.02V but only for 2 minutes after which it reverted to oscillating. I did not put it under any load and only did it for 2 seconds to measure the output.

author: gica69
author_rank: Contributor
timestamp: June 15, 2025, 06:26:55 am
content: I ordered some good caps from Digi Key and also bought some temp caps on Amazon replaced them and it did not make a difference.

author: gica69
author_rank: Contributor
timestamp: June 15, 2025, 06:44:06 am
content: Thanks for the reply and the concerns heads up, when I said I am a newbie I meant as far as vast testing experience. I am familiar with all the precautions in testing on live environments. I have developed a hobby for this type of stuff and now I am trying to figure out this particular power supply for a friend, I do know that changing parts without finding the faults and knowing the circuit is not the way to go. I have not been able to get the power supply diagram and probably won't either. It is supplied with the Led system, and they won't share it. I do have every possible equipment for testing and soldering. I just need guidance in testing for this particular issue. Again, this was completely dead on the output. Main cap was being charged fine, but the transformer was not switching. The 2 resistor I changed turned-on the switching, but it is not stable as it oscillates constantly, and it causes the load to start and stop constantly. I can test most of the components except for the control chips. Don't give up on me guys I really want to do this.You're saying the primary should be fine if it is supplying voltage. Everything on the primary side is oscillating as well, from the rectifier to most all the components. I will recheck all the secondary system components. As I understand it this 2ndary part has a floating ground right?

author: gica69
author_rank: Contributor
timestamp: June 15, 2025, 06:49:43 am
content: Here are some better pictures

author: Sorama
author_rank: Frequent Contributor
timestamp: June 15, 2025, 06:56:31 am
content: The story -at least for me- is still not clear.What exactly do you mean by oscillating?Amplitude variation? Between what voltages?An adequate load is probably necessary while measuring/testing.

author: gica69
author_rank: Contributor
timestamp: June 15, 2025, 07:06:03 am
content: Here is a picture of the old resistors.One is brown green gray gold green so 15.8 ohms replaced with a 16ohmsThe other is brown brown grey gold green 11.8 ohms replaced with a 12ohmsI am including a picture of the new ones as well.Both tested 0.3 on and off the board. New ones measure 16.3 and 12.2 respectively.

author: gica69
author_rank: Contributor
timestamp: June 15, 2025, 07:36:29 am
content: Here is a link to a video of the modulation of output voltage. Is not oscillation? My badhttps://www.dropbox.com/scl/fi/b989wlmuogwe3of0o6v3e/IMG_2629.MOV?rlkey=2aytbd7hye3zbs63xpmqgl9nv&st=jaa90df8&dl=0

author: Sorama
author_rank: Frequent Contributor
timestamp: June 15, 2025, 07:40:44 am
content: Quote from: gica69 on June 15, 2025, 07:36:29 amNot an issue for me, given no load.What happens to that voltage when loaded?

author: gica69
author_rank: Contributor
timestamp: June 15, 2025, 07:43:12 am
content: I took it to where the Lite Panel is don't have it with me, and upon plugin it in it turns on and then off then on then off. It turns on at 24v and off below that. Tehe panel has a max power draw of 105W. The power supply 120W 5A.

author: Sorama
author_rank: Frequent Contributor
timestamp: June 15, 2025, 07:47:37 am
content: How low does the voltage drops?Probably your power supply is not able to deliver power (current) or goes into protection.Try with some other appropriate load.Your display may have a problem.

author: gica69
author_rank: Contributor
timestamp: June 15, 2025, 07:49:24 am
content: No, the display is fine I have a new power supply, and it stays stable at 24. something lights up the panel no problem. Something is causing the variation in output voltage.

author: EggertEnjoyer123
author_rank: Frequent Contributor
timestamp: June 15, 2025, 07:59:55 am
content: Quote from: gica69 on June 15, 2025, 07:06:03 amAre you sure these aren't inductors?It does look like they have wire wound around a core. You should probably put them back.The first thing to check on power supplies which haven't failed catastrophically is electrolytic capacitors. Generally a few of them going high ESR is enough to cause it to shut off randomly.

author: gica69
author_rank: Contributor
timestamp: June 15, 2025, 08:06:29 am
content: Wait what has wires around the core!! The old looking resistors? Now you got me thinking, they say inductors are magnetic but so are resistors right? And it does say R1 and R2 right next to them. An inductor would be under L1 right?

author: Sorama
author_rank: Frequent Contributor
timestamp: June 15, 2025, 09:14:18 am
content: I would expect resistors going short to be visually damaged, which they are not.Can you measure inductance ?

author: gica69
author_rank: Contributor
timestamp: June 15, 2025, 09:28:05 am
content: I do have 2 LCR meters a DE5000 and a BK 879B. I think I can use these for it in LCR L mode. One only has 10KHz but the 5000 has 100KHz. Which should I choose and what do I measure?And where are you from is that a Romanian flag?

author: Sorama
author_rank: Frequent Contributor
timestamp: June 15, 2025, 09:39:37 am
content: Quote from: gica69 on June 15, 2025, 09:28:05 amI don’t know if you asking me that flag question, but I am from Belgium.You can see that when you hover over the flag symbol.Any L measurement is fine, no matter frequency in this case.

author: gica69
author_rank: Contributor
timestamp: June 15, 2025, 09:57:27 am
content: Sorry, for a second I thought it was a different flag, it's 3AM here and my eyes are deceiving me, was just curious because I am Romanian but live in the States. Belgium is great especially soccer I have some friends there hahaha. Back to our program.So you want me to measure the old resistors in induction mode?

author: Sorama
author_rank: Frequent Contributor
timestamp: June 15, 2025, 10:02:34 am
content: Quote from: gica69 on June 15, 2025, 09:57:27 amYes, just to be sure they are resistors …or not.

author: gica69
author_rank: Contributor
timestamp: June 15, 2025, 10:19:27 am
content: I used the DE-5000 at different frequencies. For some reason the BK at 10KHz gives a different number. Don't know which is good. The readings are pretty low in uH for a inductor right?Was watching this video and a similar issue with fluctuation he looked at the PFC controller circuit was causing fluctuations in the primary also. Mine fluctuates everywhere primary and secondary. He changed a capacitor and a transistor.https://youtu.be/h5oTSEvIGcQ?si=Ad373hIcxiLEqR7iAnyways really appreciate you guys for the help I am going to sleep I am beat. It's 3:30AM here. Let me know if you think of anything. Tomorrow is Father's Day here so happy father's day to all.

author: gica69
author_rank: Contributor
timestamp: June 15, 2025, 08:49:39 pm
content: Guys thanks for all the pointers I decided to put this aside for now so thanks againf.