Overheating Coil and Regulator - Page 1
Forum section: electronics / repair

author: ajp8868
author_rank: Regular Contributor
timestamp: October 09, 2025, 04:34:39 pm
content: Hi All,I'm having an issue with a buck circuit on a moving head/DJ light board. It seems to be since I accidentally shorted 28v to the 5v rail on the 2576-5.0 regulator..I replaced this part and a couple of damaged logic ICs down the line and all started working again, resistance looks ok from gnd to 5v rail again.However, the regulator, multiple caps and the coil are all heating up way too much and the regulator shuts down due to overheating after about 2 mins.I've checked and reflowed the pad for the chip which is packaged as TO-263-5, so I don't think it's because of limited contact yo the board.I've tried using my scope to see what's going on but I'm really not sure, as there seems to be a 4v pk-pk range on the 5v rail (so maybe a lot of noise?)I've also tried powering the 5v rail from my bench supply (with the regulator removed) and this works fine and pulls around 0.31a, which the regulator should be fine with I believe.Any tips on what to try next?Picture attached of the board, please ignore the melted yellow cap, I've tested it out of circuit and it's still correct value.Thank you in advance

author: Uunoctium
author_rank: Frequent Contributor
timestamp: October 09, 2025, 05:21:42 pm
content: Especially while you mentioned the huge ripple on the 5V rail, i suggest to replace all electrolytic capacitors rated below 35V. (C11, C34!, C36)

author: inse
author_rank: Super Contributor
timestamp: October 09, 2025, 05:38:01 pm
content: Can you check the current consumption of the 5V rail (maybe by supplying it externally)?Maybe there is still some damaged parts that draw too much current

author: Uunoctium
author_rank: Frequent Contributor
timestamp: October 09, 2025, 06:34:20 pm
content: If I have understood correctly, he disconnected the the circuit from 5 rail and powered it from a lab supply. Draws approx 300mA, what is full in specs of the regulator (fine up to 1.5A).Also suspect is the switching diode (D7 ?). If it gots an reverse current of several Amps during its conducting phase. Maybe damaged

author: ajp8868
author_rank: Regular Contributor
timestamp: October 09, 2025, 06:44:44 pm
content: Exactly this, I've now replaced those few caps and that diode but still seeing the same result of those overheating.The casing on the coil is damaged (i.e the coil moves separately from its base) but it tests ok out of circuit, about 0.3-0.4ohm. Do you think this could this still be a suspect?I'm not completely certain that I'm using the oscilloscope correctly... Ground to the main input gnd and probing an output from the regulator. But this shows about 4v of pk-pk (calculated by the scope).Not new to repairing, but I've not had a scope for very long!

author: Uunoctium
author_rank: Frequent Contributor
timestamp: October 09, 2025, 07:19:15 pm
content: Can you determine, what the shining part exactly behind Pin 1 is? A Short? Solder? damaged trace?

author: inse
author_rank: Super Contributor
timestamp: October 10, 2025, 07:13:31 am
content: The regulator appears not to be a LM2576, is this the original part?

author: ajp8868
author_rank: Regular Contributor
timestamp: October 10, 2025, 07:50:48 am
content: Quote from: inse on October 10, 2025, 07:13:31 amI believe it is, it's just marked differently for some reason. This is a replacement with the same markings from a donor board. I can't find anything online for a DP2576-5.0, when you search that, only LM2576 datasheets are found.

author: ajp8868
author_rank: Regular Contributor
timestamp: October 10, 2025, 07:30:11 pm
content: Quote from: Uunoctium on October 09, 2025, 07:19:15 pmI've had a look and this is just the back of the pad, the angle makes it look raised but it's flat

author: ajp8868
author_rank: Regular Contributor
timestamp: October 10, 2025, 07:58:27 pm
content: So I've taken a couple readings from the 5v rail before and after the inductor and I'm seeing huge spikes of 15v on the output pin of the regulator and still about 1.3v after the regulator and capacitor.I'm guessing it really shouldn't be swinging that hard on the output side?I've replaced the main cap, the regulator, coil (albeit the casing is damaged), the diode d7 and the inline capacitor too. What else should I try?Pics attached of the readings.

author: ajp8868
author_rank: Regular Contributor
timestamp: October 11, 2025, 08:20:46 pm
content: Sorry if a noob question but do those above readings look like I'm using the scope correctly?Any thoughts on next steps? I'm at a loss

author: inse
author_rank: Super Contributor
timestamp: October 11, 2025, 08:43:41 pm
content: When probing on switchmode powersupplies, you should always take care of your ground connection.Placing the ground clip anywhere might give you false readings.For really reliable measurements skip the clip lead and use a ground spring or a silver wire wound around the probe shield.Then select the closest ground connection.If you want to probe more signals at once, it will get difficult.

author: ajp8868
author_rank: Regular Contributor
timestamp: October 11, 2025, 08:51:39 pm
content: Quote from: inse on October 11, 2025, 08:43:41 pmThank you for the tips! Though do my above readings shed any light on a faulty component, or is it still quite a broad range of things it could be?With this board, all the components are so close it's difficult to get anything closer ground wise without soldering in a connection wire, or accidentally shorting something (which is why I'm now in this situation lol)

author: nfb
author_rank: Contributor
timestamp: October 12, 2025, 02:42:16 pm
content: The yellow 100n capacitors are melted. I would check them.

author: Kean
author_rank: Unknown
timestamp: October 12, 2025, 03:17:22 pm
content: If the coil L5 is broken as you mention then it could be saturating.

author: tooki
author_rank: Super Contributor
timestamp: October 12, 2025, 08:48:43 pm
content: Quote from: nfb on October 12, 2025, 02:42:16 pmRight in the original post:Quote from: ajp8868 on October 09, 2025, 04:34:39 pm