Wire in switching transfomer - Page 1
Forum section: electronics / beginners

author: giosif
author_rank: Frequent Contributor
timestamp: July 04, 2025, 04:43:30 pm
content: Hi,I am trying to repair a Delta DPS-250AB-44D power supply which has suffered a catastrophic failure and I am attempting to determine if the main switching transformer is still good.What I find puzzling is that, on the primary side, the transformer has a pin with a wire connected to it, but the other end of the wire is not connected to any of the primary (or secondary) windings (please see attached photos).I also checked under the microscope and each of the other pins has the same number of wires connected to it as its matching pin (i.e. there is no pin which has the other end of the wire under question connected to it).At the same time, on the PCB, the pad corresponding to that pin is connected to the main positive rail (i.e. the one after the rectification and smoothing of mains), so I don't think my wire is connected to a shield layer inside the transformer.Anyone has any idea where this wire is supposed to go and what its function may be?

author: jonpaul
author_rank: Super Contributor
timestamp: July 04, 2025, 04:57:44 pm
content: shieldmost likely the transformer is fine.Blown up switch transistorbad lyticIf you aret experienced in troubleshooting off line SMPS, be careful.Easy to blow test gear or get a shock.j

author: RoGeorge
author_rank: Super Contributor
timestamp: July 04, 2025, 05:17:45 pm
content: That single ended pin is most probably a shield foil between primary and secondary.That transformer looks OK (no melt or heat discoloration/browning signs). Also, smell it. A burnt transformer strongly smells like smoke of fried wire/isolation/plastic.Usually the active components fail, or there are some dried out or even bulged/leaked electrolytic capacitor, or electrolytic caps that look OK but developed high ESR. SMPS transformers almost never fail.

author: giosif
author_rank: Frequent Contributor
timestamp: July 04, 2025, 05:27:05 pm
content: Thanks for the confirmation!That was the only explanation which I could think of as well, although the connection to the positive rail confused me.In terms of failed components, it's a laundry list- fuse (blown)- main bridge rectifier (shorted)- main smoothing electrolytic (very little capacity, high ESR - dried?)- switch transistors (one blown, one short circuit)- two diodes on primary side (shorted)- switching IC (short between some pins)- double diodes on the secondary side (kinda shorted)I am contemplating just declaring the power supply as BER but, on the other hand, I have this "itch" of trying to bring it back to life...I am aware of (most of) the dangers around PSUs and I am trying to be as careful as possible.

author: giosif
author_rank: Frequent Contributor
timestamp: July 04, 2025, 05:34:08 pm
content: Quote from: RoGeorge on July 04, 2025, 05:17:45 pmYeah... I too thought that, for a switching transformer, it looks too nice/clean to be blown.The smell test is a good idea - tried it now and it doesn't smell at all.As per my list above, multiple things failed; too many for me to figure out what caused it all.Also, the fact that even the double diodes on the secondary side have failed puzzles me.Maybe a lightning strike hitting the power lines near where this PSU was located (I bought it as non-working)?

author: jonpaul
author_rank: Super Contributor
timestamp: July 04, 2025, 05:50:37 pm
content: Bravo for the effort.Even after decades of experience, we retired EE and techs do not easily fix a dead SMPS, often needs special probes, isolation trsf, etc.Suggest to see some sites and forums specific to off line SMPS repair.Jon

author: Gyro
author_rank: Super Contributor
timestamp: July 04, 2025, 06:01:16 pm
content: Quote from: giosif on July 04, 2025, 05:27:05 pmJust for explanation, the screen is connected to the positive rail because, in that SMPS (and most others) it's the quietest in RF terms.

author: giosif
author_rank: Frequent Contributor
timestamp: July 04, 2025, 06:51:08 pm
content: Quote from: jonpaul on July 04, 2025, 05:50:37 pmI guess it depends on how simple or complicated the SMPS is, or how overly simplistic my view/understanding of the complexity of the SMPS is.I am still somewhat undecided whether to try to repair it and if it makes economical sense to do so.Quote from: Gyro on July 04, 2025, 06:01:16 pmAh, ok.That's good to know.Thanks for that!