Help needed to fix a  S-40-12 Mean Well SMPS - Page 1
Forum section: electronics / repair

author: JacquesBBB
author_rank: Frequent Contributor
timestamp: January 21, 2018, 02:02:39 pm
content: Dear All,I tested a non functioning S-40-12 SMPS. Fortunately, with a 40w bulb in series.There was a strong BANG ! and the C9 electrolytic exploded.Can someone tell me its value ?ThanksEdit (21/1/2018): I have changed the original title of the thread

author: wraper
author_rank: Unknown
timestamp: January 21, 2018, 02:09:45 pm
content: 47uF or 100uF 35V+ is around what usually is used with UC3842 and clones. But I would expect that cap exploded because of some other fault.I would guess that SG3842 failed open on Vcc pin which caused uncontrolled voltage rise on the capacitor.

author: Bashstreet
author_rank: Frequent Contributor
timestamp: January 21, 2018, 05:59:51 pm
content: Quote from: wraper on January 21, 2018, 02:09:45 pmbeautiful Diagnostics and knowledge.

author: coromonadalix
author_rank: Super Contributor
timestamp: January 21, 2018, 10:19:23 pm
content: and your power mosfet on the photo is not properly monted on the heatsinkthe screw with metal bar ...

author: JacquesBBB
author_rank: Frequent Contributor
timestamp: January 22, 2018, 05:17:57 am
content: Quote from: coromonadalix on January 21, 2018, 10:19:23 pmThis is just for testing purpose. Will put it back when fiXed.

author: JacquesBBB
author_rank: Frequent Contributor
timestamp: January 22, 2018, 03:22:11 pm
content: Thanks for these comments.I have tried several things but without success so far. As wraper said, the C9 cap was not the source of the problem.Does anybody has a schematic of this SMPS or approaching ? I could not find something obvious on the web.

author: wraper
author_rank: Unknown
timestamp: January 22, 2018, 08:56:06 pm
content: Replace SG3842 (UC3842), IMO it has very slim chances to be working.

author: JacquesBBB
author_rank: Frequent Contributor
timestamp: January 22, 2018, 09:30:16 pm
content: Quote from: wraper on January 22, 2018, 08:56:06 pmThanks, I will do that. Will need to wait until week end, as I am out of my house during the week.Here are the additional observations I made.- The Ohm meter at the + and - of the bridge gives 28 ohm. The value looks very small,but I could not identify any dead component in the area.After removing a few components, I had an open circuit at +/-, but none of the component I removed appeared to be damaged.Is the main mosfet staying ON for some reason ?, even if the circuit is not powered ?- When plugged, with the bulb, the bulb stays on. Did not wait long.Are these compatible with your diagnostic ?

author: wraper
author_rank: Unknown
timestamp: January 23, 2018, 04:51:53 am
content: Quote from: JacquesBBB on January 22, 2018, 09:30:16 pmIf mosfet has low resistance, it should be dead. If it failed, PWM controller IC will be dead in vast majority cases as well. Now thinking about it, probably exploded cap was back powered through the gate of faulty mosfet and then through SG3842. In such case you also need to check any diodes, resistors, zeners in gate circuit as they might fail as well.

author: JacquesBBB
author_rank: Frequent Contributor
timestamp: January 23, 2018, 05:46:38 am
content: But when I test the mosfet alone with the ohm meter, it has not low resistance. It is open circuit.With the transistor tester, it looks ok, and recognised as a mosfet.

author: wraper
author_rank: Unknown
timestamp: January 23, 2018, 02:44:13 pm
content: did you check rectifier?

author: JacquesBBB
author_rank: Frequent Contributor
timestamp: January 23, 2018, 04:00:24 pm
content: Yes, I desoldered the rectifier, and it was all OK.

author: JacquesBBB
author_rank: Frequent Contributor
timestamp: January 23, 2018, 04:08:41 pm
content: The components I checked for possible short cut ( checked by desoldering them first)all in the right part of the SMPS board:Bridge retifier : BD1Big cap : C5blown cap : C9 ( replaced)power resistors R3 and the one nearbyThe blue cap near J2The green resistor R4 ( 1 ohm)power mosfet Q1The diode below the green resistor R4.The C2 and C3 grey caps.All these components were desoldered and OK.After I removed them, then the +/- at the bridge rectifier vias show : open circuitBefore : 28 ohmI have no memory of all the intermediary tests.Quote from: JacquesBBB on January 22, 2018, 09:30:16 pmCan the ohm meter power the mosfet to make it open ?

author: JacquesBBB
author_rank: Frequent Contributor
timestamp: January 31, 2018, 10:08:51 am
content: This past days, I had not much time.I changed the main mosfet although it did not look faulty, Changed the UC3842 (which was not good), and there is still problems.I desoldered the TL431, but had no time to check it.Next step will be to make a full schematics. Does anybody has already one that is close ?

author: Armadillo
author_rank: Super Contributor
timestamp: January 31, 2018, 10:45:27 am
content: I don't know if he will bestow with a response;Anyway, do share with the community once you are done with the full schematic, yeah!.