Possible reasons for 2 blown SMPS? - Page 1
Forum section: electronics / repair

author: Kilrah
author_rank: Unknown
timestamp: July 10, 2016, 06:39:46 pm
content: A friend of mine has 2 R/C AC/DC battery chargers (i.e can be supplied by 12V, but also have a built-in main-powered 12V PSU for ease of use) on which said PSUs failed at the same time. I believe they were both connected to mains on the same multi-socket bar and working at the time. 2 completely different brand and model, one is a single 50W model while the other is a quad 100W unit.I've got the 4x100W unit here for troubleshooting, first observation was a blown fuse, nothing else visible and the PSU was a pain to take out of the case so as a first step I just tried replacing the fuse, but the new one of course blew as well. 5324 screws later + 5min of the first troubleshooting I got to do with my new BM235 pointed to one of the 2 soft-start MOSFETs (located between rectifier and main DC cap) being blown (all 3 pins shorted).Pretty confident the PSU will be fine once that's replaced (will change his friend along, especially since I can't easily find the exact same part and will have to get a "close enough" replacement) and the blown fuses were just due to the inrush current largely exceeding the 1.6A fuse rating, but still wondering what could have gotten 2 separate devices (but nothing else in the house) to fail at the same time. I'll grab the 2nd charger next time I see my friend and examine that too, but all I could think of is that since they were connected to mains with a short path one of them (I'd guess the other one) decided to fail out of the blue and caused a spike that took out the other, but anything further away wasn't affected due to dampening from the inductance of the longer length of mains wiring?

author: uncle_bob
author_rank: Unknown
timestamp: July 10, 2016, 07:45:02 pm
content: HiWhat else was plugged into the strip?What else was on the branch circuit that the strip was on?Where did all the grounds run to?Lots of variables.Bob

author: Kilrah
author_rank: Unknown
timestamp: July 10, 2016, 07:50:18 pm
content: I unfortunately don't know, and my friend probably won't either since it happened several weeks ago while I was away travelling. Typical 3-phase European house install though, house is actually brand new. Both devices and the strip/circuit are earthed.

author: uncle_bob
author_rank: Unknown
timestamp: July 10, 2016, 07:53:01 pm
content: Quote from: Kilrah on July 10, 2016, 07:50:18 pmHiYou probably could add to the list:What else was powered on in the house and how sure are you it survived?Best guess, the power supplies were not very well protected and they failed at a lower voltage than your typical consumer electronics gear.Bob

author: Curtis
author_rank: Contributor
timestamp: July 10, 2016, 08:12:54 pm
content: I purchased a new"Astron"35 amp linear power supply a few years back, and every time I turned it on it made a rather loud"thump"sound, which was the current in rush to the transformer. It bothered me, so i installed a couple"MOV"from the line side to the ground, and from the neutral side to the ground. Now when I turn it on it makes no sound at all!!In America according to the"NEC"(National electrical code) we are not allowed to have more than "3%" voltage drop to any load from the main panel board.Resisitive loads simply use less power if fed with less voltage, but"inductive"loads will draw more power if fed with less voltage, which could and often will burn up the device.

author: uncle_bob
author_rank: Unknown
timestamp: July 10, 2016, 08:20:00 pm
content: Quote from: Curtis on July 10, 2016, 08:12:54 pmHiSo if I was to supply an inductive load with zero volts AC(ok, 0.01V), it would draw infinite power?If I supply an inductive load with a 10KV 60Hz signal, I don't have to worry because it will draw no power at all?Bob

author: Kilrah
author_rank: Unknown
timestamp: July 10, 2016, 08:21:21 pm
content: Quote from: uncle_bob on July 10, 2016, 07:53:01 pmDozens of things (at least 4 computers, 2 TVs, 4 routers, network switches, consoles / TV boxes, recent (i.e. electronics-containing) household devices...) nothing of which failed, they're all being used daily. Roughly 1/4th in the same room, 3/4 on the same floor, the rest on other floors (might not be relevant, I haven't seen how the house is wired).Could be subpar designs, but being 2 completely different devices and the one I've taken apart looking much cleaner and sophisticated than I'd have expected (I had never seen an SMPS with such a soft-start circuit, all high voltage sections are well separated, have insulation slots where you'd want them, high-voltage carrying TO220s have their legs protected with heatshrink which I've never seen before either) it would be a little surprising that both failed randomly...Guess a lot more will be learnt when I can examine the other broken device.

author: uncle_bob
author_rank: Unknown
timestamp: July 10, 2016, 08:25:14 pm
content: Quote from: Kilrah on July 10, 2016, 08:21:21 pmHiMany switchers are designed quite well for their intended use:Buried inside a piece of gear *after* proper protection circuits.The assumption is that they are part of a system. That system has multiple supplies. There is a spike protection and filtering block in common to all of them. It's cheaper to do that way and makes a lot of sense.What happens is somebody grabs a supply board that was intended for system use and puts it in a box. They don't bother with the protection stuff. The result is a well designed supply that is not protected against spikes.====Here's a guess:One supply took out the other one.Bob

author: Curtis
author_rank: Contributor
timestamp: July 10, 2016, 08:36:26 pm
content: Quote from: uncle_bob on July 10, 2016, 08:20:00 pm

author: uncle_bob
author_rank: Unknown
timestamp: July 10, 2016, 08:42:20 pm
content: Quote from: Curtis on July 10, 2016, 08:36:26 pmHiNo, not quite. If you short the inductor it is quite happy. It's a constant current device in the sense that a capacitor is a constant voltage device. It does *not* violate any laws of physics doing this. In an AC power circuit, it will pull more energy at a higher voltage than at a lower voltage. Putting a MOV (metal oxide varistor = over voltage crowbar) across the primary does absolutely nothing at turn on, so your thump went away for some other reason.Bob