Power Adapter Failure - Page 1
Forum section: electronics / general

author: wilheldp
author_rank: Regular Contributor
timestamp: December 13, 2013, 02:10:15 am
content: I was looking at an RC helicopter today with two exploded lithium polymer cells. It started a fire in somebody's house during the first charge of the battery pack out of the box. They included the power adapter that came with the helicopter. The battery pack was rated for 7.4 VDC at 1100 mAh. The power adapter was rated for 8.4 VDC at 850 mA. I couldn't tell if there was any charging protection circuitry on the battery pack due to the extent of the damage.The power adapter was undamaged, so I plugged it in and measured the output voltage. Open circuit voltage was 22.9x VDC. When I loaded it with a 6.7k resistor, the output only dropped to 22.8x VDC. I opened the adapter housing, and found some oxidation on the leads attached to an unidentifiable IC and a transistor...no heat damage or anything else amiss. What could cause a power adapter to put out that much more voltage than its rating? Did they just mis-rate a higher power adapter? It's a Chinese POS, so the latter is certainly a possibility.BTW, this is in America, so mains is 120 VAC, 60 Hz.

author: NiHaoMike
author_rank: Super Contributor
timestamp: December 13, 2013, 02:25:35 am
content: Sounds like it was mislabeled. As it is a safety related defect, report it to the CPSC.

author: wilheldp
author_rank: Regular Contributor
timestamp: December 13, 2013, 02:29:12 am
content: I've been the spark behind a few CPSC recalls, but I have to wait for this case to settle first.

author: Kohanbash
author_rank: Regular Contributor
timestamp: December 13, 2013, 02:30:50 am
content: Depends on the construction of the power supply.There could be something that is shorted (transistor, etc..) letting it bypass the regulator or an intermediate conversion stage. Have you checked if there is any AC voltage on the output?

author: wilheldp
author_rank: Regular Contributor
timestamp: December 13, 2013, 02:34:34 am
content: Quote from: sdk32285 on December 13, 2013, 02:30:50 amNo, I didn't think to check for AC voltage on the output. I'd love to take it apart completely and troubleshoot it, but I'm not allowed to.

author: wilheldp
author_rank: Regular Contributor
timestamp: December 13, 2013, 03:06:07 am
content: The power adapter is Jing Heng brand, model JH0840850, if anybody can find a schematic on it. I've come up empty in my searches, but there is an Australian version of it on eBay right now.

author: SeanB
author_rank: Super Contributor
timestamp: December 13, 2013, 06:18:25 pm
content: A SMPS will put out a very high voltage if the feedback is faulty and turns the driver on into it's maximum duty cycle. This is often caused by failing output capacitors ( or for a new one fakes or DOA capacitors) or by there being an open circuit feedback loop or a faulty optoisolator. I measured a 5V supply that was happy to deliver nearly 60v when open circuit, it needed new capacitors ( in put in some used reasonable ones to test it first at no cost) but the high voltage had destroyed the connected unit, so it was scrapped and replaced with a whole new kit, at a pretty steep cost.

author: amyk
author_rank: Super Contributor
timestamp: December 14, 2013, 11:47:27 am
content: Quote from: wilheldp on December 13, 2013, 02:10:15 amVery likely not - if it had a protection circuit it would not have overcharged and exploded. Most of them for 2s packs can stand up to 30V at the inputs.QuoteTry giving it a higher load than 3mA - if it drops significantly then it could be a stuck-open feedback loop, if it remains at ~23V then more likely that it's either been mislabeled or had the wrong components installed (need pictures/schematic to be sure - e.g. wrong divider resistor in a TL431 design could cause this)

author: wilheldp
author_rank: Regular Contributor
timestamp: December 14, 2013, 11:17:36 pm
content: I'm hoping the customer lets me do some additional testing on it, but I doubt they will because they can't sue the Chinese manufacturer anyway. I may get to play with it after the case is over though. I will post back with updates if I do get to do more testing.

author: Zero999
author_rank: Super Contributor
timestamp: December 15, 2013, 01:29:13 pm
content: Quote from: SeanB on December 13, 2013, 06:18:25 pmIn an application where fire, explosion and death could result from such a failure, there should be a protection circuit to stop that from happening, for example a simple crowbar circuit would stop that.

author: SeanB
author_rank: Super Contributor
timestamp: December 15, 2013, 01:59:42 pm
content: Bean counters will say the crowbar adds unneeded cost, and will happily leave it out as it will still fail after the initial warranty period ( or after they have been paid for the devices, whichever comes first) so causing them to sell a new one at a profit.

author: Unknown
author_rank: Unknown
timestamp: December 15, 2013, 02:07:37 pm
content: Until said bean counters figure out the costs of liability and bad publicity when their products damage their customers' property or injure/kill customers. Try selling Apple or Sony a charger $1 under the price of their other suppliers, with the notice that it will probably catch fire for 0.01% of the customers.

author: all_repair
author_rank: Frequent Contributor
timestamp: December 15, 2013, 04:02:38 pm
content: More reliable does not mean it is more safe. The fuse approach that feed straight into the capacitor is a less reliable design, IMHO, it is a safer design. Surge, adnormality, just blow the fuse. No need to add in so many parts which may make iy more reliable but less safe. The added parts also can malfunction, and the adnormalities can be out of calculation. Especially due yo mishandlings.

author: Zero999
author_rank: Super Contributor
timestamp: December 15, 2013, 06:16:38 pm
content: Quote from: alm on December 15, 2013, 02:07:37 pmIf there's adequate current limiting, a crowbar would only brick it if the voltage feedback loop is broken.