I keep my Kindle charger plugged into the wall all the time, and just plug the end of the cable into my Kindle as required. The rest of the time the cable just lies around behind the night stand.

So, imagine my surprise when I pulled it out, blew the dust bunnies off of it, and... voila!

I didn't think that a five-volt 0.85A output could make that much heat.

What do you suppose happened here?

tanstaafl.

Urmmmmmm you came very close to a house fire ???

Cheers

Cris

My guess is that the charger itself failed internally and generated much more heat than the wattage output specification.

Consider yourself fortunate that the damage was limited to the cable and the charger. I would not trust the charger ever again and would consign both cable and charger to the trash immediately.

Up to you whether you want to pursue a warranty claim with Amazon or search online for a failure trend among other Kindle owners.

Clarify: Which end of the cable is that? The end that plugs into the charger or the end that plugs into the kindle? You said that the cable lies around behind the night stand and that you discovered this when you pulled "it" out, but you didn't specify that "it" was. Was "it" the charger, the cable, the charger end of the cable, or the kindle end of the cable?

If it's the end that plugs into the charger, then yes, the charger failed, and you need to discard it and the cable. Perhaps water got on the charger, either from leakage/condensation in the house, or from a water spill on the nightstand?

If it's the end that plugs into the kindle, then it's possible that something shorted out the power pins on the kindle end of the cable while it was lying around behind the night stand. For instance a small piece of metal or other conductive material got into the end of the cable and laid across the power and ground pins.

It's also possible that the kindle end of the connector failed in such a way as to cause the pins to touch each other directly. For instance if the plastic in the center of the connector had cracked, split, or chipped, and that allowed the power pin to bend and touch the shroud or to touch another pin. Lying like that over time would slowly build up heat and cause the melting you observed.

No matter how it happened, though, you can no longer trust the charger, and obviously you need a new cable too.

That looks like the small/Kindle end of the cable. So most likely it somehow ended up in a small pool of water, or some really big wet bug tried to crawl into it.

Bingo! Last week I knocked a glass of cranberry juice over on my nightstand, some of it spilled down into the area between the bed and the nightstand where the Kindle-end of the cable was lying.

I see no evidence of damage with the charger, it charges the Kindle and shuts off when the Kindle is fully charged.

I had forgotten about the spill until you reminded me.

tanstaafl.

A decent charger should shut itself off into a dead short i.e. if pins where shorted well enough.

I suspect here the juice provided enough of a short to not cause the supply to shutdown but enough for a reasonable amount of current to flow through. Effectively a small heater.

5V/1A is only 5W so somewhat unlikely to cause any larger damage. 5W is definitely enough to cause something to melt.

It shouldn't have gone bang or anything though.

Not even 5 watts according to the .85 amp spec posted initially, barely over four watts.

Perfect storm for cranberry juice electrical resistance, enough heat to melt plastic but not quickly evaporate the liquid?

Do USB chargers crowbar themselves into a low current output mode or simply keep pushing the rated current into whatever low resistance the load offers?

"Did we get a new air freshener? Somehow I can smell fried cranberries..."

Peter

Wow. I've deployed a bunch of Leviton USB outlets (http://www.amazon.com/Leviton-T5632-W-Charger-Resistant-Receptacle/dp/B00J3PMU4C/) because i really dislike having these stupid wall warts everywhere. This sort of begs the question of whether the Leviton boxes any smarter about shorted connections. If nothing else, the outlet is properly grounded, and hopefully when they designed the USB side of the circuit they took advantage of having a proper ground wire nearby. They were at least smart enough to support 2A charging on both Apple and everything else.

(That said, there's no externally visible fuse. Hmm.)

Grounding is easily tested with a multimeter and an uninstalled outlet module.

Excess current draw response can be tested with a current meter and a variable test load. Maximum wattage is circa ten watts so a handful of resistors can be incrementally combined to monitor response.

My guess/expectation would be that it either crowbars down to a minimal current upon overload or it limits current at some max level and degrades voltage to maintain that max current.

i would not expect the USB output to blow an internal fuse, but that is certainly possible. This used to be a common problem on early generation computer main boards with 500mA USB ports. Once the fuse blew, the port never had power again until repaired or replaced.

I have a different brand combo USB with AC outlet and it simply limits at 1.1 Amps and 5 volts. Further load demand causes voltage decline while maintaining max current. I only did superficial testing in an attempt to allow 2 Amp charging of my iPad.

The USB host ports on equipment I've designed would in theory never do that, as there is USB overcurrent detection that would shut the port off completely when anything odd was detected and not turn it back on until reset. This is standard for computers, they normally all do much the same, as I've found out the hard way once or twice!

That said, I can easily see a cheap charger not having that amount of protection built in for cost reasons. Probably it only has a PTC resettable fuse at best, which is more or less fine most of the time. In a case such as this it looks like the goo that resulted from the spill was just conductive enough to allow significant current to flow without going over the limit required to kill the charger completely, at least for some considerable time. If it did have a resettable fuse, it probably just slowly heated up, went pop, cooled down, reset and turned the power back on, etc...

Or if they really cheaped out it's just a biggish low-ohm resistor.

The problem is that pretty much all main chargers nowadays are cheaply made to a minimum spec that's only just adequate, even from large companies. You can certainly get very good ones, but they're normally only aftermarket upgrades, not supplied with the equipment. I'm forever being given ones that have simply died with a request to fix them, on the basis that it "Cost ten quid, it must be repairable." People get annoyed when I tell them modern electronics, especially something you bought off ebay, are consumable parts with a generally short lifespan.

Although I am slightly surprised that the heat was enough to do that much damage from a liquid spill. Normally they electrolyse enough of the conductor away that the short clears itself quite quickly.

Possibly the problem is that it actually WAS a decent charger, and therefore could produce reasonable power for some time even into a cranberry-induced heating element! A cheaper one would simply have died quietly... Or simply exploded. Based on the ones I've seen supplied with raspberry pi kits, it probably would have done that even left completely alone

Note to self: Check conductivity of cranberry juice, especially when tacky

Nice to see you make an appearance again Patrick

Conductivity of cranberry juice:

www.eolss.net/sample-chapters/c10/E5-10-01-02.pdf

Really, wow, I guess all human knowledge is now on the internet. It is only a matter of time before it becomes self-aware.

Thanks

Haven't been here for a while, been busy.

Busy, busy, busy...

pca