EEVblog #1219 - Don't Trust Switches - Toy Repair

[EEVblog]

Sagan's toy remote control wouldn't work, and it lead to a most unexpected fault that released the magic smoke.

[Electro Detective]

No pocket multimeter to take out the guess work? 

Dud switch with dry contacts probably bent/shattered cheap metal bits inside shorted to buggery,
which turned the unit into a wannabee soldering station

Pry the switch apart and likely that's the deal

I'll bet it started to get stiff and or wonky beforehand

Just what you need, leaving kids to play with innocent looking and transparent plastic based toys
and copping a worst case scenario = something catches fire, like a bed...   

OneHungLow Inc. have outdone themselves on this one,
one wonders what .0000? percentage of 1 cent that switch cost to produce   

Wack in a new or quality salvaged switch, check that it's ok and lubed
and maybe an inline protective measure for 'next time' (not!)   


EDIT: the soldering process may have weakened the switch's cheap/flimsy internals, so a fail was bound to happen even with occasional use

[ergya]

Beginner player here. We are talking about 2 AAA batteries presumably. Can these things melt a relatively thick plastic? I mean that must be enough to light up a paper nearby and start a fire or something.

[SparkyFX]

Probably stating the obvious, but these sliding switches are not even closely specified to conduct high currents (let alone short circuit of two AA cells) and do fail by smearing the conductive dust from their contacts and any dirt across their terminals. High humidity might kill them too.

I got some of them malfunctioning here too in an alarm clock, but haven´t yet checked what they connect to exactly.
Guess the failure mode of them is to short.

[Gary350z]

What type of batteries were in the device?

To have enough current to melt the wire insulation, the batteries must have been NiMH, NiCad, or lithium primary. I don't think alkalines would have enough current to melt the wire insulation.

This is scary, it could light your house on fire. Think of all the low power devices in your house; remote controls, clocks, etc. Low power devices should have current limits designed in to them, especially since people can use NiMH, NiCad, or lithium primary batteries in these devices.

I have seen an unwound paper clip put across a NiCad D cell, and it glowed red hot.

As  said above:
"Just what you need, leaving kids to play with innocent looking and transparent plastic based toys
and copping a worst case scenario = something catches fire, like a bed... "

[Cnoob]

I'd like to see the car working going through the tube circuit.

[Wollvieh]

Obviously they use the same crap switch in an $80 toy as in the $1,99 one-hung-low copy of it. And as in billions of battery-operated devices. But normally the switch can only fail open oder closed when something gets stuck or broken. To produce a short cut it takes a broken design in advance.

I found 7 A as an example short cut current for an Alkaline AA.
So the failure could have ended worse:

https://youtu.be/7rdHKDMex90?t=288

Unfortunately no english subtitles. A german documentary about the operations of a fire brigade. In that episode: Evacuation of an old people's home due to heavy smoke from one resident's room. Cause: Shortcut in a TV remote control with 2 AA batteries. The batteries can be seen at 4:42. Luckily the remote control was placed on a quite flame-resistant table with tiles and not on a bed or sofa. One of the firemen reports in the aftermath that he once even experienced an exploding battery while replacing them in his own remote control.

Switches are somehow misunderestimated (place Texan-Hat-Smily here) sources of failure:

Just today I opened an older HP Procurve switch with PoE. One of the counter-sunk test/reset-buttons was missing (nothing behind the hole.) They use sub-micro-miniature right angle tactile switches with a hardly 1x2 mm button. As the top part of one of switches was missing I guess some strong guy just ripped off the whole cover and guts by hammering in a a big knitting needle or whatever. Unfortunately I found only a springy metal inlay laying loose under a PCB. The metal cover itself is still missing. 50:50 chance that it was removed unrecognized by me shaking the whole thing or it is still hanging around inside the switch. According to Murphy most likely close to the primary capacitor of the PoE switching supply.

I would conclude the switches that HP chose for their switches are way too tiny and mechanically weak for the size of the front panel hole and the tools one can insert. In consumer gear with counter-sunk reset-buttons i always found the large version of these switches with aprox. 5 mm round button and angled metal case.