Exploded plug transformer

[Tom41]

After losing the mains adapter to charge my Nintendo DS, I bought another, fairly cheap one off Amazon. It worked perfectly, charging my DS up about 4 times when the battery was low.

This evening, however, I was using the DS while the mains adapter was plugged in (I didn't want to run out of power in the middle of an online game). After saving the game, I went to pull the mains adapter out of the wall outlet...

There was an almighty BANG, and the circuit breaker for the whole house tripped. Closer inspection of the plug transformer revealed the two halves of the plastic had started to separate, there was a bit of soot on my finger, and a burnt plastic smell coming from it.

After removing the transformer and restoring power to the house (everything else survived), I opened up the transformer to have a look at it - photos are attached.

You can clearly see that one of the regulator ICs had the top blown off it, and the capacitors are either burnt or just covered in soot from the initial explosion. What I'd like to know is, what could possibly have caused the explosion in the first place? I'd assume some kind of short circuit, but I don't see anywhere that components could have touched...

I did notice that the flimsy board bent fairly easily. Is it possible that as I pulled the plug out of the socket, the wires caused the board to bend and two components to touch?

[Wizard]

Re: Exploded plug transformer

Is there a metal foil under the board on solder side? If not, cheap barrel connector is the blame. Should be buying geniune power brick for your machine. Only 20-30 dollars. Direct short is what you get on output side for the switcher to explode in violent manner.

Cheers, Wizard

[Tom41]

Re: Exploded plug transformer

Judging from where the board is charred, I'd say it was a short on the input side rather than the output. The two red wires on the left go straight to the mains Live and Neutral pins. The red and black wires on the right go to the DS charging plug.

Also, you can't see it on the photo, but the lead on the far end of the power resistor also looks very 'thin' as if it was about to blow like a fuse.

[japlytic]

Re: Exploded plug transformer

That power supply design is unsafe, because it does not have its own fuse.

[kc8adu]

Re: Exploded plug transformer

could the back of the plug blades have hit the foil side of the board when it fell apart?
and i agree these cheap aftermarket power supplies are trouble.
i see the cheap counterfiets all the time.some melt down after 1 hour.get an original.one customer who thought they were slick bought one off ebay for $20 delivered.i offered her a real unit for $25 that was used but good.that $5 savings was lost when the cheapie overshot and killed the mobo.rated at 19v.was delivering 32v.POOF!

[Krankshaft]

Re: Exploded plug transformer

That crappy PSU is actually better designed than most of the aftermarket switching warts I've seen.

Looks like the IC that blew was a PWM/FET combo you usually see discrete designs on these cheap supplies.

I agree with KC if the casing was coming apart I vote for the rear of the plug blades shorting out on the board. That is the most likely cause for the tripped breaker.

This would also be a sure fire way to quickly turn the IC into the pile of mush before you.

As for the power resistor I'm willing to bet it was part of the PWMs startup circuit and not a fusible resistor.

[c_hegge]

Re: Exploded plug transformer

That is nasty. Goes to show why some cheap generic power adapters are for temporary use only, while others are too bad to use at all.

[severach]

Re: Exploded plug transformer

Power supply aside you have a problem in your breaker box. Had you said 'I turned on my 8th air conditioner and the mains tripped' your sanity would be in question but not the breaker box. But you said 'I plugged in a transformer for a toy and the mains tripped' and I'm left wondering why didn't a smaller breaker trip.

If your breaker box says Zinsco, Federal Pacific, or Pushmatic then you need to read up on why they are fire hazards. I've seen the Pushmatic contacts welded together to where they can't trip and a fault melted through the metal to nearby breakers. Don't let your next purchase of an el-cheapo wall wart result in a visit from the fire department.

[kc8adu]

Re: Exploded plug transformer

on that side of the pond mains is the common name for ac power.
should have only tripped that circuit.
if the main breaker tripped instead of the one on that branch you would have an issue.
and i still see far too many fpe panels still in use and worse yet passed by housing inspectors at the time of sale.they are a replace on sight item.