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You thought those cheap no-name USB/Phone charger were safe?

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Stoic Joker:
I'd be inclined to think the second yellow wire came loose and shorted to something (first yellow?) cooking that corner of the board. There doesn't really appear to be any thing else there it could have hit.

The 4 diodes amount to a bridge rectifier, and give you a DC square wave, then the transformer drops the voltage. The rest is just for the green lite and power conditioning (to avoid the garbage in, garbage out scenario). Not a really complicated widget..

Just thinking out loud. :)

4wd:
 Looks like it got a little warm.   ;)-Tinman57 (November 15, 2012, 04:05 PM)
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What usually happens with these cheap things is they use cheap/marginal/under-rated electrolytic capacitors.  They dry out due to ripple in the waveform which generates heat, which causes them to go high-ESR, which generates more heat, they bulge and then go BANG!

Or they cause other things to go bang as the voltages, (either within the regulator or at output), are no longer within the design spec - this regulator, (came with a USB hub), caused a AU$130 DVB-T tuner to burn out, (not mine).

In the image above, you can see the capacitor on the right has just started to bulge, note the top of the can has started to split along the fracture lines compared to the larger one on the left.

I found out the secret to electronics a long time ago, it's "Blue Smoke" that makes it all work.  If there's a break and all the blue smoke escapes, the electronic item will cease to work....   :D
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We call it "Magic Smoke" :D

I'd be inclined to think the second yellow wire came loose and shorted to something (first yellow?) cooking that corner of the board. There doesn't really appear to be any thing else there it could have hit.-Stoic Joker (November 15, 2012, 05:31 PM)
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All that damage, (including the cutting of the yellow wire), was caused by a component that explosively destroyed itself.  The circular shimmer on the left of the photo, (above the remaining yellow wire), is the end view of what's left of it.

Here's a better view of the bulging capacitor, (now on the left), and the ex-component on the right.

You thought those cheap no-name USB/Phone charger were safe?

I might pull the electrolytics out later and test them with my ESR meter just to see what they read.

Results:
Small capacitor:
Ratings: 680uF 10V 105deg C
ESR:
Expected: ~0.2 ohms (worst case)
Reality:    47 ohms

Large capacitor:
Ratings: 10uF 400V 105deg C
ESR:
Expected: ~2.0 ohms (worst case)
Reality:    3.0 ohms

SeraphimLabs:
That burned stub looks like a coil or a resistor that burned out.

Most likely the transistor in the thing is cooked, and the resistor turned into a fuse as a result of the regulator transistor shorting out.

4wd:
That burned stub looks like a coil or a resistor that burned out.-SeraphimLabs (November 15, 2012, 09:37 PM)
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Resistor.  It blew hard enough to shatter the case and break the encapsulation of one of the TO-92 transistors.

Most likely the transistor in the thing is cooked, and the resistor turned into a fuse as a result of the regulator transistor shorting out.
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Yes, my idea also - the state of the output capacitor, (235 times it's supposed ESR), may have caused the feedback to be off possibly leading the transistor to over-regulate and fail.

I've got another cheap adapter here that's never been used - think I'll measure the capacitors and see what they're like beforehand.

Electronics is so much fun  ;D

superboyac:
I've found it a good rule of thumb to err on the side of caution and not be too frugal when it comes to AC current or anything that has enough electrical potential to kill or maim.

If it plugs into a wall, I generally stick with what the manufacturer recommends using. 8)
-40hz (November 01, 2012, 09:18 PM)
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Very wise.

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