Thursday, February 13, 2014

Fixing a broken medium-current power brick

Some time ago, I bought a 7-port USB hub from Plugable, and it came with a pretty decent power supply. Most cheap hubs provide a correspondingly cheap wall-wart which will only give half an amp of current. But this one from Plugable was a proper beast that gave me 4 whole amps of current! Wow! Until it broke after 6 months of use.

Plugable was really on the ball, and immediately sent me a new power supply. So props to them. But I was curious about what went wrong in the faulty supply, so I popped it open. Here are the pics:

On the operating table.

Lightly tapping with a hammer and a
small flathead driver on the seam
broke open the case with minimal
I think that the seams were ultrasonically welded, because the halves were joined continuously, all around. I think it's really cool that we can seal plastic with sound; I wonder what frequency they use?
Checking the caps. Nice shielding!
I approve.
The first thing I checked after popping it open were the capacitors, which will leak if they are used at too high temperatures or at too high voltages. They all seemed fine, and none of them were swollen. No burn marks anywhere.
On the copper side.
The next suspect was a broken connection somewhere, probably due to thermal stress. (I was there when the power brick stopped working, and it was completely still, so there was no mechanical shock.) After a little while, I found this:
Can you find the lifted pad?
It looks like the thermal stress popped a pad off the board and broke its connection. Would reconnecting it fix the problem? I desoldered the lifted pad and cleaned the component lead with my solder sucker. Then I clinched the lead in the direction of the trace and soldered it to a nearby component which it used to be connected to:
The fix. Look at the top of R12, where a
solder bridge reconnects the component.
I buttoned up the enclosure without glue just in case it didn't work to test it. And it passed the smoke test! The hub was powered as if nothing had happened! SUCCESS!!

I then dabbed a bit with my hot glue gun to make a more permanent seal, and now I have a nice 5V@4A power brick!

Plugable's replacement power brick came soon after and I switched to that one. I sent my diagnosis and repair pics to them, and they said that they would forward it on to their engineering team. Great!

I think that this illustrates how important it is to seat components properly on the board, and avoid stressing them at all during the initial assembly process. Thermal changes during operation will only make the stress worse and weaken the connections. Also, if you can, try to use a through-plated double-sided circuit board, since the through-plating helps lock pads to the substrate and mechanically reinforces the traces. These tips can help you avoid a circuit failure like this power brick.

That's all till next time! And Plugable's service and gadgets are great - check them out!

EDIT (02/15/2014): Featured on Hackaday!


  1. Ultra sonic welds are at, well, ultrasonic frequencies: 44kHz and above, which is out of hear range for us humans anyway.

    good way of pinpointing a fault.

    1. Chris,

      yes technically 44kHz and above are ultrasonic, however the range begins closer to 22kHz which is at the top end of audible frequencies for humans. I'm only making an assumption here but i'm assuming that you may be confusing that with the sample rate at which we record audio to a digital device 44kHz being twice frequency (minimal sample rate required to reproduce the original signal, ie. the Nyquist Rate).