Computing stuff tied to the physical world

Inside the USB supply

In Hardware on May 6, 2011 at 00:01

The USB power adapter is a nice little unit pumping out 5V:

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It’s no doubt intended mostly as charger for USB devices, but with a 1A output current, it can actually power lots of things, including JeeLinks and JeeNodes.

I was curious as to what’s inside this CE-approved device, so I took one apart:

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A chip, a bridge rectifier, lots of electrolytic capacitors (with limited lifetimes if things get hot), an isolation transformer of some kind, some inductors, and a USB jack:

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The 400V rating on the 4.7 µF caps sounds a bit far-fetched, though.

The bottom side shows an interesting mix of SMD components and solder joints:

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I haven’t looked into the circuit itself yet. I can’t help but wonder whether a 50mA supply couldn’t be done with fewer components.

Oh well. This unit works, is certified, comes from an official local wholesale supplier, and nothing I can come up with could be made for less.

  1. Quite a complex little device inside. I would be interesting to know the temperature rating of the caps. Are they the more expensive 105 degree rated ones, or the cheaper 85?

    I suspect the 400v rating is due to the peak voltage of mains reaching over 310v (220v is the RMS).

    With the way if connects onto the mains pins, I can’t help but wonder if they do the same unit with a “real” UK plug on the front… Pretty please! Then again, if it comes apart this easily, I can just make one :-)

    Last, but by no means least, what size T-shirt would you like?

  2. As TankSlappa noted, you’ll find 400V caps in any device filtering 220VAC mains, so not farfetched at all.

    It looks like a pretty standard flyback converter design. 220VAC -> Bridge rectifier -> filtering caps -> switch -> transformer -> diode and output filtering caps. There’s also a specialized IC to control the switch and an optocoupler to get feedback from the output and keep the low voltage part isolated.

    You can find much cheaper options, like this and numerous other items on DX:

    Unfortunately you have no chance of beating the price of the Chinese manufacturing machine, unless you make millions (and you make then in china :) ). From the above link, if you buy more than 10 the price is < 2EUR each! For this, especially in EU, you can’t even buy the plastic enclosure.

    • The only problem with things from China is they often have CE and other such markings on them, which are nothing more than the sticker. They haven’t been tested, and you’re lucky if they even perform to spec, let alone perform safely.

      I recently purchased a Chinese solar panel with built in battery. The panel was supposed to be 0.7w and the battery 2600mAh. I took it all with a pinch of salt, and then took it apart. The battery was labeled 2000mAh, I never saw the solar panel produce more than 0.4w and when you actually tried to use it to charge a device it could barely put 15% extra charge into a 1400mAh equipped mobile before the battery (even when charged from a USB supply for 10 hours) would clap out. Then there was the circuit design, low light (artificial interior lighting) would actually cause it to discharge the battery! The solar panel would turn on a transistor which would light an LED. Unfortunately the LED was powered by the battery, which wasn’t receiving any charge from the solar panel at the time because the panel wasn’t producing a high enough voltage!

      Plus of course there is the lawyer angle. If JC goes and buys a cheap Chinese “approved” adapter, sells it, and it catches fire and burns a house down, then he could be in trouble. If he had the bad fortune of having sold it to a customer in the USA he would be lucky to escape without having to surrender several limbs and his first born child!

      The little adapter he is supplying here looks well made and I would expect it to have a long and reliable life.

  3. Sometimes electricity corporations make mistakes and wire up 270 or even 380 Volt lines up to household lines. See: this for example They can usually not wire up higher voltages by accident (unless the mechanic is really not looking at what he’s doing).

    So the 400V is just something to make sure the device wont explode when it breaks due to something like this.

  4. Well it looks like a standaard SMPS to me. And the 400V caps are actually on the save side of the operation voltage. They are operated at the rectified 230V which gives 324,3V. But need to handle some extra for grid issues.

    The caps are rubycon YXA and they are standard 105 degree types.

  5. What intrigues me is that these 4.7 µF 400V caps are so small, when a 400V X2 ceramic cap of say 0.47 µF is so much larger.

  6. jcw: These are electrolytic caps so I guess you should expect them to be much smaller then ceramics or film capacitors. These are also not X2 (X2 rating adds considerably to the size).

    Btw. Are you sure the X2s are ceramic? I always though X2 are only made as (polyester or polypropylene) film capacitors.

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