I've recently bought 5V/1A USB power supplies, and since the packaging was slightly damaged (specified in the auction I bought it from), I thought about testing it before connecting to anything sensitive.

Since it's a USB-A receptacle and I have the corresponding socket in my parts supply, I was thinking simply about soldering leads to Pin 1 and 4, connecting those through a resistor, and the connecting a multimeter in series or in parallel to the resistor for measuring current and voltage output. I'll be using 10W resistors to stay on the safe side (with the lowest resistance of 5 Ohms, of course).

My questions is: is there something inherent in the USB standard that will cause problems, or worse, hazards, with this test setup?

Also, a side-question: is it safe to base this type of setup on a typical breadboard and 22 AWG cables?


2 Answers 2


I don't believe USB power supplies require any negotiation to ask for current. When working with a PC, devices are supposed to draw no more than 100 mA and ask permission to draw more (and in practice they usually just take what they need up to 500 mA). I only mention that because you can't expect this same test to work with your computer.

The test you're considering should not damage the power supply. On a cautionary note, the 5W in a 10W-rated resistor will generate plenty of heat over a small area, more than enough to cause skin burns and start fires. Left alone for several minutes, even in free air, it will easily exceed 100ºC. There's a reason power resistors are usually made of ceramic materials. Take some precautions. Work on a metal surface, wear gloves, use a small fan or attach a heat sink, and you should be fine.

22 AWG can tolerate 8-13A depending on the insulation. Another StackExchange question indicates that breadboards have about a 1A current limit, so I refer you to that question for alternatives. (The voltage in breadboard does not matter as long as it's low.)

  • \$\begingroup\$ Thanks for the links, especially to the breadboard question that I somehow missed. \$\endgroup\$
    – mikołak
    Commented Dec 16, 2012 at 22:27

One thing to be aware of (and watch out for) is that many of the Chinese e-bay USB power-supplies tend to outright lie about their specifications.
Some are so poorly designed and constructed that they're not even really safe to plug in, let alone trust to charge a device unattended.

There are a number of different articles around the web that have tests and teardowns of many various USB chargers: (I've added some excerpts for longevity)

  • http://www.lygte-info.dk/info/usbPowerSupplyTest%20UK.html

    That load test shows a very interesting looking curve, the ps start at 6 volt, drops to 4.5 volt, goes up to 5.5 volt and drops again. This is not good!
    [... For a different adapter ...]
    I have marked the mains connected parts with red and the low volt parts with blue. There is less than 0.5 mm between them. This is extremely unsafe.
    [... Another adapter ...]
    Notice the twisting PCB trace at the red wire, it is probably supposed to be a fuse.

  • http://www.righto.com/2012/10/a-dozen-usb-chargers-in-lab-apple-is.html

    Counterfeit chargers pose a safety hazard as well as a hazard to your phone. You can buy a charger that looks just like an Apple charger for about $2, but the charger is nothing like an Apple charger internally. The power is extremely bad quality (as I will show below). But more importantly, these chargers ignore safety standards. Since chargers have hundreds of volts internally, there's a big risk if a charger doesn't have proper insulation. You're putting your phone, and more importantly yourself, at risk if you use one of these chargers.

  • http://www.righto.com/2012/03/inside-cheap-phone-charger-and-why-you.html

    A 75 cent controller IC[3] would be a huge expense for a $2.79 power supply, so they used a minimal circuit instead.
    Stay away from super-cheap AC adapters built by mystery manufacturers. Spend the extra few dollars to get a brand-name AC adapter. It will be safer, produce less interference, and your device's touchscreen will perform better.

The "quality" (or lack thereof) in many of these cheap USB power adapters is somewhere between hilarious and terrifying.

Mike from http://www.Mikeselectricstuff.co.uk did a video teardown and analysis of a USB power supply included with a USB hub here.

Frankly, if the USB power supply you have came out of china, I wouldn't be willing to plug it in without taking a look at the internals first.

  • \$\begingroup\$ "The resistor with brown-black-gold might be working as a fuse." is also nice. Thanks for your input, those are certainly sobering pieces of information! \$\endgroup\$
    – mikołak
    Commented Apr 19, 2013 at 19:32

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