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I'm trying to design a circuit from scratch with a 5V supply, USB connectivity and an AVR chip, for the sake of learning more about building power supplies, and implementing over-voltage protection on for example USB ports.

The last thing one would want to do is burn a USB port on quite an expensive MacBook Pro.

So where can I get a fairly decent explanation on how overvoltage protection works, and how I could design my circuit?

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  • \$\begingroup\$ Just a warning: I experimented with the USB port on an old IBM machine, and I burned the whole processor by connecting + and - per accident... so please be careful and try to get an old PC for testing! \$\endgroup\$
    – user17592
    Commented Jan 8, 2013 at 14:52
  • \$\begingroup\$ @CamilStaps You're saying that short-circuiting the USB caused the computer's processor to blow up? \$\endgroup\$
    – Anindo Ghosh
    Commented Jan 8, 2013 at 15:23
  • \$\begingroup\$ Yes. But I have to say this was an old machine so it might have changed in the meanwhile. Also, Josef is using an Apple machine, so it might be different. \$\endgroup\$
    – user17592
    Commented Jan 8, 2013 at 15:26
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    \$\begingroup\$ Use A not barebones cheap usb hub. Spend 15 dollars on a retail hub with full features, not a 2 dollar ebay one. Overcurrent protection is normally done on the host/supply side. \$\endgroup\$
    – Passerby
    Commented Jan 8, 2013 at 18:11

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Overvoltage isn't so much the issue as over-current. The host supplies the voltage and you can reasonably assume that it's not suddenly going to supply more than 5V.

The two most important failure modes to protect against are:

  • if your device is connected to anything else with a power supply of its own, that supply must not feed back into the USB port.

  • if there is a short circuit or overcurrent in the device, it must not draw more power than is reasonable for a USB device. (normally 500mA, although technically you have to negotiate the use of more than 100mA, and some standalone chargers may give you up to 2A)

A standard means of protecting against overcurrent damage is a polyfuse (resettable fuse). Current limiting in normal use can be done by one of many power controller ICs, which are also useful for supplying other voltages, charging batteries, identifying when a high-current charger is plugged in, etc.

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