user31361: I want to use a mini usb jack to power my device off board, but I want to make sure that the usb power coming in is not any higher than the 5V. I've read a bit about polyfuses but I am unsure if they would be useful for this scenario. Thanks!

user3879: A polyfuse will limit current not voltage.
There are transient protection diodes which switch on if voltage exceeds a given level, which are often used in conjunction with a polyfuse. The diodes cause excessive current to flow, effectively turning on the polyfuse for extended overload, or just absorbing shorter transients. The Raspberry Pi uses a SMBJ5.0A device to suppress transients.
You should NOT need any of this if you use a reasonable power supply.

31361: <p>I want to use a mini usb jack to power my device off board, but I want to make sure that the usb power coming in is not any higher than the 5V. I've read a bit about polyfuses but I am unsure if they would be useful for this scenario. Thanks!</p>
 3879: <p>A polyfuse will limit <strong>current</strong> not voltage.</p>

<p>There are transient protection diodes which switch on if voltage exceeds a given level, which are often used in conjunction with a polyfuse. The diodes cause excessive current to flow, effectively turning on the polyfuse for extended overload, or just absorbing shorter transients. The Raspberry Pi uses a SMBJ5.0A device to suppress transients.</p>

<p>You should NOT need any of this if you use a reasonable power supply.</p>
