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I am new to electronics so I hope this question is acceptable. I have been playing around with an Arduino. I have use the USB output from a computer as a power source. I also have a 9V battery. I am a bit concerned about creating a short somewhere and damaging the computer or the battery. I am guessing that this is what a fuse is for, is that right?

For small projects (not necessarily involving an Arduino) what type of fuses should be used? Is it, for example, possible to get a 1A 9V fuse? I have searched online and I only see things like 1A 250B fuses. Will these work well for small electronics projects?

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    \$\begingroup\$ possibly take a look into resettable fuses/polyfuses and/or bench power supplies with current limiting \$\endgroup\$
    – Wesley Lee
    Commented May 12, 2016 at 21:19

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To answer your question, yes, those 1 Amp 250 Volt fuse will work fine in small projects, as the Voltage is a maximum. A 1 Amp 250V fast fuse should blow even at lower voltages. The important number here would be the Current rating.

For example, my multimeter has a 500 mA 250 Volt fast fuse, with a recommended/listed max current measuring of 200 mA. It has blown when I shorted out a 5V 1 Amp (USB) supply, and a 12V 750 mA supply by mistake.

You can test this with a spare fuse, and a resistor. Take your 9V supply and a 100 ohm resistor. At roughly 90 mA, the fuse won't blow. Short the fuse out, and it should.

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Look up 'Pico Fuses' on the web. They are small PC mount fuses, in a package that looks like a 1/4 watt resistor. They are rated for 50 to 125 volts max, with currents as low as 1/20th of an amp to 5 amps. These are fast-blow uses, so be sure to use them in places where the in-rush current is low. Digi-Key and Mouser supply them in small lots or in bulk.

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Fuses have voltage drop. A garden-variety 1A 250V 5x20mm fuse will drop about 200mV, so not too bad, but it might put the USB voltage out of spec with a long cable.

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'Most' modern USB ports are protected against shorts (and the computer knows if they have been overloaded), but I have seen old motherboards with soldered in one-time fuses.

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These glass tube 250V fuses are fine. Make sure they are fast-blow fuses. Some have lower voltage but it's less common. Prior to choosing a fuse, evaluate approximately the extreme maximum current your project may take. If you think it will never use more than 500 mA, then use a 500 mA fuse. Some projects may work with 250 mA or even 125 mA fuses. The lower the safer. A fuse will help you debug your circuit on top of protecting it.

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