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Most circuits that I have designed in the past deal with low voltages...sensors, communicating with other chips, etc. When would I need to consider the addition of fuses into my designs? Sometimes I need to control higher voltage devices like motors that require 24V for example. Fuses protect against shorts and large spikes in current. When should these be introduced?

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Fuses protect against fires. If you have a non-current limited supply (like a connection to mains power or a large battery) and you develop a short on your PCB, it's possible to ignite something.

A fuse won't:

  • Operate at its rated current indefinitely.
  • Blow at currents above its rating instantly.
  • Protect against very fast current spikes.
  • Protect against overvoltage conditions if the current isn't abnormal.
  • Protect semiconductors or ICs against damage - The fuse is slower than the silicon.
  • Protect a person against damage - You can still be electrocuted by a fused circuit.
  • Protect anything if it's on the wrong side of the short.

But it will usually prevent large currents from flowing through it long enough to start a fire.

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    \$\begingroup\$ Also won't protect you from electrocution (common misconception of "safety"). A fuse is a deliberately weak point in the circuit designed to blow first, before the wiring. If you don't have one the wiring will blow, in an uncontrolled location and probably slowly, melting its insulation and causing a fire. \$\endgroup\$ – Martin Feb 25 '11 at 13:51
  • \$\begingroup\$ @Martin - Good point! I've added it to my answer. \$\endgroup\$ – Kevin Vermeer Feb 26 '11 at 21:38
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I tell people to always use a fuse when using a non-current limited source, or a source that can provide more current then you will ever want.

The 2 main examples that come to mind first are batteries and wall power. In either situation there is usually enough current available to causing components to actually catch on fire if something went wrong. A fuse is a nice last resort fail safe, but shouldn't be treated as anything more then that.

If you are accepting a digital input I am not sure how you can prevent them from shorting power to ground. They will just be providing you signal that is power or a signal that is ground. There is not much you can do to prevent them from shorting power to ground. If you are providing them with power and ground then you can add some current limiting protection to them, but this is a slightly different case.

If you are providing a dedicated Power that is supposed to be used only for controlling button presses then a simple series resistor will make it so that if they do short power to ground you at least have some resistance to limit the current. However, this situation could cause a voltage divider situation that wouldn't give a true high or low signal. You also wouldn't want to do this on an output that someone actually expects to be able to pull some power from as it will cause it to be very inefficient.

I am having a hard time figuring out where you are wanting to put a pull-up or pull-down resistor. All this will do for you is cause your input to be pulled high/low when the external device is leaving it floating.

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