Smart or Over Kill On Safety? (Fuses)

Discussion in 'General Electronics Chat' started by ajm113, Jun 2, 2011.

  1. ajm113

    Thread Starter Member

    Feb 19, 2011
    Hello I'm currently working on a schematic that allows you to test different parts on one board and I want to add a little power supply feature to it.

    What worries me since the board will be fully exposed as for it's prototype stages and even final version, I was thinking it maybe safe to add 2x 1 amp fuses encase something causes the board to short circuit.

    One fuse on the positive side of where the circuit board will get it's power from the 12v power adapter. Then one on the positive out for the power supply feature.

    Also I would like to add encase you think I'm going to allow the board sit on the floor, it will have rubber bases on the corners of it and middle to keep the circuit board from touching the floor. (Just like the good old PicKit 1 ;) )

    Is it kinda a over kill adding fuses or is there other options I can do too keep my circuit "safe"?

    Thanks, Andrew.
  2. SgtWookie


    Jul 17, 2007
    It's a good idea to include fuses. You should select fuses as the "weakest link" - that is, you want the fuses to blow before your circuit board traces get fried, or wires melt.

    Selecting proper size fuses can take a good bit of research on your part.

    Using a fuse too low of a current rating means that you will go through a lot of fuses. Using a fuse too high of a current rating means that you will go through a lot of circuit boards.
  3. tom66

    Senior Member

    May 9, 2009
    One thing to consider is that a fuse might not save a power supply if it does not blow fast enough.

    For convenience you could have a 1A polyswitch (a fuse which can reset itself; also called a PTC fuse, resettable fuse, polyfuse, and many other names) in line with a bigger fuse, say a 2-3A or so in case the 1A doesn't trip.
  4. marshallf3

    Well-Known Member

    Jul 26, 2010
    For convenience you could have a 1A polyswitch

    Probably one of the most misunderstood and forgotten devices out there - sadly.

    They can actually be faster than a regular fuse.
  5. tom66

    Senior Member

    May 9, 2009
    Yes, I know. However, I have experienced what happens when you try and pump 40A through one, they just melt and short. A normal fuse can handle a much higher "interrupting" current.
  6. ErnieM

    AAC Fanatic!

    Apr 24, 2011
    Sorry, I need to ask what you mean by "Safety?"

    The only safety a fuse provides is perhaps by preventing an ensuing fire. It might prevent cascading failures by preventing what first blew up and drew extra current from making something else do the same.

    Something does not become "safe" just because there is a fuse or two in it. There is no magic here.
  7. ajm113

    Thread Starter Member

    Feb 19, 2011
    Thanks for the replies and sorry about the delay, I'm pretty much busy all the time. :(

    Well I heard they make fast acting fuses, so I'm not sure if thats considered 100% reliable (Never tested out fuse before) but the polyswitches sound interesting and seem like something more in up in my ally and easier to work with and sounds cheaper to manufacturer with then a regular fuse, but then again I'm not sure if they have machines that can place them on printed circuit boards for you.

    I'll have to do a little more research on my part on the polyswitches and see which would fit my needs.

    @ErnieM, safe as in nothing kills the circuit board or the user's project if a wire or conductive material touches a bad spot on the board frying important parts such as the small microcontroller. Thats why I was wondering if using a fuse was kinda a over kill or not or if there were other options.
  8. tom66

    Senior Member

    May 9, 2009
    Yes, I use surface mount PTCs in my projects. You can get 0603 and 0805 sized ones; about 50 cents each in 100 quantity.

    You have to be aware though how a PTC works. In order to interrupt a current it needs to heat up past its trip temperature and within the trip time. If you take a 500mA PTC and pump 10A through it, it will obviously heat up very quickly and trip. However, if you keep doing this, the PTC may be damaged, but only over a large number of cycles. If you consider a real world short circuit current, e.g. from a large capacitor, around 100A, your PTC will be incinerated! Also, if you run the PTC near its trip point but not so far as for it to actually trip, it can be damaged. PTCs are nice devices, but they do not replace fuses. I use them to prevent damage caused by the occasional short circuit, both to the attached devices and the power supply.