I am experimenting with making a PCB and have created some short circuits when wiring everything up which have fried one of the traces. i was wondering whether it would be a good idea to put a fuse in the next iteration to avoid breaking the whole PCB if i should make another mistake. could anyone recommend whether this is a good idea and if so, how would i choose the right fuse? the circuit runs of a lithium battery so 4.2v -3.3v thereabouts and i am not expecting current draw about 0.5A. Many thanks.
Check out a resettable fuse such as a PTC:
The PTC comes in regular packages like 0805 and through-hole just like your other capacitors and resistors. When the current through the PTC exceeds a certain limit it heats up and the resistance increases, reducing the current to your board.
Once you turn off the power and the PTC cools down then the resistance decreases and after a while you can use it again, making it resettable. It's not perfect so if you keep tripping the PTC repeatedly you may need to replace it eventually, but it's good for a couple of uses.
I'd recommend having a LED before and after the PTC so you can tell when input power is applied (first LED turns on), and when the PTC has tripped (second LED turns off).
To pick your PTC you need to look at:
- The voltage used by the circuitry on your PCB
- The hold current, how much current can go through the PTC normally (your expected maximum load)
- The trip current, how much current is needed to make the PTC start increasing resistance and limiting current to your PCB.
You can do a parametric search from most manufacturers to focus on the products they offer that fit your requirements.
A fuse could help, but first the traces on the PCB should be designed to handle whatever the maximum fault current can be. There are trace width / temperature rise calculators you can use to do this. An alternative to fuses are electronic circuit breakers that have the advantage of being more precise and act much quicker than a fuse. Linear Tech, TI and others make ICs that do this.
You've probably already done this, but just in case: If your using CAD to design it (I say that because I once saw a guy design an etching mask on a blank PCB with a marker), sometimes the auto-route feature on Eagle or other CAD softwares will have issues and route something wrong or have a few traces too close. I remember eagle once tried to auto label all of my wires as ground. always double check and maybe allow more space between traces in the next iteration if in doubt.
I agree with the others,especially raisin-wrangler, I would always use a fuse of some type (maybe add a crowbar circuit in the final iteration for added robustness if you want), recheck your PCB wiring AND I would also figure out a way to check the thickness of the copper cladding that's on the PCB.
I don't know where you got the copper clad boards from, but I know Some Chinese manufacturers often cut corners and have bad quality control. So you could have gotten a PCB that has thinner cladding than you thought, which might throw the PCB thickness calculations out of whack thus causing the PCB traces to not be able to handle the current.
An example of Chinese quality: I once got a Chinese knock off solder station as a gift from a non-Elec. Eng. relative. I took a look inside to find that some of the heatsinks for some finger-burning-hot T0-220 transistors were missing, the soldering iron and heat gun were not ESD safe like advertised and the heat shrink tubing on some connections had not had heat applied yet.
However, I'm guessing a micrometer could check the thickness of the cladding? I've designed boards before, but have usually had a board manufacturer do the manufacturing in the past. Nevertheless its an option to keep in mind when ordering boards on the next iteration. Perhaps you could beef up the cladding thickness or select a different manufacturer of the copper cladded boards to combat a cladding thickness issue if it is present.
That's everything I can suggest. Hopefully this helps. Good luck on the next iteration.
EDIT: wanted to clarify on the copper clad board manufacturer part and put the micrometer part in bold