I have a bunch of capacitors that have the leads cut off. How can I recognise which are the positive and negative leads? I know the longer lead is supposed to be the positive but this does not help after they have been cut.
Electrolytic capacitors usually have the negative identified on the side of the can, with quite an obvious marking. SMT electrolytics usually have a black 'segment' painted on the top of the can, along with the value & voltage rating
Tantalum capacitors, be they through hole or SMT, usually have the positive identified.
Ceramic disc, mylar, mica, polystyrene etc. capacitors are not polarised, so are allowed to be mounted how you choose, however, if a tip were to be offered it would be to mount all non-polarised caps with the id marks on the same side....makes for easy reading without having to constantly rotate the PCB.
The can is normally negative on axial electrolytic and an arrow marked on the packaging clearly points towards the negative. This caught me out when I was a child, even though it seems pretty obvious to me now. **broken link removed**