voltage for electrolictic condensers

[Simon]

what sort of margin should be used in selecting capacitor voltages ? I opened up a peice of equipment at work meant to go on 24 V vehicle systems and found a 25 V capacitor on the input  me figures it's a time bomb ? maybe 35 V is ok but perhaps 50 being a vehicle with spikes floating around ?

[mlaargh]

For anything more than a one of, I think the standard is to double the expected voltage value.  I've seen some pretty tight margins in commercial products as well, though.

Have also seen a lot of exploded caps...

[Simon]

and you don't really want an exploded cap on a circuit in an air con system in a military vehicle !

[jahonen]

25 V rated caps are definitely not enough for a 24 volts nominal system, as the actual voltage can be quite a bit higher (more likely something like 28 volts). And, for critical automotive systems, there is a requirement for tolerance of so-called load dump, which can generate transients of 200 volts (during the test). For normal use, 50 volts rated voltage would be ok for a conservative design.

Regards,
Janne

[Simon]

yea well as it is we know there is a "spiky" power supply and as far as i know 24 V vehilcle = 30 V full charge power before you think of spikes etc