questions on Capacitor rating?

[djk]

All parts last longer if you de-rate them from their maximum, caps included.

How much?

For caps I like to see 10%, unless the transformer is smaller than 300VA, then 15%, smaller than 100VA, then 20%. If you live in an area with poor line regulation you may need more de-rating than this.

The 'surge' voltage rating is for 1/2 of the 50/60 hz line cycle and is for choke input filters (tube stuff) only.

[poobah]

All parts last longer if you de-rate them from their maximum, caps included.

Very GOOD point... that is true for nearly all parts because of temperature which applies to electrolytic caps as well... the life of most devices tends to halve with every 10-11 C in operating temperature. Electrolytics have one quirk though... they need voltage to maintain their insulating layer.

Maximum life for an electrolytic is derived from:

1) Best practice when mounting, wiring, and cleaning... i.e. do not damage the elastomeric seals (this is often the a big culprit). Keep them away from ozone, UV, and chlorine in all its forms.

2) Maintain the lowest ambient temp (we are not talking cryo here).

3) Reduce the RMS currents as budget will allow (more capacitance per unit of RMS current).

4) Run as close to, but do not exceed, the WVDC rating. NEVER exceed the surge rating (this means proper consideration of line/circuit variations as "dkj" said above.



PS... don't hook them up backwards (dramatically shorter working life).