Lots of opinions on this I see. Here's what I understand (*but then, I may be wrong :-) ) I base my design decisions on this information :-) 1. Electrolytic capacitors are lifetime rated at their maximum design temperature. As noted by others, lifetimes will approximately double for every 10 degrees Celsius / kelvins drop below this temperature. eg Lifetime = (Manufacturers stated lifetime) x 2^( (Trated - Tactual) / 10 ) eg 85 Celsius capacitor 35 Celsius working 2,000 hour rated Actual lifetime = 2000 x 2^(85-35)/10) = 64,000 hours = 8 years In practice it's not exactly double per 10 degrees C and manufacturers provide a graph of lifetime versus temperature. 2. Electrolytics operated at high temperature will die MUCH FASTER if the power is OFF !!!! The electrolyte dries out. Manufacturers provide a rating figure on this too. AFAIR an 85C cap will die in about 10% of its lifetime if run at 85C with power off !!!!!!!!!!!!! 3. Electrolytic capacitors are best run at voltages below but near their rated voltage (rather than well below their rated voltage) , all things being equal. It may be that other factors such as ESR, ripple current rating etc lead you to other conditions but if these are adequate then it is better to use a 10V cap for a 5V supply than a 50V cap for a 5 V supply. General: - Self heating due to eg ripple current must be taken into account. - Ripple current specs must be observed to achieve rated lifetimes. - Avoid polarity reversal - Most cheap ecaps are rated at 85C but remember that 105C rated caps exist. These are dearer but may make a vast difference to lifetimes in high ripple and/or high temperature applications. regards Russell McMahon _____________________________ >From other worlds - www.easttimor.com www.sudan.com What can one man* do? Help the hungry at no cost to yourself! at http://www.thehungersite.com/ (* - or woman, child or internet enabled intelligent entity :-)) -----Original Message----- From: Roland Andrag To: PICLIST@MITVMA.MIT.EDU Date: Thursday, 9 March 2000 01:33 Subject: [OT] Short life on electrolytic caps >Hello Everyone, > >I have just (with somewhat of a shock) found out that electrolytic caps are >rated for only 2000 to 7000 hours of operation. These ratings seem to be at >high temperatures, but nonetheless they make me feel very uncomfortable >about using them. Is this some unspoken rule that somehow passed me by? > >I would like to hear some comments on what caps are usually used in >differenent situations.. > >Thanks > Roland >