# Estimating Electrolytic Capacitor Lifetime¶

From this StackExchange question. The capacitor in question is C5. It’s a 400V/470uF/105°C rated cap. The question was asking why one brand (Nichicon) failed when another (UUCap) did not when they were otherwise comparable. The burnin test was at 180V 6A.

Russel McMahon commented and gave some valuable calculations:

VAC = 220V so Vpeak = 220*1.414 =~ 310V. 180V DC/310 =~ 0.58 This is the sine of thge angle when the rectifiers start (or end ) conducting + 35 degrees. For 35/90 of the cycle the voltage in is below Vdc so the cap MUST provide the motor current. If you do not have any energy storage in inductors then the cap is seeing a ripple current of in the order of the motor current and peak currents will very likely be higher (depending on transformer and wiring resistsance and more.)

As dissipation will be in the order of proportional to current squared you probably have about 10 x rated dissiation due to excess ripple current.

Note that cap lifetime ~+ Rated hours x 2 ^ [(Trated-Trun) / 10 ]
It is usual to run a cap at WELL below rated temperature.
30C below = 2 ^ (30/10) = 8 x rated lifetime.
So a 2000 hour rated cap would last about 2000 x 8 = 16000 hours ~= 2 years.
The larger margin the better.

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