Re: Choosing the right replacement cap


 

Dave,

That this is the amount of ripple current the cap can take to make the lifetime spec? YES
A spec with twice the ripple for a given lifetime can take more power supply noise? YES
A UCY2C470MPD1TD with 375mA ripple and 10000 hr lifetime can take more power supply noise than a ULD1H470MED with 190mA ripple and 10000 hr lifetime? YES

The UCY is cruising in the 465 HV oscillator supply while the ULD might suffer degradation despite being well within voltage and temperature specs? MAYBE, and MAYBE not.

It depends on the actual ripple current. If say, there's only 100mA of ripple current, then both the UCY and ULD are cruising. If there's 250mA of ripple current, then that removes the ULD from the equation.

You can measure ripple current on a(nother) scope. You can FEEL how well the cap is handling ripple current by its temperature. All other factors being equal, a hot cap is battling to handle the ripple current, and a cool cap is handling the ripple current just fine. And if the cap is running hot, then the 10,000hr lifetime is meaningless.

ESR is the "internal resistance" of the cap. The lower the ESR, the more efficiently the cap is passing the current onwards. It's all about circuit efficiency, translated into MTBF, and signal cleanliness down the line. SMPS's require absolutely the lowest ESR caps possible that the bean-counters will authorize (and will stop working if a cap's ESR rises above a certain threshold), but a Linear PSU is far more tolerant of higher ESR caps.

Menahem

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