DIYbanter - View Single Post

Michael A. Terrell

wrote:

I recently replaced a couple of electrolytics in a flat screen TV for a customer. The caps were in the power supply and were of course rated for 105 degrees C. So this brought to mind a question. Could this possibly be an operating temperature? Or is it a storage temperature? Or perhaps it's an internal temperature? It would seem like it would have to be a very high frequency component to ever cause an electrolytic to ever approach anything like this. Could one of these parts rated as such actually get this hot and remain operational? Would this actually be within prudent design parameters for the device? In theory if the caps are not actually operating at even 85 degrees C then why wouldn't you be able to use a lower rated temperature cap for that application?

It would seem to me that if a piece of equipment were designed to run a capacitor that hot or even at 85 degrees C for whatever reason then in my mind that would certainly constitute a very poor design. I have been repairing TV's for many years and the only capacitors I've ever seen get too hot to touch were bad ones. Could someone please explain this rating to me? Thanks, Lenny

It's called 'Derating', or not operating a component right at it's
limits. The more of a margin, the longer it lasts. That 85°C, 105°C or
125°C rating is how hot you can run it for its rated useful life. That
can range from 500 to 25000+ hours, depending on what you want to
spend. Input capacitors in the power supply have high ripple current,
which generates heat. The more heat they have to deal with, the shorter
their lives.

Digikey offers 74,859 different types & brands of electrolytics for a
reason.

http://www.digikey.com/product-search/en/capacitors/aluminum-capacitors/131081?k=capacitor

Pinching pennies reduces reliability, like the several years of crap
computer motherboards that were built with substandard crapacitors.