Tweakers' Asylum

25% of the voltge not the value

71.227.220.13

Tweekeng

In Reply to: RE: Problem making the change posted by wishfish on March 03, 2009 at 19:41:59

Do not exceed the voltage rating by more than 25% of the original cap.

The value is another thing.

You really should not decrease the value this will cause more ripple and reduce the transient response of the power supply. It is fine to parallel capacitors to get closer the the value you need.

Follow Ups:

RE: 25% of the voltge not the value - Uncle Mike 06:42:08 03/04/09 (2)
"Do not exceed the voltage rating by more than 25% of the original cap."
Where does this come from? On what practical application or knowledge do you base this? It seems to me it would be fine as long as size doesn't become an issue installing it.
- RE: 25% of the voltge not the value - Tweekeng 12:09:56 03/04/09 (0)
  I base this on what I learned in school.
  
  The Aerovox engineers also stated that over rating too high can cause premature failure because the layer of aluminum oxide will not form properly on the plates when our company was looking into a rash of electrolytic capacitor failures.
- Cornell-Dubilier recommends 50% derating for mil/space applications. - Al Sekela 11:51:02 03/04/09 (0)
  From their down-loadable application guide (courtesy of the Electrochemistry Encyclopedia (http://electrochem.cwru.edu/ed/encycl/):
  
  "Voltage Derating
  
  Voltage derating is expressed as the percentage that the
  applied voltage is less than rated voltage, e.g., a 450 V capacitor
  operating at 400 V would have 11% voltage derating.
  Aluminum electrolytic capacitors made with formation voltages
  at least 35% higher than rated voltage and with rated temperatures
  of 85 �C or higher, don�t require much voltage
  derating. In applications operating at less than 45 �C no
  derating is needed, and with up to 75 �C, 10% is sufficient.
  For higher temperatures and with high ripple current, 15%
  or 20% is appropriate. Since operating life continues to
  increase for further derating, military and space applications
  use 50% voltage derating.
  Photoflash capacitors may be used at full rated voltage at
  normal room temperatures because they are designed for
  such duty. Strobe capacitors benefit from at least 10%
  voltage derating because their continuous operation makes
  them run hot."
  
  There is nothing in this guide that suggests that capacitance is compromised by operating the devices below the rated voltage.
  - Application guide pdf (Open in New Window)

Post a Followup:

FAQ

Post a Message!

Forgot Password?

Moniker (Username):
Password (Optional):
	Remember my Moniker & Password (What's this?) Eat Me
E-Mail (Optional):
Subject:

Message:	(Posts are subject to Content Rules)

Optional Link URL:
Optional Link Title:
Optional Image URL:
Upload Image:
E-mail Replies:	Automagically notify you when someone responds.

To view your new posting or follow-up, click on the RELOAD or REFRESH button on your browser.

This post is made possible by the generous support of people like you and our sponsors:

Parts Connexion