Electronics > Beginners
15v circuit, 35v capacitor, but only have 25v
Mp3:
I just wanted to ask what you guys think about this. It decides whether i finish a repair tonight or place a mouser order for just a couple parts lol.
I'm replacing a failed capacitor on a power supply (rated 4700uf, tested about 800-900uf) and doing the other two while i'm at it. It's for a 5v line and is rated 16v... no problem.... i have a 16v 4700uf here.
The other two are 35v 1000uf. One is for -15v and one is for +15v. I have 25v 1000uf on hand. I don't know if 25v is okay because i think 15v is 30v peak-to-peak, but each capacitor is only having +15 or -15 running through it, so isn't 25v enough room to be safe?
I don't want to push it too close so i'll buy 35v's and wait if you all think it's best to wait and use 35v.
Whales:
--- Quote from: Mp3 on February 10, 2020, 02:21:13 am ---because i think 15v is 30v peak-to-peak
--- End quote ---
What? Are these capacitors for DC power rails or AC signal filtering?
EDIT: possibly you mean that -15 to +15 is a 30V difference? If the capacitors are only wired from 0 to +15 and 0 to -15 then the capacitors only see 15V, not 30V. You would have to wire a cap from -15 to +15 to get the 30V (but that's probably not what this circuit requires).
Mp3:
--- Quote from: Whales on February 10, 2020, 03:19:26 am ---
--- Quote from: Mp3 on February 10, 2020, 02:21:13 am ---because i think 15v is 30v peak-to-peak
--- End quote ---
What? Are these capacitors for DC power rails or AC signal filtering?
EDIT: possibly you mean that -15 to +15 is a 30V difference? If the capacitors are only wired from 0 to +15 and 0 to -15 then the capacitors only see 15V, not 30V. You would have to wire a cap from -15 to +15 to get the 30V (but that's probably not what this circuit requires).
--- End quote ---
Yeah, i don't think they are actually seeing peak to peak voltage. These are DC power rail capacitors coming after some voltage regulators, so i was quite sure these capacitors only see 15v. But, i kept second guessing myself if they were 35v for a reason. But then i thought 35v is too close a margin if they actually saw 30v all the time. Thanks!
TerminalJack505:
Some designers like to double or even triple the voltage rating for electrolytic caps. This probably isn't necessary for name brand caps but I guess the designer will sleep better at night if they use cheap caps and derate the maximum voltage by a large factor.
EEEnthusiast:
Sometimes the capacitor ratings are chosen based on availability. May be during the company had surplus stock of 35V capacitors and hence they decided to use that. It may also depend on the cost. It may so happen that if the distributor had 35V in large quantity, they may actually sell the 35V cheaper than or equal to the 25V one.
So in my opinion, it is perfectly safe to use a 25V cap for a 15V DC line.
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