Electronics > Beginners

15v circuit, 35v capacitor, but only have 25v

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Siwastaja:

--- Quote from: rdl on February 10, 2020, 01:16:24 pm ---Dielectric deterioration of aluminum electrolytic capacitors used at voltages far below the WVDC rating is something I read about probably 40 years ago and has stuck in my mind. I don't know how much of a problem it is because it's rarely mentioned anymore. Maybe modern dielectrics are better and less affected or maybe it's becoming lost knowledge. I've always tried to stay at around 20-50% just to be safe.

--- End quote ---

As far as I understand the mechanism correctly, if it is a problem at all, then it's only a problem if, at some point after long usage at low voltage, the actual voltage is suddenly increased by a lot. This wouldn't happen with fixed rail voltages.

pcmad:
if the psu has no load in it it can rasie the voltage keep to the rating set or higher

Zero999:

--- Quote from: tggzzz on February 10, 2020, 10:04:00 am ---
--- Quote from: Mp3 on February 10, 2020, 02:21:13 am ---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?

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Ripple current can be a consideration, since it causes internal heating. That's why capacitors include that in their specification.

Apart from that, you need to understand the DC and ripple voltages on the capacitor. In other words, disambiguate your statement  "i think 15v is 30v peak-to-peak"

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I think he might be confused by negative voltages. According to his first post, it's a bipolar power supply, +/-15V and seems to be confused about the fact that the total voltage is 30V.


--- Quote from: Mp3 on February 10, 2020, 02:21:13 am ---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?
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Yes, that's fine. You're right, each capacitor will only have 15V across it, which will be safe.

Another thing which can catch people out is the physical dimensions: it's easy to order a capacitor which is electrically suitable, but won't fit the PCB. Measure the original capacitors and the distance between the legs, i.e. pitch, before ordering replacements.

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