Capacitors in a pinch - Page 1
Forum section: electronics / beginners

author: LoveLaika
author_rank: Frequent Contributor
timestamp: July 08, 2020, 11:10:09 pm
content: I know that there are a lot of different types of capacitors out there, but when repairing equipment in a pinch (i.e., you don't have a capacitor of that exact same type), if a capacitor isn't polarized like an electrolytic capacitor is, is it fine to use a simple ceramic capacitor in place of another of a different type (assuming that the ceramic replacement meets the values of capacitance and voltage)?

author: george.b
author_rank: Frequent Contributor
timestamp: July 09, 2020, 12:06:26 am
content: In the case of ceramic capacitors specifically, you often can't use them all willy-nilly. Most ceramic capacitors will show a decline in capacitance as a function of DC bias, and you must account for that (or use a more adequate capacitor type). See image below.

author: Nerull
author_rank: Frequent Contributor
timestamp: July 09, 2020, 02:24:47 am
content: The cheaper dialectics also change drastically with temperature and frequency.

author: Vovk_Z
author_rank: Super Contributor
timestamp: July 09, 2020, 10:38:38 am
content: Quote from: LoveLaika on July 08, 2020, 11:10:09 pm- It depends on application. Rated voltage and capacity are not the only parameters. It is hard to describe everything about capacitor types in one post. Sometimes ceramic caps go fine.

author: Siwastaja
author_rank: Super Contributor
timestamp: July 09, 2020, 12:00:34 pm
content: No. There was a reason why the capacitor failed. You need to perform better than the original designer if you want it to last longer. Even if you accept it to last as long as it did, you'd need to perform at least equally well.If you want the product to work at all, you need to at least understand which parameters were important for the design choices and supply at least equal or better values. For example, if the design requires an ESR between 1 and 10 ohms, then 0.1 ohms or 20 ohms are both bad choices.All this requires quite a lot of understanding the circuits case-by-case.

author: LoveLaika
author_rank: Frequent Contributor
timestamp: July 09, 2020, 01:08:59 pm
content: Thanks for your reply. Right now, I'm in the middle of repairing a power supply. With regards to this question, the capacitors I'm working with are metal polyester capacitors and polypropylene capacitors, and I' thinking of replacing them with polyester film capacitors, considering that ceramic isn't always an adequate replacement.

author: Vovk_Z
author_rank: Super Contributor
timestamp: July 09, 2020, 02:24:03 pm
content: May be there is schematics (some similar to yours?) so we may say something concrete.

author: TimFox
author_rank: Super Contributor
timestamp: July 09, 2020, 03:54:39 pm
content: Usually, if the designer used (more expensive and larger) polypropylene instead of (less expensive and smaller) polyester film capacitors, there was a reason, such as lower ESR in a high-current application. The ESR of polyester is very roughly the same as X7R ceramic, but the Y and Z ceramics have worse properties.

author: tggzzz
author_rank: Super Contributor
timestamp: July 09, 2020, 05:19:33 pm
content: I'm surprised that nobody has mentioned the X and Y safety ratings related to the behaviour when some capacitorsfail.

author: LoveLaika
author_rank: Frequent Contributor
timestamp: July 09, 2020, 07:40:21 pm
content: Schematics for what I'm attempting to use capacitors for are here, P382.https://www.electronica-pt.com/esquema/lcd/power-supply-delta-sm1540-d-sm7020-d-sm3004-d-25395/I posted this on the repair forum, but I'm attempting to repair a power supply, and support told me to check at board P382 as a first step. Hence, the various caps on that board (for a PWM).

author: Vovk_Z
author_rank: Super Contributor
timestamp: July 09, 2020, 08:47:27 pm
content: It is to much circuits for meThe probability that film caps will fail is low enough.You may use RLC and / or ESR meter to check caps.

author: tggzzz
author_rank: Super Contributor
timestamp: July 09, 2020, 09:42:40 pm
content: Quote from: Vovk_Z on July 09, 2020, 08:47:27 pmDo a search for "RIFA capacitor" and "delayed action smoke grenade". Personally I've "only" witnessed two such failures; certainly I replace them on sight.

author: Vovk_Z
author_rank: Super Contributor
timestamp: July 10, 2020, 11:13:44 am
content: I know about old film RIFA (and similar caps) with problems. Anyway - thank you.For example, Tektronix 24xx oscilloscopes have such a problem. I've replaced all several such caps in power supply of my 2465A (now it is ok).Maybe, I wanted to say: Generally film caps are quite reliable except several (old, 70-80's) types.