Capacitor replacment help - Page 1
Forum section: electronics / beginners

author: andrewlapham
author_rank: Contributor
timestamp: May 21, 2020, 06:25:32 am
content: Hey, im fixing a board with a damaged capacitor 100uf 400v and currently on hand i only have two 270uf 400v caps. If i use these if series this works out to 135uf capacitance. Do you think the extra 35uf is going to be a problem? --just realised i also have a 180uf cap i could use in series with one of the 270uf caps to get 108uf. Same question, is the 8uf extra going to be a problem?

author: rdl
author_rank: Super Contributor
timestamp: May 21, 2020, 07:03:27 am
content: Depending one what the cap is actually doing, even an extra 170uf might be okay.

author: andrewlapham
author_rank: Contributor
timestamp: May 21, 2020, 07:14:56 am
content: The capacitor was sitting directly after the bridge rectifier.

author: gbaddeley
author_rank: Regular Contributor
timestamp: May 21, 2020, 09:10:03 am
content: Looks like a fairly beefy set of rectifier diodes. The bigger cap will probably be ok, provided it fits the space. (Usual disclaimers apply, use at your own risk)

author: gbaddeley
author_rank: Regular Contributor
timestamp: May 21, 2020, 09:11:25 am
content: I wouldn’t use series caps in this application.

author: andrewlapham
author_rank: Contributor
timestamp: May 21, 2020, 09:34:22 am
content: Noted, I wont use series caps. The closest value i have is the 180uf to replace it with but the physical size between the two is what im a bit cofused about. They are both rated at 400v but the physical size are completly different. Here is a pic of the original next to what im thinking of using

author: gbaddeley
author_rank: Regular Contributor
timestamp: May 21, 2020, 12:11:08 pm
content: The original is HP(M), what type is the new one? You should be able to find the data sheets to confirm correct size and correct usage.

author: andrewlapham
author_rank: Contributor
timestamp: May 21, 2020, 12:29:57 pm
content: Its got KM(M) printed on it. But i cant find a datasheet

author: Labrat101
author_rank: Unknown
timestamp: May 21, 2020, 01:27:11 pm
content: Hi I would also NOT putting caps in series in A SMPS . these caps work with High ripple .putting a slightly high uf @ same voltage maybe ok depending if the mosfets that come after it are up to it.May put excess strain on the rest of the circuit .As someone else also mentioned . Should only use High ripple caps (surge) without getting too technical .KM are Surge rated .If you do try. keep your face clear these 400v caps can turn nastyspecially if you use second hand .First Remember the first Law of Murphy What can happen Will Happen .All Safety measures should be used .. ( get some one else to turn it on )

author: TimFox
author_rank: Super Contributor
timestamp: May 21, 2020, 01:38:04 pm
content: Sometimes, it is necessary to use electrolytic capacitors in series when you need a higher DC voltage rating than available in single units. For example, if you need 1000 V you can use three 450 V units in series. However, in that case you must add resistors in parallel with each capacitor that pass a current much higher than the leakage current of the capacitor at the resulting voltage. The leakage current will vary substantially from unit to unit, and the final DC voltage across each capacitor will depend directly on these currents as a DC voltage divider. See examples in the ham-radio literature for vacuum-tube RF power amplifiers.

author: RoGeorge
author_rank: Super Contributor
timestamp: May 21, 2020, 01:43:41 pm
content: You can safely put only one 270uF/400V. More capacity won't hurt. The inrush current would be different, but I wouldn't worry about that. At most, you'll get a better power factor, which is good.Two of 270uF/400V in series would be a waste.

author: andrewlapham
author_rank: Contributor
timestamp: May 21, 2020, 02:54:24 pm
content: Awesome ill go with the 270uf cap i have as it is of the same physical size of the original so i know it'll fit.