Electrolytic capacitor voltage ratings. Will 35V caps work at 32V? - Page 1
Forum section: electronics / projects

author: Hobgoblin612
author_rank: Contributor
timestamp: October 28, 2012, 12:50:54 pm
content: I am making a linear variable power supply and am having trouble with the capacitors.First some background.The circuit is a standard transformer, bridge rectifier, capacitor design.I have already ordered my boards and when I went to order the rest of the parts from Digi-Key, I found that the two 4700uF 50V caps I was going to use were out of stock. The problem is that the board has a 7.5mm lead pitch for these caps and this is the only cap I can find exactly like this. If I had known before, I would have changed the board but all of the other caps I can find have a 10 or 12.5mm pitch.My main question is, can I use 35V caps.(which have a 7.5mm pitch) The reason I ask this is because the transformer is 18V and has a 25% regulation factor. This means that after the bridge rectifier there will be about 32V across these caps at no load. (18*125%*1.414=31.185V) Is this a big enough safety margin or will I have to find another solution? What about spikes?The capacitors:http://www.digikey.com.au/product-search/en?x=0&y=0&lang=en&site=au&KeyWords=1189-1397-NDThe transformer: (I will be using each of the secondaries for a separate supply, so I have 2*18V@4.8A)http://www.digikey.com.au/scripts/DkSearch/dksus.dll?WT.z_header=search_go&lang=en&keywords=237-1287-ND&x=0&y=0If this will not work can I use two 3300uF caps? The most that will be drawn is 3A.Thanks in advance, Timothy.

author: SeanB
author_rank: Super Contributor
timestamp: October 28, 2012, 01:02:46 pm
content: It will work, but the capacitors will have the rated life only, and you will find that they might fail in a year or two especially if they get warm inside. It would be better to use different capacitors with a higher ripple current, or use 2 in parallel. Can you not just drill the holes for the caps at the larger pitch and solder them in by bending the leadout over to the pad? Your specced 4700/50 ones are marginal at the ripple current in your application, and should be higher rated and 105C units.

author: Hobgoblin612
author_rank: Contributor
timestamp: October 28, 2012, 01:47:35 pm
content: My design already has 2 of the 4700uF 50V caps in parallel. (That wasn't clear in the OP, sorry.)

author: SeanB
author_rank: Super Contributor
timestamp: October 28, 2012, 01:52:33 pm
content: If you can drill the hole at the larger pitch and use a 105C unit it will last longer but just using the 35V units will do.

author: Hobgoblin612
author_rank: Contributor
timestamp: October 28, 2012, 01:57:13 pm
content: Will 2 3300uF 105C 50V ones in parallel work or is that not enough capacitance?

author: SeanB
author_rank: Super Contributor
timestamp: October 28, 2012, 02:11:04 pm
content: Will work, but will have a lot of ripple at full load., you will find that full output current will top out at around 25V or so. Add an extra 2 on short thick leads if possible.

author: Hobgoblin612
author_rank: Contributor
timestamp: October 28, 2012, 02:33:11 pm
content: I have found some 4700uF 35V 105C 2.67A ripple current caps that will fit. I may have to re-spin the board anyway so this will do for now. Thanks for your help SeanB.

author: Bloch
author_rank: Unknown
timestamp: October 28, 2012, 03:11:50 pm
content: Quote from: Hobgoblin612 on October 28, 2012, 12:50:54 pmI just want to mention that the mains voltage also swing. In my country are the rule -10/+6% 230 volt (207 to 244 volt).That will give about 33 volt in worst case.

author: SeanB
author_rank: Super Contributor
timestamp: October 28, 2012, 03:17:16 pm
content: Yes mains will vary, but this has already been taken care of in the original specs. As well most caps have a surge voltage rating of a volt or three above the rating. Not good to rely on for long term reliability but still there.

author: Hobgoblin612
author_rank: Contributor
timestamp: October 28, 2012, 03:34:11 pm
content: Also I forgot the drop across the diodes, but I'm not sure it's as simple as 1.2v for 2 diodes.

author: SeanB
author_rank: Super Contributor
timestamp: October 28, 2012, 03:38:00 pm
content: It will be roughly that, a little more as the current increases but then the diodes get hotter so it lowers a little.

author: G7PSK
author_rank: Super Contributor
timestamp: October 29, 2012, 03:56:24 pm
content: How many items are you making. If only a few could you not get a plastic spacer and bend the wire of the larger capacitors to fit the board. I have used buttons as spacers when doing this repairing old radios.

author: robrenz
author_rank: Super Contributor
timestamp: October 29, 2012, 05:05:17 pm
content: Following that thought you could pre bend the wires for the hole spacing and use a O-ring almost the same diameter as the cap as a spacer and silicone the cap and oring to the board.

author: ptricks
author_rank: Frequent Contributor
timestamp: October 29, 2012, 08:50:01 pm
content: It will work , but expect to be repairing it in the very near future. Surge ratings are not presumed to be from normal over voltage conditions,each time a surge occurs you lessen the chance that the next surge will take out the cap. Capacitors are far too cheap and readily available that using a cap you know isn't correct doesn't make much sense.