Polyester film capacitor life - Page 1
Forum section: electronics / projects

author: Rufus
author_rank: Super Contributor
timestamp: May 10, 2014, 03:33:42 am
content: A friend has a control PCB for a heating boiler which had a developing problem.The board is powered from a mains dropper with 470R series resistor and a 470nF class X2 polyester film capacitor. The problem appears to be the 470nF capacitor which (removed from the PCB) on his multimeter measures about 230nF.Such a drop in capacitance would produce the observed symptoms.The boiler is about 12 years old and the circuit would have had 230v mains applied pretty much 24/7.I was surprised, you don't generally expect film capacitors to fail that way or wear out. I found one document talking about 100k hours life at elevated temperatures but it didn't define what the end of life conditions were.Anyone have more knowledge?

author: Andreas
author_rank: Super Contributor
timestamp: May 10, 2014, 04:10:15 am
content: Hello,X2 capacitors usually have a "self healing" function.If there is overvoltage on mains. You may get a local short witin the capacitor.Due to self healing the short is "burned out". This creates capacity loss.Witout self healing you would have immediately a problem (shorted capacitor).So ageing depends largely on how often there are high voltage transients on mains.A VDR would help to limit the transients. But usually they have ageing too.At end of life they are likely to overheat. They may burn.With best regardsAndreas

author: SeanB
author_rank: Super Contributor
timestamp: May 10, 2014, 05:19:32 am
content: Typical of those class X2 capacitors. Replace with another class X2 and it will be fine. Caused by self healing, and the only way to solve it is to have a VDR across the supply lines, preferably AFTER the mains fuse. I have gotten used to changing motor capacitors at around 2-3 year intervals as they slowly age and drop in capacitance. Opening them and unwinding the foils shows normally large areas of blown off metallisation, and often areas where the foils have melted together and have been isolated by self healing burning away the connections.I do have some capacitors from old units that are spot on capacity wise after 40 years of use, but I am not opening those as they do contain PCB oil. Those likely are not metallised foil but a proper polyester foil construction.

author: Towger
author_rank: Super Contributor
timestamp: May 10, 2014, 05:56:59 am
content: I have 4 or 5 'new' X2 caps in a parts draw for 20+ years. The body on all but 1 has split. So it would appear they age, whether used or not.

author: SeanB
author_rank: Super Contributor
timestamp: May 10, 2014, 05:59:44 am
content: Rifa with a clear casing? Those do age badly. The red Wima ones do not do that.

author: Towger
author_rank: Super Contributor
timestamp: May 10, 2014, 09:14:59 am
content: They have a clear casing. Not sure of the make.

author: SeanB
author_rank: Super Contributor
timestamp: May 10, 2014, 09:44:25 am
content: That will be Rifa then, the clear does craze with time and fail from moisture ingress.

author: tom66
author_rank: Super Contributor
timestamp: May 10, 2014, 12:02:26 pm
content: I have replaced quite a few film caps with this problem, almost all measuring low capacitance; one measured open circuit.

author: SeanB
author_rank: Super Contributor
timestamp: May 10, 2014, 12:36:46 pm
content: Open circuit is really an extreme case of low capacitance. The funny thing is these crazed caps will survive for years with power applied with no problems, as the heat keeps them dry. Only when not used will they absorb moisture and fail by either catching fire or blowing themselves apart.

author: BravoV
author_rank: Super Contributor
timestamp: May 10, 2014, 12:50:51 pm
content: Quote from: SeanB on May 10, 2014, 09:44:25 am+1, examples taken from known culprits at certain old Tek analog scopes, thanks to Franky & m1kta for these close up photos.