----- Original Message ----- 
From: "Olin Lathrop" <olin_piclist@embedinc.com>
To: "Microcontroller discussion list - Public." <piclist@MIT.EDU>
Sent: Thursday, July 15, 2010 12:24 AM
Subject: Re: [EE] Bulging Electrolytics


> Roger Weichert wrote:
>> I've been going to ask for a while about possible causes for the
>> failure of some 10,000uF 35v electros that are used in a basic power
>> supply for a pulsation controller.
>
> The basic causes for electrolytic cap failures are:
>
> 1 - Poor manufactured quality.  This is one of those things where
>    brand does matter.
>

I'm not sure about the brand of the originals, and just for a change I threw 
the crook ones away rather than leave them lying around the bench. I guess 
if I get any more, I can try and look up their specs.

While these still look reasonably new they have been in daily service for 
around 7 years, so have lasted reasonably well  ...  it's just their way of 
failing that surprised me.


> 2 - Voltage stress.  There isn't a hard limit at the voltage rating,
>    so creative specsmanship can be a factor (see #1).  For long
>    life it's a good idea to derate the max voltage by a comfortable
>    margin.
>
> 3 - Temperature.  Again, no hard limit, reputable suppliers may
>    leave more margin in their specs.  Derate for long life.  Note
>    that high ripple current causes dissipation, which increases
>    temperature, which decreases life.  Caps meant for this kind of
>    application have a ripple current spec, like Panasonic FK
>    series.


I actually ordered Panasonic caps from Farnell, but the ones I got had no 
Panasonic markings on them that I recognised, and they have supplied two 
different types so far.

I went for 105 degree caps, which limited my choices due to size restraints. 
Maybe I should have been worrying more about ripple current.

Looks like I better study up on it.


>
>> I've uploaded an image at http://imagebin.ca/view/HX_1Ati.html
>
> It's a little hard to tell how tall these caps are, but they look
> surprisingly small for 10mF and 35V.  Does it look like 6 Joules can fit 
> in
> there to you?  Look up the size of caps of the same rating from reputable
> vendors.  If those turn out to be bigger, then it's pretty clear what
> happened.


The originals are 25mm diam by 40mm high. The 105 degree replacements are 
45mm high. They just :) fit with 2mm headroom!

Thanks to everyone for the response so far.

Regards,  Roger

-- 
http://www.piclist.com PIC/SX FAQ & list archive
View/change your membership options at
http://mailman.mit.edu/mailman/listinfo/piclist