21 May
2012
21 May
'12
10:19 a.m.
On 21 May 2012 at 11:55, JP Hindin wrote:
I am curious, now, as to why they used this
glass-encased "metallised
paper" capacitor instead of, say, an electrolytic. There are many
electrolytics inside the PSU, so it clearly wasn't a lack of
availability. Presumably there was a specific reason they chose this
type of cap over another. I'd hate to be wrong and do worse damage.
Are you talking about the infamous Rifa caps?
That's not glass--I think it might be polystyrene. At any rate,
they're "safety" capacitors in that their failure mode is limited to
a set of known conditions, none of which can lead to electrical shock
hazard. Generally, these are X- and Y- rated capacitors, with
several numbered subcategories.
X-capacitors are designed for "across the line" applications, i.e.,
from one side of the line supply to the other.
Y-capacitors are
designed for "line to ground" applications.
Here's a brief FAQ on that subject:
http://www.okaya.com/FAQ1.html
Rifa caps are still with us, though there are X- and Y- rated
polypropylene and polyester caps as well.
Do not use an electrolytic, non-polar or otherwise. They're not
designed for the application.
--Chuck