[MakeLV] Makelehighvalley Digest, Vol 14, Issue 14

[Mr Thomas Rumpf]

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 Message: 1
Date: Fri, 23 Dec 2011 12:36:22 -0500
From: Josiah Ritchie <josiah at josiahritchie.com>
To: makelehighvalley <makelehighvalley at makelehighvalley.com>
Subject: [MakeLV] Cap going bad?
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    <CANyh6VteFfFOchktD-jc-YJto0w1j+AK51qJ2ALfKR5HZ=V3OA at mail.gmail.com>
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I'm messing around with another old pc power supply and found that the top
of one of the 200v 330mF caps is giving 150v when connected to ground. Is
this normal? The other caps have no problems.

Yes, I discovered this through physical contact with the cap. My wife
thought a small animal was released in the room by the yelp I made. :-)

JSR/

The case of electrolytic caps is usually connected to the negative terminal and negative doesn't always mean ground.
The voltage rating is a maximum working voltage, normally the actual voltage is 10 to 40 % lower than the max.
Old, and new, PC power supplies operating off 120VAC use a doubler circuit to generate the HOT side DC voltage supply.
This means one of the two big caps can have its positive terminal connect to ground and the negative terminal can be -150 to -170 VDC with respect to ground.
Together they make about 300 to 340 VDC on the HOT side for conversion to the low operating voltages.
When these supplies are used on 240VAC the don't need to double, just rectifying gives the 300 - 340 VDC.
That's what the 120V/240V switch does, selects whether the doubling takes place.

Tom R.

( sometimes I think I get carried away )
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