<html><body><div style="color:#000; background-color:#fff; font-family:arial, helvetica, sans-serif;font-size:10pt"><div><span><br></span></div><div><br></div> <div style="font-family: arial, helvetica, sans-serif; font-size: 10pt;"> <div style="font-family: times new roman, new york, times, serif; font-size: 12pt;"> <font face="Arial" size="2"> <hr size="1"> <b><span style="font-weight:bold;"></span></b></font>Message: 1<br>Date: Fri, 23 Dec 2011 12:36:22 -0500<br>From: Josiah Ritchie <<a ymailto="mailto:josiah@josiahritchie.com" href="mailto:josiah@josiahritchie.com">josiah@josiahritchie.com</a>><br>To: makelehighvalley <<a ymailto="mailto:makelehighvalley@makelehighvalley.com" href="mailto:makelehighvalley@makelehighvalley.com">makelehighvalley@makelehighvalley.com</a>><br>Subject: [MakeLV] Cap going bad?<br>Message-ID:<br> <CANyh6VteFfFOchktD-jc-YJto0w1j+AK51qJ2ALfKR5HZ=<a ymailto="mailto:V3OA@mail.gmail.com"
href="mailto:V3OA@mail.gmail.com">V3OA@mail.gmail.com</a>><br>Content-Type: text/plain; charset="utf-8"<br><br>I'm messing around with another old pc power supply and found that the top<br>of one of the 200v 330mF caps is giving 150v when connected to ground. Is<br>this normal? The other caps have no problems.<br><br>Yes, I discovered this through physical contact with the cap. My wife<br>thought a small animal was released in the room by the yelp I made. :-)<br><br>JSR/<br><br><span style="font-family: arial,helvetica,sans-serif;">The case of electrolytic caps is usually connected to the negative terminal and negative doesn't always mean ground.</span><br style="font-family: arial,helvetica,sans-serif;"><span style="font-family: arial,helvetica,sans-serif;">The voltage rating is a maximum working voltage, normally the actual voltage is 10 to 40 % lower than the max.</span><br style="font-family: arial,helvetica,sans-serif;"><span style="font-family:
arial,helvetica,sans-serif;">Old, and new, PC power supplies operating off 120VAC use a doubler circuit to generate the HOT side DC voltage supply.</span><br style="font-family: arial,helvetica,sans-serif;"><span style="font-family: arial,helvetica,sans-serif;">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.</span><br style="font-family: arial,helvetica,sans-serif;"><span style="font-family: arial,helvetica,sans-serif;">Together they make about 300 to 340 VDC on the HOT side for conversion to the low operating voltages.</span><br style="font-family: arial,helvetica,sans-serif;"><span style="font-family: arial,helvetica,sans-serif;">When these supplies are used on 240VAC the don't need to double, just rectifying gives the 300 - 340 VDC.</span><br style="font-family: arial,helvetica,sans-serif;"><span style="font-family:
arial,helvetica,sans-serif;">That's what the 120V/240V switch does, selects whether the doubling takes place.</span><br style="font-family: arial,helvetica,sans-serif;"><br style="font-family: arial,helvetica,sans-serif;"><span style="font-family: arial,helvetica,sans-serif;">Tom R.</span><br style="font-family: arial,helvetica,sans-serif;"><br style="font-family: arial,helvetica,sans-serif;"><span style="font-family: arial,helvetica,sans-serif;">( sometimes I think I get carried away )</span><br style="font-family: arial,helvetica,sans-serif;"><br> </div> </div> </div></body></html>