At 12:57 PM 4/20/2007, Nate Duehr wrote:

>I have a linear 13.8 VDC 20A continuous/25A peak power supply that blows
>input fuses on the 110V AC side as soon as it's powered up.

Things I'd be checking:

1) measure resistance across C1.  Very low resistance suggests input 
rectifier diodes shorted - just like you thought.
a) If low resistance across C1, disconnect both input terminals from 
the bridge, continue with following tests.

2) measure resistance across output terminals.  Very low resistance 
suggests DZ-2 is shorted.

3) measure resistance across Q6 E-C.  Very low resistance means that 
you need to not only replace the output pass transistors but also 
check all the other transistors as well.

When you think that you've found and fixed the problems, connect a 
60W or 100W 120V incandescent lamp in series with the input.  No load 
on the output.

Power it up.  Dim bulb means no more shorts.  Bright bulb means to 
keep checking.

dwayne

PS - if you suspect a problem with the power transformer, disconnect 
the secondary leads, tape them so they can't short, then power up the 
primary using a 120V lamp for current limiting as above.  Dim lamp: 
no problems.

dwayne

-- 
Dwayne Reid   <dwayner@planet.eon.net>
Trinity Electronics Systems Ltd    Edmonton, AB, CANADA
(780) 489-3199 voice          (780) 487-6397 fax

Celebrating 22 years of Engineering Innovation (1984 - 2006)
  .-.   .-.   .-.   .-.   .-.   .-.   .-.   .-.   .-.   .-
     `-'   `-'   `-'   `-'   `-'   `-'   `-'   `-'   `-'
Do NOT send unsolicited commercial email to this email address.
This message neither grants consent to receive unsolicited
commercial email nor is intended to solicit commercial email.

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