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PWM Control IC Problem (SG3525)

Discussion in 'Electronic Repair' started by alphamnemonic, Jul 2, 2008.

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  1. Hi Everybody

    This chip normally puts out square pulses. In the power supply I am
    working on (http://www.laserfaq.org/sam/150rcsch.pdf) it is putting
    out a sawtooth wave. I am testing it in 'standby mode', in which Q4 is
    off and R33 is in control of the duty cycle. Replacing the IC1 has no
    effect. Does anyone have any ideas?
     
  2. Guest

    Where are you seeing sawtooths? Pin 13?

    --- sam | Sci.Electronics.Repair FAQ: http://www.repairfaq.org/
    Repair | Main Table of Contents: http://www.repairfaq.org/REPAIR/
    +Lasers | Sam's Laser FAQ: http://www.repairfaq.org/sam/lasersam.htm
    | Mirror Sites: http://www.repairfaq.org/REPAIR/F_mirror.html

    Important: Anything sent to the email address in the message header
    above is
    ignored unless my full name AND either lasers or electronics is
    included in the
    subject line. Or, you can contact me via the Feedback Form in the
    FAQs.
     
  3. Yes the sawtooth is on pin 13. Pin 13 is being used as an output and
    pins 11 and 14 are grounded because the chip is being used in single-
    ended supply mode (see http://www.datasheetcatalog.org/datasheet/motorola/SG3525AN.pdf
    p.6 for an example of this configuration). I can see why you find that
    strange--I did too!
     
  4. It's not my design. I am trying to repair a commercial unit. I will
    keep you posted.
     
  5. Guest

    First, I've never scoped pin 13 so don't know for sure what should be
    there.

    But, my guess is that one of the 3 transistors is not behaving
    properly.and you're
    seeing one of the caps charging.

    --- sam | Sci.Electronics.Repair FAQ: http://www.repairfaq.org/
    Repair | Main Table of Contents: http://www.repairfaq.org/REPAIR/
    +Lasers | Sam's Laser FAQ: http://www.repairfaq.org/sam/lasersam.htm
    | Mirror Sites: http://www.repairfaq.org/REPAIR/F_mirror.html

    Important: Anything sent to the email address in the message header
    above is
    ignored unless my full name AND either lasers or electronics is
    included in the
    subject line. Or, you can contact me via the Feedback Form in the
    FAQs.
     
  6. The schematic is drawn correctly. This mode of operation is described
    on p.6 of the datasheet (link is given in my post from July 3). I
    removed the board and re-soldered R1 and R2, and the supply is now
    working properly again. A rather boring fix, but if it works long term
    I won't complain... If anyone else is having trouble with this supply,
    I can send you waveforms captured with FlukeView from the working
    unit. Thanks for the suggestions everyone.
     
  7. Yes: .rtf
     
  8. I don't have a website, but I will post them here if you tell me how.
     
  9. The pics are up at http://sites.google.com/site/scientificelectronicsrepair/
     
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