TL494 missing capacitor?

Discussion in 'General Electronics Chat' started by Eddiie, Oct 12, 2014.

  1. Eddiie

    Thread Starter New Member

    Oct 12, 2014
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    Hello,
    I am working on troubleshooting a problematic SMPS that kicks off after a few moments of load.
    Looking around the PS for loose/broken solder joints, and notice there is a missing capacitor (C5) between pins 2 (Error Amp1 - ) and 3 (feedback pwm comparator input) on the TL494.

    Often I find unused solder pads on circuit boards, but not on this one. This is the only designated area on the whole board with nothing on it. I cannot say with any certainty if there was a capacitor there or not.

    Looking at the datasheet, I think it should be a 0.01 Cap (uf?) and only see this cap present for a push-pull converter and I think a step-down converter.
    My suspicion is this missing capacitor is screwing up the voltage/power sensor circuit and tripping the cut-off.

    Does someone out there with a lot of experience with TL494's confirm there should be a cap between these two pins? If so, confirm 0.01uf is the correct value. Or possibly theorize if this cap has anything to do with the cut-off.

    Let me attach a picture of the 'area'.

    Anyone have a spare? :)
     
  2. Eddiie

    Thread Starter New Member

    Oct 12, 2014
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    0
    When zooming in on the C5 solder pads, I can make out an L shape on the left solder pad. As if there was a capacitor there.
     
  3. Eddiie

    Thread Starter New Member

    Oct 12, 2014
    16
    0
    No one?
     
  4. paulktreg

    Distinguished Member

    Jun 2, 2008
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    The solder on the C5 pads is domed and clean and I'm fairly confident in saying that there's never been a component installed.

    Edit: On a second look there may of been but it's difficult to say from the photograph.
     
  5. takao21203

    Distinguished Member

    Apr 28, 2012
    3,577
    463
    need a better photo, need to check datasheet
     
  6. Eddiie

    Thread Starter New Member

    Oct 12, 2014
    16
    0
    Hi,

    Yes, I am convinced there was cap there. I've studied the datasheet, compared near by components with diagrams on datasheet, none match exactly.
    This capacitor will go between pins 2 and 3 on the TL494. According to the datasheet, pin 2 is the -- negative side of the Error Amp 1 (Not exactly sure what this is), and pin 3 is the Feedback Pwm comparator input / Inv.. Not sure what this is, either. The datasheet did not mention much, but I gather it may be the error detection circuit ???

    What I am afraid of is this modifying the pwm pulses, modifying the output. I am also concerned my replacement cap not being the correct spec... I think this requires 1% tolerance? I have 10%. Also have 25V cap, it looks like this can be up to 40v according to the datasheet.

    Also, one diagram on the datasheet shows .001 uf cap, the other shows .01 uf cap.

    Was hoping someone out there knew the TL494 really well.. Or well enough to confirm pins2/3 are for error detection? What an Error Amp is (I will Google it). How do I find out what the max rating is of the capacitor I should use? The picture shows similar caps. (no values)

    Thanks!
     
  7. Alec_t

    AAC Fanatic!

    Sep 17, 2013
    5,783
    1,103
    Have a look at the application notes for the IC:
    http://www.google.co.uk/url?sa=t&rct=j&q=&esrc=s&source=web&cd=1&cad=rja&uact=8&ved=0CCMQFjAA&url=http://www.ti.com/lit/an/slva001e/slva001e.pdf&ei=XEtGVIuyF8aP7AbB5IHgBQ&usg=AFQjCNGgBZnSyixuHWgaHtnkiZ0J-ny55g&bvm=bv.77880786,d.ZGU
    Not sure why you'd want a cap between pins 2 and 3, since the notes section 3.3.4 about the error amps says "Precautions should be taken to minimize capacitive loading of the amplifier outputs. Because the amplifiers employ active pullup only, the amplifiers’ ability to respond to an increasing load demand can be degraded severely by capacitive loads."
     
  8. ErnieM

    AAC Fanatic!

    Apr 24, 2011
    7,386
    1,605
    Seems to me that cap was never ever there.

    But just below it there is a series R and C that are quite definitely still there.
     
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