Help opamp getting hot!

Discussion in 'The Projects Forum' started by gvardan, Feb 27, 2013.

  1. gvardan

    Thread Starter New Member

    Feb 27, 2013
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    Hi,
    I am trying to make a circuit that would convert a pwm signal (0-5v) to a 12v to -12v analog signal. Schematic and the layout is attached. There have been several changes made to the initial circuit, and the changes are marked in the diagrams.

    The circuit consists of a differential amplifier and a Sallen Key filter that suppose to get two pwm signals with complement duty cycles and output analog voltage. Dual channel LT1678 opamps are used for both Diff.amp and Sallen Key.

    Issue.
    We powered up the board, without any PWM inputs to the board we checked all the rails; voltage levels are fine. But when we touched the opamp it is pretty damn hot. Also theoretically when 0V(no pwm) is given into both inputs, the output should be 0v. But we are measuring output voltage around -13.5v, this is strange.

    I double checked the circuit, there's nothing wrong with soldering and no short circuits. I'm not sure what's wrong here. I have another spare opamp, but before I swap it and test, i want to double check it.

    I would really appreciate if someone could help me debug this.

    Thanks in advance.
     
    Last edited: Feb 27, 2013
  2. MrChips

    Moderator

    Oct 2, 2009
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    You probably have the power connected to the wrong pins or the wrong way around.
    Your schematic shows the negative supply incorrectly connected.
     
  3. gvardan

    Thread Starter New Member

    Feb 27, 2013
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    Well, the -15v is connected to the -V and the +15 is connected to the +V. And on the connector the 1st pin is the -15V.
     
  4. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
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    What happens if you take C1 out of circuit?
     
  5. gvardan

    Thread Starter New Member

    Feb 27, 2013
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    I could try that, just wondering how to C1 affect the circuit. It's part of the Sallen Key.
     
  6. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
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    If it is oscillating at power up, ,that positive feedback could keep the op amp oscillating rail to rail without a signal.

    If all your other connections are good, removing positive feedback for initial condition is a quick test, it'd be easier if it were built on a breadboard and debugged prior to PCB.

    If you have a scope, you can leave C1 in and see what the outputs are, I'd guess there is oscillation, or a short in soldering, I'm assuming you checked the board with an ohm meter to ensure it matches the fixed schematic.
     
  7. gvardan

    Thread Starter New Member

    Feb 27, 2013
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    When I checked the output with multimeter, as I mentioned before it is measuring
    (-)13.658 to 13.670. I'll remove the cap and see.
     
  8. gvardan

    Thread Starter New Member

    Feb 27, 2013
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    So I removed the cap and checked. Now the output initially shows ~12v and keeps reducing. Looks like some other cap is discharging/charging..

    I'm not sure whether opamp is in working condition at this point. Afraid of swapping a new one too.
     
  9. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
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    Is the op amp getting to hot to hold your finger on for a few seconds?

    Or is it getting warm?

    I'm thinking the PCB isn't matching your schematic. With C1 out, U2 should be close to a voltage buffer for the input voltage divider. With 0 VDC input, it should be a zero output. If it is non-zero, there is another issue, test the voltages vs. simulation to see where they begin to diverge.
     
  10. crutschow

    Expert

    Mar 14, 2008
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    If you reverse connect the op amp in the dip package you will also reverse connect the power supplies which will short the op amp through the substrate diode and likely zap the amp.
     
  11. gvardan

    Thread Starter New Member

    Feb 27, 2013
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    I understand. I have attached a picture of the final board. I did double check, I don't think I have reversed the connection. :confused:
     
  12. gvardan

    Thread Starter New Member

    Feb 27, 2013
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    I did check it. When no input signals are given, first amp output and the inputs to the second amps are almost zero(~25mv) and there's no osculations in rails. Only the final output is measuring ~12V.
     
  13. crutschow

    Expert

    Mar 14, 2008
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    I don't see a picture.

    Where is the dot on the DIP package? That's pin 1 as you probably know.
     
  14. gvardan

    Thread Starter New Member

    Feb 27, 2013
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    There are two pictures and two pdfs attached, the yellow picture shows the final board.

    This chip does not have a dot, they use a chamfer shape to indicate the sides.
    http://cds.linear.com/docs/en/datasheet/16789fs.pdf
     
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