Smoke test - intriguing outcome.

Discussion in 'General Electronics Chat' started by atferrari, Feb 10, 2016.

  1. atferrari

    Thread Starter AAC Fanatic!

    Jan 6, 2004
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    Around six months ago I managed to burn my middle finger big time, by touching a TO220 transistor in a constant current source out of control. Yesterday I repeated the burning experience and dare to say the effect was even worst when testing the circuit here below.

    Smoke test.png

    Both channels were outputting a continuous negative voltage of around -3,76V. The test lasted maybe two or three minutes until I checked how warm both opamps were. Imagine cursing!

    After finding out the reason I was resigned to replace both opamps TL071 and TL074 so when they had cooled down completely I connected them properly to see how much they were damaged. Long story short, both channels are giving the expected 5 V (+ 3 to 4 mv) for an input of +3,7V right now on my bench.

    How comes...?

    SANY9377.JPG
     
  2. paulktreg

    Distinguished Member

    Jun 2, 2008
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    Because you connected them correctly? ;-)
     
  3. Lestraveled

    Well-Known Member

    May 19, 2014
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    By now I am sure the fingerprint of my right index finger looks like a mirrored transistor part number. As far as what was going on with your circuit, don't have a clue. But....In your schematic there are no TLO71 op-amps.
     
  4. atferrari

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    Jan 6, 2004
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    Yes, as I said, I did it later.

    The schematic shows the power supplies reversed. Sorry, I should have said that explicitly. Minimum 2 minutes!

    Yes, there is one TL074 and one TL071.
     
  5. SLK001

    Well-Known Member

    Nov 29, 2011
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    I imagine that the op amps have some form of reverse polarity protection diode that was getting hot. Your schematic shows the powers are connected in reverse. Is that the way you connected on your protoboard?
     
  6. crutschow

    Expert

    Mar 14, 2008
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    The "reverse protection" diode is simply the normal parasitic substrate diode that most IC's have, which conducts when the power is reversed.
     
  7. #12

    Expert

    Nov 30, 2010
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    I think you should learn to wet your finger before you touch IC's. You will get burnt less.;)
     
  8. atferrari

    Thread Starter AAC Fanatic!

    Jan 6, 2004
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    I used to do that in the past. Call it overconfidence this time.:)
     
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