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IC overheating.

Discussion in 'Electronic Design' started by [email protected], Feb 18, 2007.

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  1. Guest

    I'm working on a little fanspeed regulator circuit. This specified
    debouncing circuit wasn't working at all (bad switch?), so I
    substituted a design known to work that used a not-gate (LS04). It
    warks fine, but the problem is that the LS04 chip heats up pretty
    quickly, then eventually gets too hot to leave it plugged in. I
    searched the layout, and checked all the connections, but couldn't
    find any shorts or bad connections. I had a different chip before (I
    think a display driver) that was set up propperly on a breadboard but
    burned up too, leading be to believe that it was just a bad chip. So
    its possible that the chip I have (04) is just bad, and may have been
    damaged during soldering. Does this theory fly with any body else?

    Is there anyhing I can do to fix it without using another chip? Ground
    outputs, etc? Has anybody had a 'bad' chip that burns up right out of
    the box? Does the idea of soldering sound like a cause of overheating?

  2. Assuming power is applied to the right pins, the only load
    situation that normally heats up TTL is to have an output
    tied to the positive rail. Of course, inputs or outputs
    connected to voltages outside the rails may cause the chip
    to turn into and SCR that shorts the supply rails.
  3. Eeyore

    Eeyore Guest

    That's normally called an inverter. 'Academics' might call it a not gate
    perhaps. They're normally fairly out of touch with reality as a rule.

    Where's your schematic ?

    Not unless your soldering is really bad.

  4. Guest

    Obviously. Is a car an automobile? Perhaps a transportation vector
    would be more 'academic'. And it's technically a "Hex-Inverter"
    anyway. Burn.
    The base design is located at:

    However the input/debouncing circut is completely different in my
    implementation. Also, I only have four output states, not five.
    The soldering suggestion was implying that I may have baked it. I did
    heat-shield the pins and all, but it is possible that I botched
    something, especially at 3am and all. I wanted to know if a baked
    chip could be prone to internal voltage shorts.
  5. Eeyore

    Eeyore Guest

    In which case you should have said a hex not-gate !

    No it's not !

    How about a link that works ?

    A 'baked' chip could do anything but you have to be pretty clumsy to to that.

  6. Guest

  7. Eeyore

    Eeyore Guest

  8. Andy

    Andy Guest

    On 18 Feb, 09:00, wrote:

    As far as I can tell both the schematics are the same and don't
    include a HC04 inverter.

    However assuming the chip where you described, is not damaged, is
    connected to correct rails and has been decoupled. My guess would be
    that as you have probably only used one of the inputs and the others
    are floating. They tend to float to the mid rail and often oscillate a
    bit this causes masses of power to be used as it switches backwards
    and forwards in the region where both the transistors in the output
    are on for a short time.

    If this is the case all you need to do is make sure all the unsued
    inputs are tied to ground / 0v (CMOS parts like the HC only). If the
    part is not decoupled a 100nF between power and ground will help to
    reduce the oscillations for unconected inputs but even when they are
    all grounded its good practice and will reduce the chance of switch

    To further reduce the chance of switch bounce I would recomend
    changing to a HC14 part as this has a schmitt trigger input. This has
    different threasholds for rising and falling signals so there is a
    region after it has changed state that even if it does drop back a
    little due to noise it won't change state again. Again unused inputs
    should be tied to ground although this is less of a problem with
    schmitt triggers.

  9. neon


    Oct 21, 2006
    no bad soldering can cause heat up and still working. my bid is that it is oscillating at some very hi frequency for some design or introduced signal. that will make it heat up. LS are fast devices try a 7404 same function without the speed. with a fast scope you could verify the oscillations.
    Last edited: Feb 19, 2007
  10. colin

    colin Guest

    Sounds like they overfilled the chips with magic smoke, has any escaped yet

    dont ground outputs, but ground any unused inputs, make sure outputs arnt
    overloaded, and supply voltage or any other inputs havnt got any over
    voltage peaks etc.

    Colin =^.^=
  11. Rich Grise

    Rich Grise Guest

    I've had chips get too hot to touch by plugging them in backwards. =:-O

    Good Luck!
  12. Lionel

    Lionel Guest

    More likely the unused gates are oscillating, or you're pulling too
    much current from the gate that you are using. Try tying all your
    unused inputs together & connecting them to +5V with a 4K7 resistor.
    Also check that Vcc (pin 14) is really at +5V relative to the ground
    pin (pin 7).
  13. jasen

    jasen Guest

    I've had them work afterwards. (it was a 7555)
  14. Guest

    This thing is getting on my nerves. I got some wire and stuff, and
    made a breadboard to molex adapter, and it worked flawlessly. No
    overheating, no problemo. I try it in the Pc however, and it burns up
    like fire. It doesn't matter if the fan is plugged into it or not So
    I start looking at the differences between the working (breadboard)
    and nonworking (PC) states. First I notice that with my little
    adapter, the two grounding rails (black wires) are tied together, but
    not in the PC. So I bridge them together. Breadboard mode still
    works, but Pc doesn't. So then I think, "with the B.B, theres only
    one ground, while the PC has two. So I remove a pin from the molex,
    same deal. So I give up for a while and watch some sopranos. A
    little later I try it in the pc again. It works. It normally gets
    hot in less than a minute, but this time it didn't. I leave it
    plugged in and keep checking it. Five minutes go by, then ten. A few
    hours later, its a little warm, but not hot. Its been plugged in for
    over eight hours and its still cool. I don't know why.

    Before I tried it again, I messed with the wires in the PC, testing
    the voltages in the cables. I don't have time to go into detail, but
    it seems as if one of the fans were plugged in upside-down,
    effectively swapping the +5 and +12 rails. Not good. I'm going to
    monitor it for a few days, and maybe add a feature or two. But this
    may be the big 'Problem Solved', and I sure hope it is.
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