Ne555d

Discussion in 'General Electronics Chat' started by richbrune, May 15, 2010.

  1. richbrune

    Thread Starter Senior Member

    Oct 28, 2005
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    a few years ago I designed a circuit using an NE555 D timer. After a few days or months, the timer fails in the follwing way:

    Pin 2 seems shorted to ground, internally.Providing a 1k path to VCC does not "pull up" the voltage, I think there's a short to ground

    Pins 6&7 Remain high, and the discharge transistor in the 555 doesn't seem to reset the external timing cap, even though it's voltage is close to VCC

    Pin 3 remains high
     
  2. Wendy

    Moderator

    Mar 24, 2008
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    Your post doesn't include a schematic.

    We really do need one.

    Without it my guess is R1 is too low, causing too much current to flow though Pin 5, eventually causing the transistor doing the switching to fail.

    [​IMG]

    [​IMG]

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    Last edited: May 15, 2010
  3. richbrune

    Thread Starter Senior Member

    Oct 28, 2005
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    Thanks, Bill, working on the sketch right now...
    pin 5 is grounded, through a .01u cap, how do you get much current through it? R1 is 100k C1 is .1u. Load (pin 3) is less than 5mA when I measure it.
    Input current (Pin 2) reads about 1mA.
     
  4. Wendy

    Moderator

    Mar 24, 2008
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    Opps, my bad. I mean pin 7. I don't remember what the specific current allowed through it is, but going over 100ma is not good.

    What is the value of C1? The current from it is transient, but it can add up.

    The cap is optional on pin 5, it also makes a dandy VCO input.
     
  5. richbrune

    Thread Starter Senior Member

    Oct 28, 2005
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    Here's the drawing of the timer and it's input/output/power connections.
     
  6. Wendy

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    Mar 24, 2008
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    I'm heading out to my favorite electronics store (Tanner's in Carrolton near Dallas), but when I get back I'll take a close look. What part of the world are you?
     
  7. richbrune

    Thread Starter Senior Member

    Oct 28, 2005
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    Costa Mesa, CA
     
  8. Wendy

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    Mar 24, 2008
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    OK, I redrew your schematic into something a bit more legible, it is obviously a monostable. You biggest problem is CR1, loose it. You need a firm voltage on pin 2, it is an input for a comparator and diodes are switches. When it is off it is an open input.

    The other thing is the color of the LED, it matters. White LEDs can drop as much as 3.8VDC, red typically 2.5VDC. Just in case I adjusted the schematic, but R4 may not be necessary.

    [​IMG]

    I would recommend you read my 555 Monostable article.

    You could also be suffering from ESD damage. Is their a way static could get to the timer?

    The power supply could also be the culprit. Is this an automotive circuit? How regulated is the power supply?

    It could also be a wiring error somewhere.
     
    Last edited: May 15, 2010
  9. richbrune

    Thread Starter Senior Member

    Oct 28, 2005
    106
    0
    Thanks, Bill!

    And thanks for the excellent drawing, I'll re-visit my decision to install crr1. My vague recollection was that I was getting a slight glow from the green LED, that was confusing as D1 is an indicator to verify the "button" (contactor) was closed. The power supply is an LM2575-12 it always looks pretty stable, but I don't have the equipment to really see small spikes. Static as you said, is also suspect. The leads that actually go to S1 are necessarily long, I'm not sure what to do about it.
    Thanks again and your further instruction is always appreciated--Rich
     
  10. Wendy

    Moderator

    Mar 24, 2008
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    You could put a resistor between pin 2 to Vcc. It would also eliminate the need for R4 on the second schematic (actually it would be R4). Designing stuff like this is choices, I don't always pick the best solution on the first try (hence my tagline).