One min timer with voltage regulator

Discussion in 'The Projects Forum' started by crafter2u, Jan 13, 2013.

  1. crafter2u

    Thread Starter New Member

    Jul 16, 2012
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    Regulator is a 7812 and works in my simulation program but when i bread board it then it does not work
     
  2. spinnaker

    AAC Fanatic!

    Oct 29, 2009
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    You are going to have to but in more effort than that if you expect help around here.

    For startes:

    1. What is your input voltage? Is it DC or AC?

    2. Is your regulator working?
     
  3. praondevou

    AAC Fanatic!

    Jul 9, 2011
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    Something is missing. Is this supposed to be an astable or a monostable?
     
  4. tracecom

    AAC Fanatic!

    Apr 16, 2010
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    The LM7812 needs a small (.33μF) ceramic cap on the input and a small (.1μF) ceramic cap on the output. In addition, a large filter capacitor may be needed on the input and a small filter capacitor may be needed on the output. There are other omissions from your circuit that may hinder its performance. What does it do or not do that you want to change?

    If you mean the regulator doesn't work at all, you probably have it wired backwards. Is it a LM7812 or a LM78L12?
     
    Last edited: Jan 13, 2013
  5. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    It looks like a relay which is activated about a minute after power is applied to the regulator.

    This may not be why it doesn't work, but there is nothing to definitively discharge C1 when power is turned off. D1 is supposed to do that, but there is no defined current path to ground when the power is turned off. C1 probably powers the IC and the relay for short time after power is turned off.
    I would add a 1kΩ resistor from the regulator output to ground. This would discharge the cap in less than 250mS, probably much less, after power is shut off.
     
  6. tracecom

    AAC Fanatic!

    Apr 16, 2010
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    I breadboarded the circuit as drawn except that I substituted 1M for R1 and 100μF for C1 (because that's what I had handy.) I also used a regulated 12V supply that I previously built. Here's what happens.

    On power up, 555 pin 3 goes high and stays high for about 110 seconds. Then, pin 3 goes low, the relay operates, and stays operated until the power is removed. When power is removed, the relay releases immediately. Apparently, the cap discharges somehow because I tried to reconnect the power quickly and make pin 3 go low prematurely, but could not. I would say the circuit works as designed, and that with a 1.1M R1 and a 47μF C1, the delay would be 56.87 seconds plus or minus component tolerances.
     
  7. spinnaker

    AAC Fanatic!

    Oct 29, 2009
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    You guys are way too helpful. Take a look at the Ops post. He has only one post. My guess the OP will not be back.

    My advice is to not help those that do the bare minimum and don't have a good reputation for doing otherwise, without a followup from the OP.

    You guys are the best. Save it for those that deserve your help.
     
  8. tracecom

    AAC Fanatic!

    Apr 16, 2010
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    You may be correct. I like to breadboard circuits for my own edification.

    In this circuit, for example, I didn't see the purpose of D1, so I removed it and the circuit's operation seems unchanged. What am I missing?

    ETA: I guess Ron H has already pointed out that D1 was useless.
     
    Last edited: Jan 14, 2013
  9. wayneh

    Expert

    Sep 9, 2010
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    I'm not sure D3 is useful either. It'd be a better use of a diode to put it in reverse around the regulator, to protect the regulator from a loss of power. The charge on C1 probably couldn't ruin the regulator, but it's good form to avoid an output voltage that exceeds the input.
     
  10. tracecom

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    Apr 16, 2010
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    You are right; I bypassed D3 and the circuit's operation is unchanged. And once you take out those two diodes, it's just a standard 555 one-shot.

    "Simplify. Simplify!"
     
    Last edited: Jan 14, 2013
  11. Ron H

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    Apr 14, 2005
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    I pointed out that D1 is supposed to discharge C1 rapidly when the power is turned off. The main reason you would want to discharge C1 rapidly is if the power were switched off and back on in a matter of milliseconds. If the cap doesn't have time to discharge through R1 and pins 2 and 6, the next delay will be less than 1 minute.
     
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