555 False Triggering

Discussion in 'General Electronics Chat' started by nevis2010, Apr 28, 2016.

  1. nevis2010

    Thread Starter New Member

    Apr 28, 2016
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    Hi, i have the following circuit. When the 25W DC motor or the Air Compressor is turned on the current to the 555 circuit drops and causes false triggering of the 555 monostable circuit and issues with the 555 astable circuit. Does anyone have any ideas?
     
  2. dl324

    Distinguished Member

    Mar 30, 2015
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    Welcome to AAC!
    Spikes on the power supply due to switching transients from the motors/pumps.

    What decoupling/bypass caps do you have in the circuit?
     
  3. AnalogKid

    Distinguished Member

    Aug 1, 2013
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    Your circuit shows no decoupling or other power management. Since the 555's drive relatively compliant loads (a relay and an LED), Consider this:
    Place a rectifier diode such as a 1N4004 between the +12 V supply and the 555 circuit.
    Place a 10 uF capacitor from the diode cathode to the 555 circuit GND.
    Place a 0.47 uF capacitor across each 555's power and GND pins.

    This gives the 555 circuits a stable power source even when the main power drops out momentarily.

    ak
     
  4. ronv

    AAC Fanatic!

    Nov 12, 2008
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    You could also benefit from some clamp diodes across the relay and pump motors.
    [​IMG]
     
  5. hp1729

    Well-Known Member

    Nov 23, 2015
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    Isolate the power to the LM555s from the 12 V line. Stick in a couple of hundred ohms and a big capacitor (1,000 uF) on the 555 side.
     
  6. nevis2010

    Thread Starter New Member

    Apr 28, 2016
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    Hi, i have taken all the comments. I have updated the diagram to what i think i should do? Any thoughts?
     
  7. hp1729

    Well-Known Member

    Nov 23, 2015
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    The LM555 circuit is good, as suggested. The need is to isolate the high power loads from the LM555 power.
     
  8. AnalogKid

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    Delete the 200 ohm resistor and connect the 1000 uF capacitor directly between the diode cathode and GND. The suggestion in post #5 was for a *series* resistor, in place of the diode. I think the diode gives more consistent 555 performance.

    ak
     
  9. hp1729

    Well-Known Member

    Nov 23, 2015
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    Agreed, the diode is a better idea than the resistor.
     
  10. ronv

    AAC Fanatic!

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    And don't forget the clamp diode on the upper pump.
     
  11. nevis2010

    Thread Starter New Member

    Apr 28, 2016
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    I have add the new parts as suggested. The issues is resolved by the 1000uF capacitor & diode as well as the clamp diodes on the AC pump. How do clamp diodes work?
     
  12. nevis2010

    Thread Starter New Member

    Apr 28, 2016
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    How would i put a clamp diode on the AC pump? The diode on the 12vDC works well. Not sure if i can use a diode on 240vAC? Any ideas?
     
  13. ronv

    AAC Fanatic!

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    I don't see an AC pump in your diagram.
    Is the coil AC or are the contacts switching AC?
     
  14. dannyf

    Well-Known Member

    Sep 13, 2015
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    Isolate the 555s from the rest of the power circuits using a diode + capacitor.
     
  15. nevis2010

    Thread Starter New Member

    Apr 28, 2016
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    Its the AC Air compressor thats causing the issue. Its connected before the AC-DC SMPS. The solutions above have stopped the 12vDC pumps causing any issues, its just the AC air compressor. Any ideas would be most welcome.
     
  16. nevis2010

    Thread Starter New Member

    Apr 28, 2016
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    This is the present circuit, the air compressor is controlled by a pressure switch, when the blow gun is pressed it turns the pump on. In addition sometimes the 555 circuits trigger.
     
  17. dannyf

    Well-Known Member

    Sep 13, 2015
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    Your capacitor is of no use for those two pumps. Take the power electronics out of power domain from which the 555s are attached.
     
  18. ronv

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    Maybe something like this:
    http://www.alliedelec.com/images/products/datasheets/bm/RED_LION_CONTROLS/70030263.pdf
    Across the compressor motor. Also make sure the compressor case itself is connected to earth ground.
     
  19. nevis2010

    Thread Starter New Member

    Apr 28, 2016
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    Thanks, the RC network looks like the right part. Any idea how i size an RC network? the AC compressor is 230vAC, 550W.
     
  20. ronv

    AAC Fanatic!

    Nov 12, 2008
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