555 monostable timer calculations are way off

Discussion in 'General Electronics Chat' started by thedrive, Oct 4, 2016.

  1. thedrive

    Thread Starter New Member

    Sep 23, 2016
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    Hi,

    I'm using a simple 555 monostable circuit to control automatic shutoff of another circuit. The problem I'm running into is the R/C calculations given result in vastly different duration then what its supposed to be. Anyone have an idea of what could be the problem? I'm currently using the attached circuit with one modification I'm using a 0.01uf cap on pin-5 to gnd. My output pin 3 is connected to a N-channel mosfet. I have used other similar circuits to this one and I'm getting the same results. Currently my R1 is 4.7Meg and C is 220uf. I believe this should result in a 1137.4 second (18 minutes) on cycle. But its currently giving me about 2100 second (35 minutes) on cycle.

    I have used a few different 555 chips so I dont think its a problem with the chip. Also Im using a 12v source from an old UPS battery as my power source.

    any ideas?

    Thanks!
     
  2. ericgibbs

    Senior Member

    Jan 29, 2010
    2,499
    380
    hi td,
    The leakage current thru the 220uF is effecting the timing period.
    4.7meg and 220uF are IMO outside the limits for getting a stable period.

    E
     
    #12 likes this.
  3. thedrive

    Thread Starter New Member

    Sep 23, 2016
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    Thanks for jumping in on this, Its appreciated. I'm looking for 45 minute delay, Do you think 45 minutes is possible with this circuit or do I need to look at something different?
     
  4. #12

    Expert

    Nov 30, 2010
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    6,745
    One of these:
     
  5. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    If you want a stable delay there are a couple of options :
    1) A small MCU suitably programmed,
    2) A shorter-period oscillator circuit driving a counter IC, e.g. based on a CD4060.

    Edit: #12 beat me to it with the CD4060 suggestion :)
     
  6. Dodgydave

    Distinguished Member

    Jun 22, 2012
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  7. crutschow

    Expert

    Mar 14, 2008
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  8. thedrive

    Thread Starter New Member

    Sep 23, 2016
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    Thanks everyone for backing me up. I will give the cd4060 a try looks like the best option. Looking at the data sheet I can add some additional features I was kicking around for my invention.
     
  9. hp1729

    Well-Known Member

    Nov 23, 2015
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    Do you have a way of measuring the actual capacitance of that220 uF cap? A -20% to +100% tolerance?
    That is a rather long time constant. Do you get the same result with a CMOS 555?
     
  10. #12

    Expert

    Nov 30, 2010
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    Published specs on aluminum electrolytics show you can't even get 5 minutes because of leakage currents in the cap.
    The only reason you CAN get 5 minutes is that the caps don't leak as much as the guaranteed max.
    This guy wants 18 minutes. The 4060 is a slam-dunk.
     
  11. thedrive

    Thread Starter New Member

    Sep 23, 2016
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    I'm sure I have a way. I just don't know how to measure a cap? I have multimeters and a oscilloscope.
     
  12. Bernard

    AAC Fanatic!

    Aug 7, 2008
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    I've had good luck with the 4060, but I have also timed 555 out to 3 hr. "pulled out 2 caps, 10,000 & 3300; first still had 1 V, second 5 V, how long since charging, months. Use your MM and read resistance of some caps. Check to be sure that red lead , on resistance setting, is really positive. A good cap will gradually climb to full scale. Disconnect lead & recheck in a minute, should be near full scale for a good low leakage C. With a low leakage C, set up 555 with a 1 M R & R & LED to monitor output. Time pulse & use formula.
     
  13. thedrive

    Thread Starter New Member

    Sep 23, 2016
    13
    0
    Thanks for your help
     
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