timing a charging cap

Discussion in 'General Electronics Chat' started by terry noakes, May 25, 2015.

  1. terry noakes

    Thread Starter New Member

    May 25, 2015
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    how to calculate component values to charge a capacitor in 5 seconds.
    The task I have is simple yet escapes me. I need to throw a switch (on a 12 volt dc circuit) hold it for 5 seconds and have a relay click and latch on. If the switch isn't held on for the full 5 seconds i.e. turned off after 4 seconds it needs to be re-thrown for the full 5 seconds to operate the relay ( and not a further 1 second) so there needs to be a back discharge part to the circuit. to bring the cap back to zero volts. Can anyone help with this? thanks. Terry
     
  2. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    Welcome to AAC!
    What is the relay coil rating (current or Ohms)? What load (current) will you switch with the relay? (Don't mention automotive or the thread will be closed!)
     
  3. MikeML

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    Oct 2, 2009
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    A 555 timer chip might work for this project.

    How accurate does the 5min period have to be? +- what tolerance?

    Can the switch be a form C (three terminal NC-C-NO)?
     
    Last edited: May 25, 2015
  4. terry noakes

    Thread Starter New Member

    May 25, 2015
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    the relay (may be replaced with a power transistor) will be switched with a transistor as the loading is very small, something in the region of 1 or 2 amps, the time is 5 seconds not 5 mins and the switch will be a simple push to make non latching. It's on a camera crane and is to avoid accidental switching on. Therefore a constant press of 5 seconds is required.
     
  5. MikeML

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    Oct 2, 2009
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    I'm confused.

    If the switch is pressed for less than 5s, and then released, nothing happens.
    If the switch is released for even a fraction of a s, then the timing is reset.
    If the switch is held for >5s, a relay turns on.
    What happens if the switch is held beyond 5s, then released? Is the relay on only for the time after 5s until the switch is released? The act of releasing the switch also drops out the relay?
     
  6. MikeML

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    Oct 2, 2009
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    Oh, and one more ?

    Does the 2A load current come through the push switch?
     
  7. Alec_t

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    What will cause the relay to un-latch?
     
  8. terry noakes

    Thread Starter New Member

    May 25, 2015
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    no the 2a load doesn't go through the switch, and yes the switch must be held for a constant 5 seconds, any longer isn't necessary. if pressed for anything less then nothing happens, to negate it being pressed for 1 sec five times over a period of time. only a full five second press will turn things on. the relay will self latch as it's only activated after the full 5s and not during. I need a capacitor to charge during this time when it will trigger the base of a transistor which will then latch up the relay. It seems to me the simplest way to create the 5 seconds.
     
  9. terry noakes

    Thread Starter New Member

    May 25, 2015
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    relay will unlatch when a push to break switch disrupts the power side of the relay
     
  10. ian field

    Distinguished Member

    Oct 27, 2012
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    A thought occured to me that if you leave off the window comparator pin of a 555 that senses when the cap gets to 2/3 Vcc, it'll latch - you can either reset it with the reset pin or take the missing pin high. Although it sounds as if the TS actually just wants the text book monostable configuration.
     
  11. AnalogKid

    Distinguished Member

    Aug 1, 2013
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    More like a missing pulse detector. Normally I'd recommend an LM393, but the timing cap for 5 seconds is pretty large, and a 393 output stage might not discharge it fast enough if it has been charged most of the way up.

    Input from switch, 2N44401 NPN transistor, 750K pullup resistor and 3.3 uF ceramic timing capacitor to GND, 1/2 of an LM393 with the trip point set to 86.5% (7.5K up, 47.5K down). Output of 393 drives whatever.

    ak
     
  12. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    Here's one possibility:
    5secPress.gif
     
  13. terry noakes

    Thread Starter New Member

    May 25, 2015
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    that looks interesting Alec, I will try a mock up tomorrow if time allows but it suits my idea of simplicity. Terry
     
  14. crutschow

    Expert

    Mar 14, 2008
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    Below is my take on a 555 design.
    An ON push-button time of >5s turns on the 555 and the output stays high until the Reset push-button is momentarily depressed.

    One advantage of this design is that the RC delay trigger point is not determined by the threshold voltage of a MOSFET, which can vary over a significant voltage range from unit to unit, but by a resistive divider network in the 555, which is fairly accurate.
    It also doesn't use any relay contacts for the latch.

    As you can see it ignores two 4s PB inputs in sequence (high PB signal is NC push-button pressed) but triggers on the 6s input after about 5.2s.
    Q1 prevents the ON PB from resetting C1 after the 555 output goes high.

    The circuit requires a NC push-button to turn on the relay and a NO reset push-button, which resets capacitor C1 to V+ instead of resetting the power.

    The relay coil is connected from output to ground.
    Make sure to add a transient suppression diode (cathode to positive) across the relay coil.

    Delay On.gif
     
    Last edited: May 26, 2015
    Alec_t likes this.
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