Relay power on- power off 1 second

Discussion in 'The Projects Forum' started by Jovyevane, Apr 6, 2018.

  1. Jovyevane

    Thread Starter New Member

    Apr 6, 2018
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    Hello everyone. I tried looking for a similar project, but I couldn't find him. I have an energized circuit with 12V dc. With a switch I open and close my power. I would like to energize a relay for about 1 second when I open the switch and when I close it. My relay is 12vdc.
    Can anyone help me with a very simple scheme? Thank you very much
     
  2. danadak

    Distinguished Member

    Mar 10, 2018
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  3. Jovyevane

    Thread Starter New Member

    Apr 6, 2018
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    Thanks for your response Dana. A timer relay is fine when I open the circuit, but when I close it of course the relay is no longer powered. I think I need a configuration with a capacitor that functions like a battery. I have a decent experience in the manufacture of PCBs. You can achieve this effect by using bjt?
     
  4. Jovyevane

    Thread Starter New Member

    Apr 6, 2018
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    Like this!
     
  5. AnalogKid

    AAC Fanatic!

    Aug 1, 2013
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    What is the resistance of the relay coil? With that you can calculate the capacitor size. Warning, it might be pretty large. For example:

    Relay coil resistance - 120 ohms
    Nominal relay coil current - 100 mA
    Dropout voltage - 6 V

    Approx capacitor size: 12,500 uF / 25 V

    ak
     
  6. danadak

    Distinguished Member

    Mar 10, 2018
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    You could always use a SPDT break before make switch to generate
    triggers to a 555 timer to generate the 1 sec on time and use the 555
    timer to drive the relay.

    Regards, Dana.
     
  7. Jovyevane

    Thread Starter New Member

    Apr 6, 2018
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    Thank you.The resistance value is 230 ohm
     
  8. AnalogKid

    AAC Fanatic!

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    Does the datasheet for the relay say what its dropout voltage is?

    ak
     
  9. Jovyevane

    Thread Starter New Member

    Apr 6, 2018
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    I'm not sure, but it seems 6v
     
  10. eetech00

    Senior Member

    Jun 8, 2013
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    HI

    Here's a optional circuit with a 555 Timer.
    I show a pushbutton but its just for the simulation and would actually be a switch contact.
    Probably could use a digital Lead/trail edge detector for the input but I didn't want to add a chip(s).

    Relay1SecOnOffTriggerTimer.png
     
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  11. AnalogKid

    AAC Fanatic!

    Aug 1, 2013
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    For this discussion, note that the capacitor charges from 0 V to 12 V through the relay coil when the switch is closed, and discharges through the coil from 12 v to 0 V when the switch is opened. I'll discuss everything in terms of charging, but these are symmetrical operations so everything applies equally to the discharge cycle.

    A first order approximation is that the capacitor voltage increases linearly rather than exponentially as the capacitor charges. We are interested only in the first 50% (12 V to 6 V) of the charge curve, and that is the more linear portion. To make my life easier, lets round up 230 ohms to 240 ohms (less than 5% change).
    Start: 12 V, 24o ohms >> i = 50 mA >> relay pulls in
    End: 6 V, 240 ohms >> i = 25 mA >> relay drops out
    Therefore, the average capacitor current is 37.5 mA
    Charge voltage = 12 V - 6 V = 6 V

    Constant current capacitor equation:

    EC = it
    charge voltage change x capacitance = charge current x time
    6 x C = 0.0375 x 1 (sec)
    C = 0.0375 / 6 = 0.00625

    C = 6800 uF - ish

    This should be pretty close. 6800 is over 8% greater than 6250, which offsets the 5% adjustment we made to the coil resistance.

    ak
     
    Last edited: Apr 7, 2018
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  12. Jovyevane

    Thread Starter New Member

    Apr 6, 2018
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    Works perfectly. I used an electrolytic capacitor from 2200 uF and 16v . Thanks!
     
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