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Power-Up Latching Relay

Discussion in 'Electronic Projects' started by JBrock, Feb 15, 2009.

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  1. JBrock

    JBrock Member

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    Location:
    Carlsbad CA
    The purpose of the circuit is to create a switch which cannot change position after power is applied to the circuit.
    The position of the switch must be set before power is applied. After power-up, the position of the switch is locked.
    The schematic shows a SPDT "RUN/OFF" switch on the left, and the coil of a DPDT relay on the right.
    If the switch is set to RUN, the relay will actuate roughly one-half second after power-up. If the switch is set to OFF, the relay will not actuate.
    In either case, the switch will have no further effect after the power-up initializing period.
    This circuit has been built and tested. It currently works reliably in a computer with a second hard drive. It prevents power from being applied to or removed from the hard drive while the computer is on.
    The switch contacts of the relay control both +5V and +12V to the power input of the hard drive. There is an LED indicator on each of these rails to confirm power.
    The heart of the circuit is a set-reset flip-flop constructed from gates B and C of a quad nand gate.
    The timing diagram shows waveforms at points labeled in the schematic for the two conditions RUN and OFF.
    The assembly diagram shows the top side with components in the top half of the picture and the bottom side with wiring in the bottom half of the picture.
    This design is completely original by yours truly and it is free use (at your own risk of course!)
    Comments and criticism welcome.
     

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    Last edited: Feb 15, 2009
  2. killivolt

    killivolt Well-Known Member

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    Jbrock, Thank you.

    I may want to use this someway to bring a relay in to power an outlet driven of a control panel for a light circuit.

    I want this outlet to have power only when people are at the University. When my operators close up the control room the will shut down the control panel thus powering down the relay.

    kv
     
  3. Chaerl

    Chaerl New Member

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    This is great. I just want to add to further robust the design. Additional protection for the BJT is needed. Either a free-wheel diode // in the relay coil or a TVS // in the CE of the BJT.
     
  4. dave

    Dave New Member

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  5. alphadog

    alphadog Banned

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    Thank you!
    Could someone please tell me how do you implement the logic gates using Mosfets or BJTs?
     
  6. Chaerl

    Chaerl New Member

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    Is this what you are looking for?

    HTML:
    http://hyperphysics.phy-astr.gsu.edu/HBASE/electronic/trangate.html
     
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