Transistor or other Device that's "Always On"

Discussion in 'The Projects Forum' started by Dan-O, Nov 15, 2009.

  1. Dan-O

    Thread Starter Active Member

    Jul 26, 2009
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    Hi folks, forgive my ignorance (amateur at work)
    Is there a transistor or simple circuit that is always on until acted upon?
    Basically I want to simulate the NC contacts of a relay.
    I'm trying to redesign a circuit I've created to not use relays.
    Thanks for the help,
    Dan
     
  2. SgtWookie

    Expert

    Jul 17, 2007
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    An N-channel power MOSFET with it's gate tied to a voltage 10v higher than the source terminal would do it. Then you could short the gate to the source terminal to turn it off.

    What are you considering powering?

    It would help if you posted a schematic of what you're considering doing, or at least describing it more fully.
     
  3. Dan-O

    Thread Starter Active Member

    Jul 26, 2009
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    Actually it's for a push button start project I've been toying with on and off.
    The output of the circuit would be +12v powering the OEM accessory relay.
    I want the one part of the circuit to be +12v powering the OEM accessory relay all the time but shut off when there's power on the start relay circuit.

    In simplest terms:
    I want to simulate the NC contacts of a relay-->When + power is applied, the contacts are open. Remove power and the contacts close again.
     
  4. SgtWookie

    Expert

    Jul 17, 2007
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    You might want to look at this thread first:
    http://forum.allaboutcircuits.com/showthread.php?t=9058

    It's a complete push-button start system.

    However, it's basically just for show cars; it has no provisions for a traditional ignition key anymore. You would need to use some method to disable the vehicle so no one could steal it.
     
  5. Dan-O

    Thread Starter Active Member

    Jul 26, 2009
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    Yup, that was my inspiration months ago.
    I just need a solid state replacement for the NC contacts of a relay.
    I'm developing a different circuit.

    My current setup:
    Press and hold button and the ignition powers up after .2 seconds and after 1 second, the starter relay engages.
    Press and hold the button while it's running and after .2 seconds the car shuts off.
    One button does it all.
    I was looking to get rid of the relays and make the circuit more reliable.
    I have the security issues already worked out. I have a passive remote alarm that disables the system if the fob is out of range.
    Basically I'm copying the OEM installs with the smart Fob and pushbutton.
     
  6. SgtWookie

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    Jul 17, 2007
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    Last edited: Nov 15, 2009
  7. Dan-O

    Thread Starter Active Member

    Jul 26, 2009
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    I appreciate the input and I thank you for taking the time to help me.

    I'm trying to avoid relays and make this more solid state.
    I currently have the power feed to the accessories fed through the second set of NC contacts on the start relay.
    When the relay is engaged, the contacts open and remove power to the accessories. It is simple and effective but I'm trying to eliminate relays (I use 4 per setup) for reliability and to lower my assembly costs.
    I've had tons of interest in assembling these for friends and am certainly not looking to retire off the small $$ I'd be making (not counting labor) on each unit.
    Like I said, I'm on prototype #3 now and am working on #4 to also include delayed accessory power that keeps the accessories powered on when you turn the key off and shuts the accessory power off when you open a door.
     
  8. eblc1388

    Senior Member

    Nov 28, 2008
    1,542
    102
    The type of relay you are looking for is called MOS relays. Some of them do have NC contacts without requiring any external voltage provided to the relay. In simple terms, the contact conducts and cut off when voltage is applied to the "relay coil" which is actually a type of LED.

    Search Google for "MOS relay" and you will get lots of hits.
     
  9. Dan-O

    Thread Starter Active Member

    Jul 26, 2009
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    OK I'll check that out.
    Thanks for the input.

    Am I nuts for trying to reinvent the wheel by wanting to eliminate mechanical relays? Technically my car already has relays in it that I'm controlling with my circuit. So technically I do still have relays in the car.

    My design does work with the relays just fine.
    I'm going to connect it to my bench supply and leave it on for a week to see if it skips a beat.

    Then off to V4.
    :D
     
  10. CDRIVE

    Senior Member

    Jul 1, 2008
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    Sgt., just curious why you chose a P channel? I just know I'm missing something obvious.
     
  11. bluebrakes

    Active Member

    Oct 17, 2009
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  12. CDRIVE

    Senior Member

    Jul 1, 2008
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  13. bluebrakes

    Active Member

    Oct 17, 2009
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    nothing to do with your question but more to do with the original thread question. It's a good explaination as to what you can do to change levels.
     
  14. CDRIVE

    Senior Member

    Jul 1, 2008
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    Oh, OK. Thanks for clarifying. ;)
     
  15. Wendy

    Moderator

    Mar 24, 2008
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    Having looked at that issue I think I can answer some of it. A series pass MOSFET for a positive power supply almost has to be a P-channel. If you know a different way I'd love to hear it. Been designing several power supplies that are positive voltages.
     
  16. SgtWookie

    Expert

    Jul 17, 2007
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    You could do it with an N-channel, but you'd need a high-side gate driver.
     
  17. CDRIVE

    Senior Member

    Jul 1, 2008
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    I would agree if Sgt.Wookie's FET was used as a pass element, but it's not. It's driving a relay coil which can be put in the Drain node of a N channel.
     
  18. CDRIVE

    Senior Member

    Jul 1, 2008
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    I was thinking of an N channel wired like this.
     
  19. Dan-O

    Thread Starter Active Member

    Jul 26, 2009
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    I already have a relay in place.
    I'm trying to eliminate the relay and simulate the NC contacts.
     
  20. CDRIVE

    Senior Member

    Jul 1, 2008
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    Yes, I realize that. My comment referred to the need for a P Chan FET to drive the relay in Sgt. Wookie's circuit. That circuit has the FET configured as a Pass Transistor and can drive your load directly. Of course your FET must be rated to handle the current that you're going to draw.
     
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