Volt-control relay

Discussion in 'The Projects Forum' started by schmoofer, Aug 27, 2013.

  1. schmoofer

    Thread Starter New Member

    Aug 26, 2013
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    Hi all,

    I'm a novice with electronics so please bear with me. I'd like to build a voltage-controlled relay, specifically a sort of "gate" that will allow current to pass (turn on) above 12.8 V or so and close at voltages below that. I'm looking to use it in my motorbike to connect it with my phone charger so it will only charge when the bike is running and being charged by the alternator (at which point the terminals will read out a voltage above 12.8 V or so).

    From perusing the forum and the web, it seems a comparator (an LM311 or so) coupled with zener diodes will do the trick, but I'm dumbfounded when it comes to building it and choosing what resistors and capacitors to use. Can anyone offer advice or point me to a circuit diagram? I've looked at the diagram here. How valid is it? Thanks!

    David

    P.S. Can a moderator change the title to "Help with Voltage-Controlled Relay"? I'm getting the bug where I can't submit a thread unless the title is 20 characters or less. And I can't PM since I just joined =( Thanks!
     
  2. MaxHeadRoom

    Expert

    Jul 18, 2013
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    The LM311 should do the trick, I used one on a 240vdc mobile generator circuit to control the field above a certain voltage.
    You just need a scaled down version.
    Max.
     
  3. #12

    Expert

    Nov 30, 2010
    16,247
    6,744
    The title is close enough. We get 'em a lot worse.:rolleyes:
     
  4. schmoofer

    Thread Starter New Member

    Aug 26, 2013
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    Is it better to use a voltage reference or zener diodes to set the voltage threshold?
     
  5. MaxHeadRoom

    Expert

    Jul 18, 2013
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    That is the route I went, when the Zener goes into conduction, the LM311 flips over.
    Max.
     
  6. strantor

    AAC Fanatic!

    Oct 3, 2010
    4,302
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    Why do you need to do this? Are you worried about power flowing the wrong direction; worried about your phone trying to charge the bike?
     
  7. schmoofer

    Thread Starter New Member

    Aug 26, 2013
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    I'm more so worried that the phone or, rather the charger will drain the bike, which has happened. I also want to power a gps tracking device on my bike, so I want it to only charge when the bike's running and use it's own battery when it's not.
     
  8. MaxHeadRoom

    Expert

    Jul 18, 2013
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    The circuit in the link is very close to what I had, I used a zener detection for the flop over point.
    Max.
     
  9. piranha3380

    Member

    May 28, 2013
    30
    3
    just hook it up to your 12v headlight, ime powering my handle warmers from 1 headlight and my gps from the other, when ya start it it goos on and when ya stop the bike it goos off, no fancy stuff that could break or freek up your bike/Phone due to fibration, just be sure to hook it up to your city light and not your high beam,

    keeping it simple is usualy the better option

    (btw it wil not pull your lighting down as the provided amp are more than your gps and headlights need, done this on my own FJR 1300 never had any problems)
     
    wayneh and #12 like this.
  10. wayneh

    Expert

    Sep 9, 2010
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    Maybe I'm missing something but you could likely replace the relay with your load and power it directly through the transistor. If you need more current, just use a beefier transistor.

    (assuming you don't just use the headlight idea above)
     
  11. schmoofer

    Thread Starter New Member

    Aug 26, 2013
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    Yeah, I'm trying to avoid tapping any existing wires for the sake of cleanliness and as an exercise. I'll see if I can simulate the circuit in LTspice before I buy the materials.
     
  12. wayneh

    Expert

    Sep 9, 2010
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    Me, I'd reverse the comparator logic (switch inputs) and use an N-channel MOSFET (such AS IRF540N) positioned below the load, dropping the relay.
     
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