Voltage Triggered Switch

Discussion in 'The Projects Forum' started by Snowwake4553, Oct 3, 2012.

  1. Snowwake4553

    Thread Starter New Member

    Oct 3, 2012
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    Hello Everyone,

    I am trying to set up a smart box that allows power to pass from the battery to a controller once it senses that there is a second power source turned out. I was thinking about using a comparator to do this, but I am not very experienced with these.

    Any suggestions?

    Thanks,
    Anthony
     
  2. wayneh

    Expert

    Sep 9, 2010
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    A comparator does sound like the right choice, at least for detecting what is going on. You may need the comparator to control a transistor or and/or relay to do the switching.

    The LM339 (quad) comparator is very common. Read the datasheet and perhaps it will trigger some ideas.

    It would help if you could sketch out your plan, with as much detail as possible about voltages, currents, power loads, etc.
     
  3. Snowwake4553

    Thread Starter New Member

    Oct 3, 2012
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    The other idea was to use a Schmitt Trigger, with 2 emitter coupled transistors.

    The easiest way to explain what I am trying to do is:
    I want a smart box that senses an input voltage, as soon it passes a threshold voltage of 11V, it will pass on the voltage, if it is below the 11V, it wont pass anything. Its basically a switch that is triggered by the voltage change from below 11V to above 11V.
     
  4. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    Are you working with a single supply, and you just want to connect it to the load when it exceeds 11V?
    What is the load?
    Is this a battery charger?
     
  5. crutschow

    Expert

    Mar 14, 2008
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    The problem can be feedback from one supply driving the other. Is that a concern? Or do you switch out the first supply when you switch in the second.
     
  6. wayneh

    Expert

    Sep 9, 2010
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    How much current at the threshold voltage? If you just need a few mA, a comparator will do it. If you need lots of amps to pass, you might need the comparator to switch a MOSFET or a relay.
     
  7. Snowwake4553

    Thread Starter New Member

    Oct 3, 2012
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    0
    It is not a battery charger, its supposed to be a smart box that allows voltage to pass from a 12 battery to a control box once a second voltage is sensed.
     
  8. wayneh

    Expert

    Sep 9, 2010
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    Voltage doesn't pass, current does. In the water analogy, voltage is like the height of the water tower. It's potential. But with no pipe out, there's no flow.

    Again, how much current needs to pass from your smart box to the "control box"?
     
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