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HELP with normally open circuit

Discussion in 'General Electronics Discussion' started by oakpark, Mar 6, 2013.

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  1. oakpark

    oakpark

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    Mar 6, 2013
    Does such a component exist:

    <> Source Voltage

    <> Control (+5V)

    <> Destination Voltage

    In essence I want voltage to NOT flow from Source Voltage to Destination Voltage (circuit open) so long as 5V is applied to Control. The moment the Control voltage drops to 0V, I want Source Voltage to flow to Destination voltage.

    I am not trained in electronics and your help with finding such a component (or combination of components) is greatly appreciated!
     
  2. BobK

    BobK

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    1,688
    Jan 5, 2010
    Most relays have normally open and normally closed contacts. You would use the normally closed ones.

    Bob
     
  3. oakpark

    oakpark

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    Mar 6, 2013
    WOW! super fast answer. Is there a solid state relay for really small load (9-12V, 100ma) that you can point me to?

    What I am trying to do is provide a backup battery to an otherwise DC transformer fed circuit. If power goes out, I want to feed the circuit from my 9V battery. Normally the DC transformer will energize the relay and block voltage to the circuit from the battery BUT when the DC voltage ceases to exist, the relay opens and the battery supplies the voltage. Does this scheme make sense? If it does, can you point me to the right component?
     
  4. rebeltaz

    rebeltaz

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    Nov 22, 2012
    You can get a SPDT solid state relay from here:

    http://www.digikey.com/scripts/dkse...ewproducts=0&ptm=0&fid=0&quantity=0&PV130=239

    I don't think that simply using a relay to switch a power source for battery backup will be fast enough though. In the amount of time that it takes for the circuit to detect the power outage and switch the relay, the circuit will have lost power for a brief instant.

    I'm not sure of your circuit, but in my microcontroller circuits, I will usually add a memory capacitor (5.5v @ 0.1 to 0.5 farad) to the power input. This will keep the processor powered long enough for the source to switch to battery.
     
  5. davenn

    davenn Moderator

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    Sep 5, 2009
    Just a note ... something you need to understand

    current flows. voltage doesnt!! :)

    Dave
     
  6. oakpark

    oakpark

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    Mar 6, 2013
    Excellent. I did some Googling before reading your reply and arrived at the same conclusion. I ordered the part from Digikey and later read about adding a capacitor or a couple of capacitors to even out the spike. I need to do some reading about capacitors and how to calculate stuff like that. THANK YOU very much for answering my question so quickly.

    This is for an Arduino project. I read a bit more and discovered that some people are powering the Arduino with a LiPoly battery and continuously charge the battery. I found that the Arduino Pro board has the LiPo charging circuitry so I can charge the battery using that board. I am investigating that route which would alleviate the need to handle power switching between the wall-DC adapter and the battery because the battery will always be primary. Any thoughts on that idea are appreciated.
     
  7. BobK

    BobK

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    1,688
    Jan 5, 2010
    You can also just use diodes if you can arrange for the wall wart supply to be a little higher than the battery voltage. Just use a diode from each supply with the anode to the battery / supply + and the cathodes connected together to the power in of your board. That way, only the higher voltage will be supplying power, and when the wall supply drops below the battery voltage, it picks up seamlessly.

    Bob
     
  8. oakpark

    oakpark

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    Mar 6, 2013
    Do you mean like this:

    [​IMG]
     
    Last edited: Mar 6, 2013
  9. BobK

    BobK

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    1,688
    Jan 5, 2010
    Yep. As long as the 12V supply is on, no power will be drawn from the battery because the diode is reverse biased (cathode at higher voltage than anode). When the 12V disappers, the current starts flowing from the battery within nanoseconds.

    Bob
     
  10. oakpark

    oakpark

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    0
    Mar 6, 2013
    Thank you very much!
     
  11. oakpark

    oakpark

    6
    0
    Mar 6, 2013
    dully noted. current flows, voltage doesn't. thanks.
     
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