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  1. #1
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    Changeover relay circuit standby power supply

    I am looking for a circuit for one of mine typical application. I have transformer power supply with 230VAC/DC input with 12V /1A and 5V/500mA output.

    Now I wanted to introduce another stand by power supply from battery which gives 12V supply & acts like standby.

    My end device will work on either AC/DC power supply. Can let me know how it can be internally combine to form common source with protection circuit.

    I would like to use recommend use of relay so it take care of such issue.

    i wanted to monitor weather its working on AC supply or dc supply. and its states judge by arduino board , display the state in serial monitor
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  2. #2
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    Re: Changeover relay circuit standby power supply

    Connect the relay coil across the input side of the regulator, using a series resistor if the voltage is too high. Then, switch your output using the relay contacts between the regulator output and the battery. When powered, the regulator gives you the output, if the relay de-energizes the contacts swap over and the battery produces the output.

    You can monitor the output voltage using the Arduino ADC, if you want to tell whether AC or the battery is producing it, either use a second set of relay contacts if it has them, or use the ADC to monitor the voltage at the input of the regulator. If the output voltage is higher than the input voltage it means you are on battery power.

    Brian.
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  3. #3
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    Re: Changeover relay circuit standby power supply

    i need right Minimum ckt to make it work.i could not able to imgine ckt here. Priciple i would use DPDT /DPST relay . how to enegersie coil voltage in this case



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  4. #4
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    Re: Changeover relay circuit standby power supply

    The coil is powered directly from the rectified AC so it de-activates as soon as the power fails. We have no idea what voltage comes from your transformer or what the relay coil is rated at so if necessary, use a resistor to drop the voltage to the coil.
    Use the relay contacts to switch the output between battery and regulator output. So it works like this:

    1. with AC present - the relay activates and the regulator connects through the relay contacts to the output.
    2. if AC is removed - the relay de-activates and the battery is connected to the output.

    The Arduino does not control the relay. You want to monitor the voltages so use any Arduino 'sketch' example for measuring voltage and connect it to the output. Note that the Arduino probably has a maximum input (measurable) voltage of 5V but your circuit produces 12V so you need to add two resistors as a potential divider to drop the voltage. Scale their values so 12V in gives 5V out or as near as you can get, for example 10K to 12V then 6.8K to ground will give ~4.9V at their junction. If you need an exact reading with those values, multiply the result in the program by 1.0205 to raise 4.9V to 5V.

    Brian.
    PLEASE - no friends requests or private emails, I simply don't have time to reply to them all.
    It's better to share your questions and answers on Edaboard so we can all benefit from each others experiences.



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