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24VAC power supply

Discussion in 'Power Electronics' started by zuiko, May 26, 2011.

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  1. zuiko

    zuiko

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    May 26, 2011
    I have a large transformer that I use to regulate line voltage (120VAC) to 24VAC to power CCTV cameras. Draw is no more than few amps (@24VAC). Since it is just a transformer in a box the output voltage is typically high and varies according to the line voltage and load. I am wondering:

    1) What I could use to regulate the output to 24VAC.
    2) If there is something I could do to make the power supply more efficient (like convert it to a switching power supply?). The efficiency of the current set up is really poor. The transformer throws off a decent amount of waste heat (core runs at about 43C).
     
  2. davenn

    davenn Moderator

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    Sep 5, 2009
    yes you could use a switching supply and supply the cameras with 12VDC,
    that is also common practice

    Dave
     
  3. zuiko

    zuiko

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    0
    May 26, 2011
    The cameras are labelled 24VAC 60Hz. There is no indication they can accept 12VDC. Is there anything I could do to regulate the output voltage and improve efficiency without going to DC?

    I've seen the voltage regulator ICs but I don't know if they would work with this setup. I have also seen a resistor and diode in series but I don't want to end up with half wave DC or anything.
     
  4. poor mystic

    poor mystic

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    Apr 8, 2011
    Voltage regulator IC's are designed to work with DC and not AC, and of course no resistor/diode solution is possible. Therefore I think the 24VAC would need to be made from scratch by a regulated pure sine wave inverter.

    This project reminds me of the UPS supplies used in modern computing. UPS supplies provide a very reliable current at mains voltage and could solve the problems with changing voltages on your mains. If you ran your existing setup from a UPS it might help?

    The other part of your problem seems to be the power that is wasted in the transformer. You could measure the lost power, preferably using a proper 3-lead wattmeter, and find out the exact extent of your losses. You might find that although the transformer does get warm, losses are too small (perhaps 5W?) to be worth trying to reduce.
     
  5. davenn

    davenn Moderator

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    Sep 5, 2009
    Im not aware of any easy way to regulate an AC voltage

    All the cctv cameras I worth with, can all accept 12VDC and 24VAC
    would pay to look up the specs on the camera(s) you have and you may find the same thing :)

    the trick with them is the 24V AC goes through a bridge rectifier and them through voltage regulators to lower DC voltages. so the inputting of 12VDC isnt a problem.
    You loose ~ 1.2V through the rectifier and it then also goes through the regulators to supply the circuitry. And as a bonus, it doesnt even matter which way around the 12VDC is connected as the bridge rectifier sorts that out :)

    The reason they like to have the 24VAC ability, is to avoid some of the voltage drop on some of the longer cable runs to distant cameras from the PSU

    cheers
    Dave
     
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