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Switched-Mode Power Supplies

  1. Apr 9, 2015 #1

    I am interested in designing a power supply that can accept 110V or 220V at 60Hz as input, and can provide an adjustable DC output of 0-20V.

    Here is the thing: I know I can use a mechanical switch so that my supply can accommodate either 110 or 220V, but is there a way to do so without using such a switch?

    I believe there must be a way to do so. My belief is motivated by an observation I made looking at my laptop adapter. My laptop adapter accepts 100-240V AC and provides a fixed 19V DC output, yet, it does this automatically without the need for a mechanical switch or any interference from the user.

    I am tempted to think that this property is inherent to Switched-Mode Power Supplies. My knowledge about such supplies is rather limited. I read wikipedia articles about such supplies, but I would be delighted to have a good source.

    To conclude: How can my supply accept 110V or 220V AC and still provide an adjustable DC output from 0 to 20V without requiring any human intervention?

    Take care
  2. jcsd
  3. Apr 9, 2015 #2


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    How much current do you want? I used to work on capacitor and inductor analyzers that had a leakage test function for caps. You dialed in the voltage you wanted anywhere up to 1000 volts. Obviously it was a low current supply but with a single switcher they could vary the voltage anywhere between close to zero and 1000. With that kind of range it should not be difficult for you to allow for the input voltage to move around when you only want up to 20 volts.
  4. Apr 9, 2015 #3
    How much output current do you need?
  5. Apr 9, 2015 #4


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    I'd guess that you could use a 220V transformer having two secondaries. If the voltage on one secondary is sensed as being half what is needed, series it with the other. Primary current rating to suit the highest current operation, viz., 110V.

    Switched mode supplies probably do this a bit differently, the changes being made on their high-frequency side.
  6. Apr 11, 2015 #5


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