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Low cost mains power supply

Discussion in 'Electronic Design' started by Nomen Nescio, Apr 8, 2013.

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  1. Nomen Nescio

    Nomen Nescio Guest

    In most DIY stores you can find these cheap Chinese digital timers to
    turn you lamps or appliances on at specified time. The thing I'm
    wondering is, how do they power the digital circuitry from
    110VAC in a device costing only $5-$10? Are they using a transformer or
    are they directly powering the digital circuitry from the AC voltage?
     
  2. Stef

    Stef Guest

    In comp.arch.embedded,
    Google for [capacitor dropper] and you will find lot's of examples.
    Efficiency is not that bad, power factor is terrible.

    And keep in mind that this type of circuit is not isolated, so your 'low
    voltage' side is live AC!
     
  3. Jasen Betts

    Jasen Betts Guest

    I've got one here it has a capacitive dropper and a tiny 4mAh NiCd cell (I
    assume NiCd because the charge rate printed on it is C/10) the AC switch
    is a 48V relay and there's a TO92 device which probably switches it.
    image in ABSE
     
  4. Robert Macy

    Robert Macy Guest

    I one time needed a super efficient 5Vdc [allowed to be directly
    connected to the mains] power supply but had NO room for the size of
    caps needed for the normal drop stage. So, I designed up a small
    circuit that connectd the mains to the rectifier diode starting at
    zero crossover and DISCONNECTING when the mains input went above
    around 7V and started to take the 5 Vdc output voltage up to 5.1, or
    so. thus I had efficiency AND regulation.
     
  5. John S

    John S Guest

    The mains voltage rises to about 7V input (5V output) in about 100us and
    to about 7.1V input (5.1V output)in 111us. So you have about 11us to
    charge your storage capacitor. Then, the storage cap must hold up the 5v
    for about 16ms until the next rise (this assumes 170V peak, 60Hz,
    half-wave).

    I must be missing something very important and I would appreciate your
    help in understanding what it might be.

    Many thanks,
    John
     
  6. Oliver Betz

    Oliver Betz Guest

    I consider it useful to mention mains voltage (and frequency) when
    presenting such calculations.
    I also think that "7.1V" is somewhat tight.

    The Harris HV-2405E used this principle, of course with wider
    thresholds.

    Oliver
     
  7. Arlet Ottens

    Arlet Ottens Guest

    They were mentioned (170V peak, 60Hz, half-wave) in the part that you
    snipped.
    And of course, it all depends on the current consumption. With very low
    currents used, the cap would only discharge slowly.
     
  8. Oliver Betz

    Oliver Betz Guest

    correct, sorry!

    Oliver
     
  9. rickman

    rickman Guest

    It may not be isolated, but it can be current limited and so not lethal.
     
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