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Reducing an Input Voltage

Discussion in 'Electronic Basics' started by Peter, Jul 28, 2003.

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  1. Peter

    Peter Guest

    Newbie question. I'm running a small timer circuit (555, CD4017 and
    some output Mosfets) from a 35v AC supply to sequence Christmas
    lights. At present I reduce the input to 15V DC as follows:

    35V AC (the Christmas lights input supply - typical Downunder outdoor
    lights).
    Bridge rectifier
    330uF smoothing cap
    2.2k 2W dropping resistor
    15V 5W zener
    100uF smoothing cap
    555 and 4017 (small cap, high resistors on 555 to reduce current
    drain).

    All of the above values were chosen by trial and error, the main
    criteria being
    - it would work (it does)
    - it would be as small as possible (eg. 330uF rather than 1000uF).

    I saw another posting referring to the use of a voltage divider
    resistor pair to reduce the voltage, with the zener in parallel with
    the grounded resistor.

    Would that allow a lower wattage zener to be used?
    Would it allow a lower wattage dropping resistor(s) to be used?
    Would it be a "better" design?
    Any other suggestions on minor improvements? (Still keeping it
    simple!)
     
  2. Baphomet

    Baphomet Guest

    I saw another posting referring to the use of a voltage divider
    resistor pair to reduce the voltage, with the zener in parallel with
    the grounded resistor.

    Would that allow a lower wattage zener to be used?

    YES (no yelling intended. Fred Abase got on my case for using HTML tags)

    Would it allow a lower wattage dropping resistor(s) to be used?

    NO

    Would it be a "better" design?

    DEPENDS upon what you mean by better. It would allow a lower wattage zener
    to be used at the expense of the voltage range of regulation. You would
    probably save a couple of cents on a lower wattage zener, or perhaps, you
    have a 15v. zener lying around but it's stand alone wattage is insufficient
    to the task, then using resistive shunting would be the way to go. If this
    isn't a consideration, I would go with an unshunted higher wattage zener and
    use one less resistor.

    Any other suggestions on minor improvements? (Still keeping it
    simple!)
     
  3. Bill Bowden

    Bill Bowden Guest

    The 2.2K resistor is only dissipating 0.56 watts, so you could
    use a 1 watt size, and the zener worst case current is 16mA,
    so you could use a 1/2 or 1 watt size.

    Also, the 330uF cap can be much smaller, at 20uF for 6 volts
    ripple, or 50uF for about 2 volts ripple. And the cap across
    the zener could be smaller at maybe 1uF since all it does
    is bypass the ICs.

    But since you have it working, might as well leave it as is.


    -Bill
     
  4. Rich Grise

    Rich Grise Guest

    (Peter) wrote in message
    ....
    It depends on what your intent is. If the goal is to light/
    sequence some lights, then if it ain't broke, don't fix it. If
    the point is to futz around with stuff to learn stuff, then
    not only go for it and see what happens, but try an LM317 or
    7815 or something as well. :)

    Have Fun!
    Rich
     
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