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Help with simple circuit

Discussion in 'Electronic Design' started by [email protected], Oct 29, 2006.

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  1. Guest

    Hi. Please take a look at this circuit here:

    This is from a toy/ornament that connects to one node of a xmas light
    string and shows 4 mini lamps through some colored plastic lenses.

    It measures about 11.2V AC at its inputs when connected. Furthermore,
    each of the 4 mini lamps seem to have a 2.35V DC drop across them. I
    just want to understand what those Zener Diodes do exactly? I don't
    have the codes of which type they are (maybe IN4148 switching diodes?).
    Can I assume that they are just there to lower the voltage that the
    lamps see by 1V or so? Finally, with a multi-meter how can I measure
    the current that this circuit is drawing?

    Also, if I apply a 12V DC source to those AC labelled inputs, it seems
    that it is too much since the Zener diodes get really hot and I start
    smelling a slight bit of smoke smell. Why is that?

    And finally, I want to do the following. I want to take say a 20V DC
    source and attach to this to make it work (without burning the lamps
    prematurely). Hence, I guess I should first get rid of the 4 bottom
    diode/bridge (IN4005 I believe). Then, can put a resistor in series
    (1/4 W or 1/2 W and where in this circuit) so that I can bring the 20V
    DC down to about say 8V DC to run this? I imagine that if I had how
    many AMPS the circuit draws, I can then use OHMs lamp to calculate the
    R value needed to lower the voltage for this.

    Thanks for any input/education you more advanced electronic guys can
    provide to me.
  2. There's no current limiting in that circuit if you hook it to a voltage
    source. My guess is that it relies on the series resistance in the rest
    of the string. You probably should measure the current through it while
    it's in its natural location, and use series resistance to limit it to
    that level or less, whatever else you do with it.
  3. JM

    JM Guest

    The zeners appear to be there to keep the remaining pair of lamps lit if a
    lamp burns out.

  4. Why do you call them zener diodes?
  5. John Fields

    John Fields Guest

  6. neon


    Oct 21, 2006
    are sure that those are indeed zeners or just plain limiting resistors if they smoke i take my bit on resostors not zeners or even diodes. measure the current as follows open one light set the meter into current mode pay attention of polarity and measure the current. i think youi may have LED string not lights in that case the to increase the voltage is simple assume 10ma as a safe current then 20[ -] the voltage [2.5] drop of light devided by .01ma gives you the resistor value required to safely operate it. the resistor power is 20-2.5 *.01= .2w or 1/4w resistors. if you tie in series then add 2.35 *LEDS and add one resistor to give you .010amps need more help ask again?
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