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Homework Help: Power dissipated in R (zener diode circuit)

  1. Aug 17, 2010 #1
    1. The problem statement, all variables and given/known data

    A zener diode runs off 10V and supplies 5V to a load that draws 100mA. The minumum zener current = 5mA. R = 47Ω What is the dissipation in the resistor if the output is short-circuited?

    2. Relevant equations


    3. The attempt at a solution

    Using R = V - Vz / Izmin + Iload:

    I said Iload = 0 (since the output is short-circuited) then:

    R = V - Vz / Izmin + 0
    = 10 - 5 / 0.005
    = 1000Ω

    Power of R = (V - Vz)^2 / R
    = (10 - 5)^2 / 1000
    = 0.025W

    But the correct answer was 2.13W. Could someone please tell me what I did wrong?
  2. jcsd
  3. Aug 17, 2010 #2
    I see two mistakes offhand. The first is you are confusing load with resistance. A short circuit may have 0 ohms of resistance which is an infinite load (theoretically).

    The second mistake is that if the load voltage drops below 5 V, the zener stops conducting and is out of the picture.

    The problem is much simpler than it seems. Draw it out and see if that helps.
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