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Homework Help: Zener diode ballast resistor question

  1. Jul 17, 2010 #1
    1. The problem statement, all variables and given/known data
    Determine the value of the ballast resistor is satisfactory if a 5Volt 0.5 watt zener diode is used in the circuit

    sorry cant upload a picture but basically

    VS = 15v min to 20v max

    R ballast = 90 ohms (series with zener)

    and R load (in parralel with Zener) is 50 ohms

    2. Relevant equations

    max zener current = watts/voltage 0.5/5v= 100Ma

    now im getting a bit stuck i keep getting values of 100ohms at min Vs and 150 ohms at max Vs.

    will update post later running out of time at the minute

    thanks guys

    3. The attempt at a solution
  2. jcsd
  3. Jul 17, 2010 #2


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    Staff Emeritus
    Science Advisor
    Homework Helper

    Looks good, I agree with your current and resistor values. So, is the 90Ω sufficient?

    If you're unsure, here is a hint: what are the current and power dissipated in the diode if 90Ω is used for the ballast resistor?
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