Zener diode ballast resistor question

[tommoturbo]

Homework Statement

Determine the value of the ballast resistor is satisfactory if a 5Volt 0.5 watt zener diode is used in the circuit

sorry can't 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

Homework Equations

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

now I am 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

The Attempt at a Solution

 

[Redbelly98]

Homework Equations

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

now I am 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

The Attempt at a Solution

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?