Zener diode : Calculating the series resistance

[n.easwaranand]

Homework Statement

: [/B]
Given a voltage regulator with 6.8V Zener diode, input voltage range of 15-20V and load current range 5mA-20mA. Calculate the series resistance R for the regulator.

Homework Equations

: [/B]
Applying KVL and no load situation, we get R = (V - Vz)/Iz where V is the input voltage,Vz is the zener voltage and Iz is the maximum current determined by the power dissipation.

The Attempt at a Solution

: [/B]
At first glance , the question seems to be simple. However, looking at the given values, the problem seems to be more trickier. Maximum and minimum currents for Zener diode is not given in the problem. Let us solve by assuming the Zener diode as ideal where minimum current is zero.
Izmax = (Vmax - Vz)/R - ILmin
Izmin = (Vmin-Vz)/R - ILmax
where Vmax = 20V, Vmin = 15V, ILmax = 20mA, ILmin=5mA and Vz = 6.8V
These are the only equations which I can think of to solve this problem. Assuming Izmin = 0, I get R=410ohms which is not the right answer.

 

[NascentOxygen]

Izmin = (Vmax-Vz)/R - ILmax

This formula doesn't look quite right. Can you explain what it is saying?

What does your textbook give as their answer?

http://www.imageshack.com/a/img109/4666/holly1756.gif

 

[n.easwaranand]

This formula doesn't look quite right. Can you explain what it is saying?

What does your textbook give as their answer?

http://www.imageshack.com/a/img109/4666/holly1756.gif

 

[CWatters]

I also got 410R. Do you have one in your box of assorted resistors :-)

 

[NascentOxygen]

I can't read pdfs (small tablet).

410Ω looks okay, it's the upper bound.

http://www.imageshack.com/a/img109/4666/holly1756.gif

 

[CWatters]

Do you have one in your box of assorted resistors :-)

In case it wasn't obvious this was a hint for why the answer might be 390 rather than 410.