# How do I find the value of the resistor with the zener diode on the image below?

#### Deshan Wolfey

Joined Aug 23, 2018
3

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#### ericgibbs

Joined Jan 29, 2010
18,209
hi DW.
Welcome to AAC.
E

#### Deshan Wolfey

Joined Aug 23, 2018
3
I attached the picture of what i tried

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#### WBahn

Joined Mar 31, 2012
29,469
So if you use a 450 Ω resistor and the load resistance is such that it is drawing 100 mA (when 7.5 V is across it), how much current is flowing in the zener diode? Is this enough to satisfy the knee current requirement in order for the diode to regulate the voltage?

#### Deshan Wolfey

Joined Aug 23, 2018
3
So if you use a 450 Ω resistor and the load resistance is such that it is drawing 100 mA (when 7.5 V is across it), how much current is flowing in the zener diode? Is this enough to satisfy the knee current requirement in order for the diode to regulate the voltage?
Current through Zener diode will be 0.09 A

#### ericgibbs

Joined Jan 29, 2010
18,209
hi DW,
A simple error check 12v/450R = 26.6'mA maximum current draw , so 450R cannot be correct.
E

#### KeithWalker

Joined Jul 10, 2017
2,914
hi DW,
A simple error check 12v/450R = 26.6'mA maximum current draw , so 450R cannot be correct.
E
Look at the problem logically:
What you know:
The Voltage across RS equals the battery voltage less the Zener Voltage: YRS = 12 - VZen
The current through RS equals the load current plus the Zener current: RS = IRL + IZen
The maximum current through RS equals the maximum load current plus the minimum knee current of the zener: IRS max = IRL max + IZen min
Therefore:
The resistance of RS equals the voltage across it divided by the maximum current through it: RS = VRS / IRS max.

#### ericgibbs

Joined Jan 29, 2010
18,209
hi Keith,
As it is an Homework question we do not normally supply the answer to the problem.
Just hints and clues.

Eric

#### ArakelTheDragon

Joined Nov 18, 2016
1,362
You need to consider will the current increase or decrease when changing the potentiometer resistance.

#### WBahn

Joined Mar 31, 2012
29,469
Current through Zener diode will be 0.09 A
Could you please show/explain how you arrived at that? Without seeing your work, we can't even begin to guess where your reasoning is going astray.

Let's assume that you are correct and that, when the load current is 100 mA, the zener diode current is 90 mA. How much total current is flowing in Rs? What is the voltage drop across Rs if it is 450 Ω and that much current is flowing in it? Is that consistent with the problem?