Zener diode - Practical Problem 1

Electricity and Magnetism Level 4

Problem statement:

A circuit is given to you and certain components are unknown within this circuit. You have decided to do a simple test with an ammeter to measure the amount of current passing through the entire circuit, while changing the input voltage between 0 - 24 Vdc.

Assume the following

Zener breakdown voltage is unknown
Zener forward voltage is 0.7V
Current represents an Ammeter connected in series with the source
There are two components with unknown characteristics.
Component 1: The zener breakdown voltage
Component 2: A resistor "R"

The following circuit is given

A tabulated representation of the measured data

A graphical representation of the measured data

Use the data provided and calculate:

Zener breakdown voltage (in Volts)
Resistance R (in ohm)
Relationship between current passing through zener and the input voltage (in mA/V)

5 and 5 and 10 5 and 3.8 and 20 5.5 and 5 and 10 5.5 and 3.8 and 20

1 solution

Brandon Louw
Jun 20, 2016

First Answer

The solution at finding the Zener breakdown voltage is by using the tabel given in the question.

It can be noted that each 2V increases the input current by 150mA
However from 20V and up the current changed to 20mA per increment, this is a sign that the zener is in its ON state

Knowing this, you can use normal ohm's law to calculate the voltage drop over the resistor by picking any voltage above 20V.

Vbd = Vdc - Vdrop = 20 - (10 * 1.5) = 5 V
or
Vbd = Vdc - Vdrop = 22 - (10 * 1.7) = 5 V

Second Answer

If the zener is off (thus open). It allows the designer to calculate the total resistance within the circuit using the following equation.

Rt = $\frac{2}{0.15}$
Rt = 13.33 ohm

Thus the single resistor R can be calculated as

Rt = 10 + $\frac{(15+R)*4}{15+R+4}$
13.33 - 10 = $\frac{(15+R)*4}{15+R+4}$
$\frac{(15+R)*4}{15+R+4}$ = 3.33
49.95 + 3.33R + 13.332 = 60 + 4R
3.282 = 0.67R
R = 4.9
R rounded to 5 ohm for simplicity

Third Answer

By observing that the current changes in the following graph

It can be noticed that 20mA is suddenly drawn and that every 2V increases the current by 20mA more. This is due to the zener diode working to regulate the voltage to 5V. Larger currents put more strain on the zener diode, therefore increasing the current drawn by the zener diode, to keep the current passing through to the resistor branch constant.

Hope you enjoy the problem, please let me know if you find issues within my wording etc.

The answers are simulated and 100% correct.

Brandon Louw - 4 years, 12 months ago

Didn't understand what we should find in the 3rd part.. P.S. : got it correct due to the 1st and 2nd part

Vignesh S - 4 years, 11 months ago

The amount of current drawn by the zener as the input voltage varies.

Brandon Louw - 4 years, 11 months ago

Zener diode - Practical Problem 1

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