Aim
To draw the characteristic curve of a Zener diode and to determine its reverse breakdown voltage.
Apparatus
A Zener diode (with small reverse breakdown voltage of about 6 volts), [i.e., Vz = 6 V], a ten volt battery, a high resistance rheostat, two 0-10 V voltmeter, one 0-100 mA ammeter, one 20 Ω. resistance, one way key, connecting wires.
Theory
Zener Diode. It is a semiconductor diode, in which the n-type and the p-type sections are heavily doped, i.e., they have more percentage of impurity atoms. This heavy doping results in a low value of reverse breakdown voltage (BVR). This value can be controlled during manufacture.
The reverse breakdown voltage of a Zener diode, is called Zener voltage (Vz). The reverse current that results after the breakdown, is called Zener current (Iz).
to-draw-the-characteristic-curve-of-a-zener-diode-and-to-determine-its-reverse-breakdown-voltage-1
At breakdown, increase of VI increases II by large amount, so that V0 = VI– RI II becomes constant. This constant value of V0 which is the reverse breakdown voltage, is called Zener voltage.
Formula used
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Diagram
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Procedure
  1. Arrange apparatus as shown in circuit diagram.
  2. Make all connections neat, clean and tight.
  3. Note least count and zero error of voltmeters and milli-ammeter. (micro-ammeter)
  4. Bring moving contact of potential divider (rheostat) near negative end and insert the key K. Voltmeters and milli-ammeter will give zero reading.
  5. Move the contact a little towards positive end to apply some reverse bias voltage (VI). Milli-ammeter reading remains zero. Voltmeters give equal readings.
    [i.e.,V0 =VI ... II = 0 (eqn. 2)]
  6. As VI is further increased, II starts flowing. Then V0 becomes less than VINote the values of Vp II and V0.
  7. Go on increasing VI in small steps of 0.5 V. Note corresponding values of II and V0 which will be found to have increased.
  8. As VI is made more and more, II and V0 are found to increase. Values are noted.
  9. At one stage, as VI is increased further, II increases by large amount and V0 does not increase. This is reverse breakdown situation.
  10. As VI is increased further, only II is found to increase, V0 becomes constant. Note values of VI, II and V0.
  11. Increase V1 to a value of 10 V, noting corresponding values.
  12. Record your observations as given ahead.
Observations
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to-draw-the-characteristic-curve-of-a-zener-diode-and-to-determine-its-reverse-breakdown-voltage-5
Calculations
Plot a graph between input voltage VI (column 2) and input current (column 3), taking VI along X-axis and IIalong Y-axis.
to-draw-the-characteristic-curve-of-a-zener-diode-and-to-determine-its-reverse-breakdown-voltage-6
Result
The reverse’breakdown voltage of given Zener diode is
Precautions
  1. All connection should be neat, clean and tight.
  2. Key should be used in circuit and opened when the circuit is not being used.