Experiment - 1 Electronics Laboratory – PH 3204 (Spring 2025) Solution

Aim : Study of Zener diode as a voltage regulator and use of IC 7805 voltage stabilizer..
Electronic Parts Required :
(i) Power supply : ± 15 V
(ii) RS = RC = 2.2 KΩ = 2 Nos
(iii) Load resistance = RL = 1.0 kΩ Potentio-meter, 1.0 W
(iv) Zener diode = 1 No with VZ = 2.7 or 3.9 or 5.1 V
(v) LM7805 IC = 1 No
(vi) Breadboard = 1 No
(vii) Two DT-830D multi-meters for current measurements. One 8007 multimeter with probe for Voltage measurements.
(viii) single stand wires = 6 Nos

Part A : Zener Diode as Voltage Regulators:
Circuit Diagram :

A Zener diode of break down voltage Vz is reverse connected to an input voltage source Vi across a load resistance RL and a series resistor RS. The voltage across the Zener will remain steady at its break down voltage VZ for all the values of Zener current iZ as long as the current remains in the breakdown region. In the above circuit, the Zener diode maintains a constant output voltage V0 = VZ independent of variations in load resistance RL or the variation of input voltage Vi ( > VZ) so long as the diode remains in the breakdown region and the input voltage remains within a minimum and maximum voltage.
From the circuit diagram we can write iS = iZ + iL
Where, iS = 𝑉𝑖 −𝑉𝑧, and iL = 𝑉𝑜
𝑅𝑠 𝑅𝑙
Basically there are two types of regulations such as:
(i) Line Regulation :
In line regulation, series resistance, RS and load resistance, RL are fixed, only input voltage, Vin is varied. Output voltage, V0 remains the same at VZ and iL remains constant as long as the input voltage is maintained above a minimum value.
 iZ =  iS
Thus when load RL is fixed and input voltage Vi varies then Zener current iZ and total current iS change in such to maintain iL and hence V0 constant. Any change in Vi appears across the series limiting resistance RS.
Table – I [Maximum Input voltage should be 20 V]
Sl.
No RL (Ohm) Vi (Volts) iS
(mA) iZ
(mA) V0 (Volts)
1. 1K or 2.2K 0 V
2. CONSTANT -
" -
20. " 20 V
You can take more than 20 readings if you like to have smooth curve.
Graph –I
(i) Plot graph between iZ and iS to show that iZ = iS (Directly proportional)
(ii) Plot graph between Vin and Vout . Find out the breakdown voltage (VZ) of Zener diode.
(ii) Load Regulation :
In load regulation, input voltage, Vi remains constant and the load resistance, RL is varied. Output voltage remains same, as long as the load resistance is maintained above a minimum value. Since the voltage VZ across the Zener remains constant, iS is independent of load. Hence in this case
 iZ = -  iL
Table – II (without RC) : RC = A current limiting constant resistor [Maximum Input voltage should be 15 V ]
Sl.
No Vi (Volts) RL (Ohm) iL
(mA) iZ
(mA) V0 (Volts)
1. 15 V 0 Ω
2. CONSTANT -
" -
20. " 1K Ω
You can take more than 20 readings if you like to have smooth curve.
Graph –II
(i) Plot graph between IZ and IL to show that IZ = - IL (Inversely proportional)
(ii) Plot graph between Vout and RL . Find out the breakdown voltage (Vout = VZ) of Zener diode.
Table – III (with RC = 2.2 K Ω) : RC = A current limiting constant resistor
Sl.
No Vi (Volts) RL (Ohm) iL
(mA) iZ
(mA) V0 (Volts)
1. "
2. CONSTANT
"
20. "

Graph –III
(i) Plot graph between IZ and IL to show that IZ = - IL (Inversely proportional)
(ii) Plot graph between Vout and RL in the presence of RC. Find out the breakdown voltage (Vout = VZ) of Zener diode which was not possible in Table II because Load RL draw more current.
Part B : Voltage Regulator IC 7805:
For LM7805 the minimum input voltage is 7.2 V and maximum input voltage is 35 V to get a stable 5.0 V output voltage. The maximum current the load can draw is about 1 Amp.
Circuit Diagram :

(i) Line Regulation :
In line regulation, output constant current limiting series resistance, RC and load resistance, RL are fixed, only input voltage, Vin is varied. Output voltage, V0 remains the same at VZ and iL remains constant as long as the input voltage is maintained above a minimum value.
Table – I [ Maximum Input voltage should be 20 V ]
Sl.
No RC + RL (Ohm) Vi (Volts) il
(mA) V0 (Volts)
1. 2.2 K 0
2. CONSTANT -
" -
20. " 20
You can take more than 20 readings if you like to have smooth curve.
Graph –I
(i) Plot graph between Vin and Vout. Find Vmin input voltage for which Vout is constant to 5 V.
(ii) Load Regulation :
In load regulation, input voltage, Vi remains constant and the load resistance, RL is varied. Output voltage remains same irrespective of load.
Table – II [ Maximum Input voltage should be 15 V, RC = 2.2 KΩ ]
Sl.
No Vi (Volts) RL (Ohm) il
(mA) V0 (Volts)
1. 15 V 0 Ω
2. CONSTANT -
" -
20. " 1K Ω
You can take more than 20 readings if you like to have smooth curve.
Graph –II
(i) Plot graph between Vout and RL.

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