Experiment Five

Measurement and control lab ( 0620444)

Supervisor: Eng. Nessreen Al-Zboon

(Measurements)

Illumination measurement

Experiment (5)

5.1 photovoltaic cell Characteristics. 5.2 phototransistor Characteristics. 5.3 Light Intensity Detector. 5.4 Characteristics of PIN photodiode.

Experiment Five

- The figure (1) shows the arrangement of the light transducers . The transducers are

contained inside a cave and they are illuminated by a lamp that is located in a central

position.

Figure 1

5.1- The photovoltaic cell

figure 2

The figure 2 shows the basic construction of a semiconductor photovoltaic cell that

consists basically on a device with two silicon layers. A fine layer of P material is built

on a type substrate N.

When the light impacts on the union a positive potential difference appears on the side

N regarding the side P.

The output voltage depends on the magnitude of the light that impacts on the device

and is as maximum in the order of 0.5 V. With a load resistance connected to the

output, will pass a current.

The magnitude the current depends on the magnitude of the light that impacts on the

device and the surface of this.

Figure 3

Experiment Five

The figure 3 shows the arrangement of the circuit for The Trainer DL 2312HG.

The main characteristics are the following:

Voltage of open circuit (Insolated) 500mV

Current in short circuit (Insolated) 10mA

Wavelength peak of spectral response 840nm I.R

Time Response 10 s

Table 1

50

Figure 4

Connect the circuit as is shown in the figure 4.

With the connected source (ON) adjust the 1KΩ conductive plastic potentiometer for a zero

output voltage in the power amplifier.

Write down the output voltage of the photovoltaic cell:

(a) With your hand covering the box, that is to say, with the cell to dark.

(b) With the cell exposed to the environmental light (b).

Experiment Five

Now adjust the control of the 1KΩ potentiometer to increase the output voltage of the power

amplifier in steps of 1V.

In each step, write down the output voltage of the photovoltaic cell and record the values in the

table 2.

Lamp´s Filament Voltage (a) (b) 1 2 3 4 5 6

Voltage photovoltaic cell

Table 2

The readings will indicate an increase of output voltage of the photovoltaic cell as the intensity of

light increases. We cannot establish an exact proportionality between the output and the level of light, but the

exercise illustrates the basic characteristics of the device.

Experiment Five

5.2- The Phototransistor

The construction and basic circuit used are shown in the figure 1.

The unit is basically a three layers semiconductor device, NPN, with three terminals:

e(emitter), b (base) and c (collector).

Figure 1

The device differs to a transistor in that allows the light falls on the union collector-

base. The basic connection of the circuit is shown in the figure 1 where the collector is

connected to the positive of a DC source whit or without load resistance R. The

connection of the base is not used in this circuit but it is available for its use in other

circuits if you wish.

Although the light doesn’t impact on the device, a small current will exist due to the

couples hole-electron generated thermally and the output voltage of the circuit will be

lightly smaller that the source value due to the fall of voltage in the load resistance R.

When the light impacts on the union collector -base, increases the current pass. With

the base terminal in open circuit, the collector-base current it should go by the base-

emitter union, and from here the current flow is amplified by the action of a regular

transistor. In this way the unit is more sensitive than a photodiode.

Experiment Five

The output voltage of the circuit falls when increases the current and the output

voltage depends this way on the intensity of the light that impacts on the device.

Voutput = V - ICR

Where V: voltage applied

IC: Collector Current

R: Collector Load resistance Figure 2

The figure 2 shows the arrangement of the circuit.

The main characteristics of the device are the following:

Current of collector To dark 100 A

(VCE=5V) Set typical 3.5mA

Table 1

Figure 3

Connect the circuit of the figure 3 and place the control of the 100KΩ carbon track potentiometer

in position 2 so that the load resistance of the phototransistor is approximately 2KΩ.

With the source connected (ON), place the control of the 1kΩ conductive plastic potentiometer

so that the voltage is null to the power amplifier output.

Experiment Five

Write the collector output voltage of the phototransistor:

(a) With your hand covering the transparent box (a), and (b) With the phototransistor exposed to the environmental light (b).

Now increase the output voltage from the power amplifier to intervals of 1V and observe the

collector voltage of the phototransistor. Record the values in the table 2.

Lamp Filament Voltage (a) (b) 1 2 3 4 5 6

Phototransistor Collector Voltage

Table 2

Trace the graph of the collector voltage versus the lamp filament voltage in the following axes.

Why is there a minimum of approximately 0.7?

Experiment Five

5.3- The Photoconductive Cell

The figure 1 shows the basic construction of a photoconductive cell, formed by a disk

semiconductor and a wound layer of gold making contact with the semiconductor

material.

Figure 1

The resistance of the semiconductor material between the contacts of gold varies when

the light strikes on it.

Without light on the material, the semiconductors resistance is high:

The light on the material causes hole-electron pair and the resistance falls.

Are available a diversity of semiconductors materials.

The device included in the Trainer uses the cadmium sulfide, who has a frequency

response similar to of the human eye.

Experiment Five

The main characteristics of the device are illustrated by the graph in figure 2.

Figure 2

Cell Resistance

Dark 10M

50 Lux 2.4k

Typical 500

Environment

100 Lux 130

Response Time

Rise 75ms

Fall 350ms

Peak of spectral 510nm

response

Table 1

When the light is removed to the device, the hole-electron pairs move slowly to be

regenerated and the answer is slow as it can be proven.

Experiment Five

Light Intensity Detector.

Figure 3

Connect the circuit as is shown in the figure 3 and adjust the 10KΩ slide potentiometer so that

the load resistance of the photoconductive cell is approximately 2KΩ.

Adjust the carbon track potentiometer of 100KΩ so that the comparator input has the voltage to

which we want to sound the Buzzer. The student can verify that on terminal B of 100KΩ potentiometer voltage should be > 1.4V.

When the luminous intensity is increasing (rotating the slide potentiometer 10KΩ) the output

voltage of the photoconductive cell goes diminishing; when falling a little below 1V, the Buzzer

begins to sound.

Note: By means of a digital voltmeter you can observe how the voltage to the output of the

photoconductive cell falls when the lamp’s filament voltage increases due to a reduction

of the resistance of the cell.

Experiment Five

5.4- The photodiode PIN

The figure 1 shows the construction of a PIN photodiode. This differs of the normal

PN diode in that it has a layer of intrinsec silicon or doped very lightly introduced

between the sections P and N. This reduces the capacitance of the device and, as a

result, decreases the response time.

Figure 1

The device can work in two ways:

(a) Voltage source. As a photovoltaic cell;

(b) Source of intensity.

The figure 2 shows the arrangement of the circuit. Figure 2

The basic characteristics of the device are the following:

Current whit out light 1 nA

Current whit light 10 nA/Lux

Capacitance 15 pF

Response time 50 ns (Whit resistance of load 5 )

Peak of spectral response 850 nm (I.R)

Table 1

Experiment Five

Figure 3a

Connect the Circuit as it is shown in the figure 3a and with the lamp‘s filament voltage in zero,

observe the output voltage of the differential amplifier: (a) With your hand covering the transparent box, and

(b) With the PIN photodiode exposed to the environmental light.

Record the values in the table 2. Lamp Filament Voltage (a) (b) 1 2 3 4 5 6

Current Amplifier Voltage Output (Volts)

Table 2

Observe that the output voltage increases when the illumination is increased.

If it is necessary, the voltage can be increased for an additional amplification.

Experiment Five

Now connect the output of the PIN photodiode to a buffer amplifier and an amplifier of variable

gain as it is shown in the figure 3b.

50

Figure 3b

Put the control of the coarse gain of the amplifier 1 to 100, and the fine gain to 0.3. Check that the offset is zero for an input zero and adjust the control if was necessary.

Repeat the test of the photodiode writing down the output voltage for lamp filament voltage from

0 to 6V in steps of 1V. Record the values in the table 3.

Lamp Filament Voltage (a) (b) 1 2 3 4 5 6

Amplifier 1 Output (Volts)

Table 3

Observe that the output voltage increases when the illumination is increased.