Lesson # 5 - Light measurement transducers.pdf

LIGHT

1. LIGHT MEASUREMENT
TRANSDUCERS
Process and Instrumentation
Lerotholi Polytechnic
©December 2012

2. Learning outcomes
•Sketch the construction and plot the
characteristics for:
•A photovoltaic cell
•A phototransistor
•A photoconductive cell
•A P.I.N photodiode
•State the application areas for each of the
devices mentioned.

3. Semiconductors
•A semiconductor has electrical conductivity
intermediate to that of a conductor and an
insulator.
•Semiconductors differ from metals in their
characteristic property of decreasing electrical
resistivity with increasing temperature.
•Semiconductor materials are useful because
their behaviour can be manipulated by the
addition of impurities, known as doping.

4. •Current conduction in a semiconductor
occurs via mobile or "free" electrons and
holes, collectively known as charge
carriers.
•Doping a semiconductor with a small
amount of impurity atoms greatly increases
the number of charge carriers within it.
• When a doped semiconductor contains
excess holes it is called "p-type", and when
it contains excess free electrons it is known
as "n-type".

5. •For semiconductors to operate as conductors,
they need to be biased.
•Biasing in electronics is a method of establishing
predetermined voltages and currents at various
points of an electronic circuit to set an
appropriate operating point
•Semiconductor-based electronic components
include:
• transistors,
•solar cells,
•many kinds of diodes including the light
emitting diodes (LED), the silicon controlled
rectifier, photo-diodes etc

6. Photovoltaic cell
Construction
•Basic construction of a semiconductor
photovoltaic cell, consisting basically of a
two layer silicon device.

7. •A thin layer of P-type material is formed on an N-
type substrate
Principle of operation
•When light falls on the junction of the two materials,
a voltage is developed with the N-type material
positive with respect to the P-type.
•The output voltage depends on the magnitude of
the light falling on the device and is a maximum of
the order of 0.6V
•With the load resistance connected to the output, a
current will flow.
•The magnitude of the current depends on the
magnitude of the light falling on the device and on
the surface area of the device

8. •The output capability of this devices can be
increased
•Series-parallel connection of the cells can be used to
increase the magnitude of the voltage and current
capabilities if desired
Example: A photovoltaic cell gives an output of 0.5V
for a certain illumination level and is capable of a
current output of 5mA. What would you expect to be
the output of capability with the three identical units
with the same level of illumination,
a)Connected in series
b)Connected in parallel?

9. The phototransistor
Construction
•The unit is basically an NPN three layer semiconductor
device as for a normal transistor, the connections to the N,
P, & N sections being labeled e (emitter), b (base), & c
(collector).

10. Principle of operation
•The device differs from the normal transistor in
allowing light to fall on the collector-base junction
•With no light falling on the device there will be a
small current flow due to thermally generated hole-
electron pairs and the output voltage from the
circuit will be slightly less than the supply value due
to the voltage drop across the load resistor R.
•With the light falling on the collector-base junction
the current flow increases.
.

11. •With the base connection open circuited, the collector base
current must flow in the base-emitter circuit and hence the
current flowing is amplified by normal transistor action
•The output voltage from the circuit falls as the current
increases and hence the output voltage is dependent on the
light falling on the device.
•V
out = V - I
cR
Where V = Supply voltage
I
c = Collector current
R = Collector load resistance

12. Example:
•A phototransistor is connected to a 10V DC supply via a
2kΩ load resistor.
a)For a certain ambient illumination the collector current is
1mA. What will be the collector voltage?
b)What would you expect the collector current and voltage
values to be when the illumination is doubled?

13. The photoconductive cell
Construction
•It consists of a semiconductor disc base with a gold
overlay pattern making contact with the semiconductor
material.

14. Principle of operation
•The resistance of the semiconductor material between the
gold contacts varies when light falls on it.
•With no light on the material, the resistance is high.
•Light falling on the material produces electron-hole pairs
and reduces the resistance.

15. •The main characteristics of the device

Cell resistance Dark 10MΩ
50 Lux 2.4 kΩ
Typical ambient 500 Ω
100 Lux 130 Ω
Response time Rising 75 ms
Falling 350 ms
Peak spectral response 610 nm (red)

16. •When light is removed from the device, the hole-electron
pairs are slow to reform and the response is sluggish

17. The P.I.N Photodiode
Construction
•The P.I.N photodiode differs from the normal diode by
having a layer of intrinsic or very lightly doped silicon
introduced between the P and N sections.
•This reduces the capacitance of the device and as a
result, the response time is reduced.

18. •The device can be operated in one of two ways:-
a) As a photovoltaic cell, measuring the voltage output, and
b) By measuring the small output current and converting
this to a voltage.