Download Transducers

Transducers
Devices which convert non-electrical quantities (e.g. temperature, pressure) into
electrical ones (e.g. voltage, resistance) whose value depends on the value of the nonelectrical quantity. Transducers exist for almost every imaginable quantity!
We will concentrate on three example transducers and the circuits which can be used
to apply them.
1. Light level -- the photodiode
Forward direction -- operates like a normal diode.
Reverse direction -- much larger leakage current than normal; its value is dependent
on the light level.
The operating principle is that the junction region is exposed to incident light (unlike
a normal diode where the device is contained in an opaque packaging). The photons of
light cause electron-hole pairs to be produced, which give rise to the light-dependent
current. The current-light level relation is nonlinear; in particular, there is a leakage
current known as the “dark current” when there is no light falling on the device.
We could produce a voltage dependent on the light level like this:
E
R
Voltage proportional
to diode current
Phototransistors also exist -- they give rather more current than photodiodes for a
given light level because of the current amplification they produce, but they also have
a greater “dark current”.
2. Temperature -- Resistance Thermometers and Thermistors
These have in common that their resistance changes with temperature, but the
difference is how rapidly!
Platinum resistance thermometer -- resistance increases by 0.39% per degree C
Thermistor (negative-temperature-coefficient) -- resistance DECREASES strongly
with temperature rise.
The platinum resistance thermometer is normally used in an unbalanced Wheatstone
Bridge circuit to give a temperature-dependent voltage.
E
Pt element
To
differential
amplifier
If all the resistances are of nominal value R, it can be shown by circuit theory that the
voltage at the differential amplifier input (assuming its Rin is high) is ER/(4R) when
the thermometer resistance has changed by R.
We only need to use such an arrangement with a thermistor if it is measuring very
small temperature ranges. More usually, something like this will suffice.
E
Vo
Hall-effect devices
The Hall effect is observed quite strongly in semiconductors and its principle is very
like that of an electric motor. Consider the rectangular block of semiconductor
material below in which a magnetic field is “flowing” into the paper.
Current I
Semiconductor
(assumed p-type
for simplicity)
Hall
Voltage
EH
Voltage source
producing the current I
+
-
The principle is that the current I, flowing at right-angles to the magnetic field of flux
density B, experiences a force in a direction at right-angles to both the field and the
current according to Fleming’s Left-Hand Rule. The “holes” carrying the current are
therefore deflected to the side, creating a Hall Voltage EH as shown. (You may like to
confirm whether the polarity of the voltage is correctly marked). We find that:
EH
=
RHBI
where RH is a constant known as the Hall Coefficient.
Would the Hall Effect also occur in an n-type material? If so, what would be
different ?
Hall-effect transducers are most frequently used for measuring magnetic fields. The
transducer is placed in the field and a known current is passed through it, resulting in
a Hall voltage proportional to the field strength. It is otherwise surprisingly difficult to
measure constant magnetic fields -- alternating ones are easier as we can insert a coil
and measure the alternating voltage developed across it. Hall-effect transducers are
also used to measure large currents non-invasively in “Clip-on” devices where a loop
of magnetic material containing a Hall-effect device is clipped round the wire carrying
the current. In each case, the Hall-effect device is often manufactured with an
amplifier in the same package, which allows a much higher output voltage to be
obtained for a given field strength and current.
Transducer Examples
1.
A particular thermistor has a resistance which can be expressed in terms of
temperature by the following graph.
Resistance, ohm
1200
1000
800
600
400
200
0
0
10
20
30
40
50
Temperature, C
60
70
80
90
100
It is to be used to produce a voltage which will vary from 0 V at 10 C to 5 V at 50 C.
Design a suitable circuit.
2.
A similar device is to be produced using a platinum resistance thermometer
having a resistance of 100 ohm at 20 C.
Design a suitable circuit.
3.
A photodiode having a dark current of 50 nA and a reverse current of 100 uA
on a sunny day in summer is to be used to make a lightmeter for use by cricket
umpires. The cricket club is short of funds and it must therefore utilise a 0-5 V
panel meter of resistance 10 k which the wicket-keeper has found in his
garden shed. The No. 11 batsman, anxious to persuade the selectors of his
claim to a higher place in the batting order, has donated a 4.5-volt battery and
a large box of resistors, fixed and variable, of most conceivable preferred
values. A free ticket (normal cost £0.00) for the opening match has been
offered to the designer of the first successful design opened ...
(a)
Design a suitable circuit.
(b)
The No. 11 batsman now additionally offers a second 4.5-V battery, an
operational amplifier and a light-emitting diode. Modify your circuit to
illuminate the LED when there is insufficient light.
4.
RS offer (or offered !) a Hall-effect i.c. incorporating amplification which
provides an output voltage of 7.5 - 10.6 mV/mT when supplied with a current
of 3.5 mA.
How could it be used in conjunction with the meter from Question 3
and any other necessary components to read directly in mT ?