Measuring
Voltage - Laboratory Exercise
Measuring
Some Sensor Voltages
In this lab exercise you will use some devices that produce a voltage that
changes with temperature. They are:
-
A thermocouple,
-
An integrated circuit
temperature sensor - the LM35.
-
A thermistor in a voltage
divider.
All of these devices are called sensors because they sense some sort of
physical quantity (temperature and light for these sensors) and produce
a voltage that changes when the physical quantity changes - although it
takes a voltage divider or a bridge circuit to make the resistance change
in the thermistor into a voltage change. (Note, not all sensors change
voltage. Some sensors - like thermistors and strain gages - change
resistance as their controlling physical quantity changes value.)
Temperature Sensors
- For The Record
The sensors are temperature sensors. Temperature sensors come in
several varieties including the following.
-
Thermocouples, which are
inexpensive and widely used. However, thermocouples produce a voltage
out that is in the millivolt range. Here is a photo of a thermocouple.
It's just two dissimilar wires that produce a voltage in the right temperature
environment.
-
Thermistors are temperature
sensitive resistors. Here is a photo of one kind of thermistor.
You need to put a thermistor into a circuit of some sort to translate the
resistance change into a voltage change.
-
Integrated circuit temperature
sensors, like the LM35, are designed to produce a voltage that is directly
proportional to temperature. Here is a photo of an LM35 integrated
circuit temperature sensor. When you add the power to the LM35, it
will produce a temperature dependent voltage.
We will look at all three
sensors.
-
Measure the output voltage
of the thermocouple, and do that at two temperatures. Remember, you
have room temperature, and your body temperature! Click
here for info on the thermocouple.
-
(Optional) The Hydra Data
Acquisition Unit (DAU) can measure temperature directly. Redo the
first part here using the temperature measurement capability of the Hydra
DAU.
-
You will need to connect
the LM35. Click
here to get the circuit for the LM35. Wire it up and measure
the same two temperatures.
-
You need to have the thermistor
in a voltage divider circuit or a bridge
circuit.
Here are some questions
for you to answer.
-
Which sensor produces
the largest voltage as temperature changes between room temperature and
body temperature? In other words, which sensor is the most sensitive?
-
Does any sensor have a
voltage so small that it might be difficult to measure?
-
Which sensor(s) would
require some circuitry in order to make it a usable sensor (one that produces
a voltage output)?
-
Order the sensors from
slowest to fastest as best you can.
-
Which sensor do you find
easiest to use? Why?
