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. Click
here for a note on using the Hydra.
-
The Hydra Data Acquisition
Unit (DAU) can measure temperature directly. Redo the first part
here using the temperature measurement capability of the Hydra DAU.
-
Then, note what you observe
there.
-
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.
Wire it up and measure the same two temperatures as you did in the thermocouple
and LM35 measurements above.
Here is a link
to a hand-in sheet.
