Experiments
With Temperature Sensors - 2
One way to measure temperature is with a thermistor in a bridge
circuit. Here is a thermistor (shown as a variable resistor
- the one with an arrow through it) in a bridge circuit.
The bridge circuit has a number of characteristics
that make it a useful circuit.
-
The output voltage can
be adjusted so that it is zero at some reference temperature.
-
To adjust the output voltage
to zero - when the bridge is said to be balanced
- requires that the resistor values satisfy a certain relationship.
-
Then, if the temperature
moves above the reference temperature the output voltage moves in one direction
- positive or negative? - and if the temperature moves below the reference
temperature, the output voltage moves in the other direction.
Two
EE grad students - Willy Nilly and Millie Farad are interested in using
this circuit to measure temperature. They are trying to help out
some friends in Chemical Engineering who need to be able to measure temperature
so that they can build a temperature sensing and control circuit.
They need your help to answer the following questions.
-
Is the circuit set up
properly so that output voltage goes positive when temperature rises -
assuming that the bridge is balanced at room temperature?
-
If it isn't set up properly,
what changes can be made?
Willy
and Millie are also puzzled by another aspect of this circuit. They
are not sure how much current can be drawn from the circuit and still be
able to get an accurate temperature measurement. They have generated
the following questions.
-
How does the output voltage
depend upon current drawn from the output terminals.
-
Does the answer to this
question depend upon temperature?
Finally,
Millie has been working with something called Mathcad, and she thinks that
she can use that to predict how the output voltage will depend upon temperature.
However, she needs help setting that up, and has called on you to help.
-
Generate a graph of output
voltage vs. temperature from the theoretical equation relating thermistor
resistance and temperature.