You have been asked to consider this problem from River City Circuits Corporation.
RCCC wants to produce and sell a line of measurement modules for various physical variables. They have decided to start with a humidity measurement system. The heart of the measurement system is a resistive humidity sensor. In the humidity sensor, the resistance is a function of the relative humidity to which the sensor is exposed.
RCCC wants to use the sensor in a bridge circuit, and amplify the output of the bridge circuit. The bridge circuit they want to use is shown below.
They have an application note from National Semiconductor (the company that produces the humidity sensor). You can download the application note from National Semiconductor at:
However, the graph of resistance vs. relative humitidy is shown below. (Relative humidity - RH - is along the horizontal axis, and resistance of the sensor is along the vertical axis.
v
The relative humidity sensor (RH sensor) can be
placed either at Rc or Rd, in the circuit above.
v
The bridge output needs to be measured with a
voltmeter, placed as shown below.
They plant to investigate whether they can measure the voltage across the bridge with a voltmeter. That would lead to this circuit. Click the button to see the equivalent circuit.
RCCC needs you to do the following:
v Determine resistance values in the bridge circuit;
v Predict output voltage as a function of relative humidity (RH);
v Determine a range of linearity for the output as a function of RH.
RC3 also will need some advice on whether they should try to amplify the output of the bridge circuit. Their usual consultant - a Mr. R. J. Conque - had suggested that they might be able to use the circuit shown below. Mr. Conque has an alcohol dependency problem, and he is presently unavailable. He may soon be permanently unavailable, so they need your opinion on at least the following questions.
v
Determine resistance values in the amplifier portion of the circuit.
(shown below)
There are several things that you will need to do for this problem.
lGiven
the first circuit, with the voltmeter,
u(A)
Define the problem.
u(B)
Determine a "plan of attack" that will lead
to a solution of the problem as defined.
u(C)
Implement the plan of attack.
u(D)
Evaluate the resulting solution.