ENGINEERING INSTRUMENTATION

Engineering Instrumentation

Engineering Instrumentation

A linear potentiometer is used to measure displacement. The potentiometer has a total resistance of 400( and a length of 25 mm. Resistance along the length of the potentiometer is linear. Calculate the output voltage for a displacement of 13 mm is the circuit shown below.

The voltage is measured with a meter that has a resistance of 10k(. What will be the apparent voltage measured by this meter and the current through the meter.

Ans: The key thing here is not to lose the linearity of the resistance versus displacement. We cannot use a voltage divider because the voltage versus resistance variation is not linear for that circuit. We can use an op amp circuit, however, because the gain and therefore the output voltage is linearly dependent on the feedback resistor (Turner & Hill 1999, 12-25).

Potentiometric displacement sensor.Circuit for Example 5.1.

Therefore, the circuit of Figure 5.2 will satisfy this problem. The output voltage isVout = -(RD)/1kW)(-10V) Vout = 0.01RDAs RD varies from 0 to 1 kW, the output will change linearly from 0 to 10 volts.

In the voltage divider circuit below, Vout is measured to be 6V. Calculate the resistance of resistor R.

Ans: P = EI60 = 26 * II = 2.31 ampsvoltage across the resistor is 120 - 26 = 94 voltsR = E/I = 94/2.31 = 40.7 ohms

Calculate the current through each of the resistors in the following circuit.

Ans: A parallel combination of a 200 ? and 120 ? resistor are placed in branch 2 (underneath the 300 ? resistor). Recalculate the supply current. Resistors can be placed in series or parallel. When placed in series the total resistance is equal to the sum of the individual resistors (McBride & Wetsel 1990, 909-916):

Resistors in series

It is also worth noting that the same current flows through each resistor, but the voltage across each resistor is proportional to the resistance of that particular resistor. For resistors placed in parallel, the arithmetic is a little more complicated because the reciprocal of the total resistance is equal to the sum of the reciprocals of the constituent resistors (Morris 2004, 14-17):

Resistors in parallel

When there are only two resistors R1 and R2 in parallel this can be simplified to:

If both these resistors have the same value it can be seen that the overall value of the resistance is half the value for the individual resistor. When resistors are placed in parallel the voltage across the resistors is the same, but the current through each one is inversely proportional to its resistance.

The bridge circuit below is used to determine the output of a resistive thermometer. The output is the voltage measured between terminals 1 and 2. Each resistor rb in the bridge has a resistance of 200(, and at 0°C the thermometer also has a resistance of 200(. The thermometer has a linear coefficient with temperature of 0.004 (/°C. Calculate the output voltage when the temperature of the gauge is 100°C

Ans: For the resistance measurements the wires were placed in a specially designed ...