Question

In: Physics

In certain textile finishing processes, the moving strip is passed through a moistening chamber to be...

In certain textile finishing processes, the moving strip is passed through a moistening chamber to be moistened and softened. The relative humidity in the chamber is maintained at a high level by air circulates. The system layout is illustrated in the figure shown below. A resistive hygrometer (humidity transducer) measures the relative humidity in the chamber.

The electronic circuit used to monitor the humidity transducer (hygrometer) output is shown in the figure below.

The resistive hygrometer is represented by resistor R3, which must be excited by an ac voltage (a dc current would chemically polarize the sensor over time). This transducer has resistance characteristics that vary exponentially with the relative humidity; important data for this textile processing application are as follows:

                                    Relative Humidity          Resistance, R3

                                                50 %                            35 kΩ

                                                60 %                            15 kΩ

                                                70 %                            7.5 kΩ

                                                80 %                            3.5 kΩ

                                                90 %                            2.0 kΩ

Description of the Situation and the Assumptions

1. Input transformer with 115-V (rms) input and a 10.0-V (rms) output (on secondary).

2. Resistance bridge with the humidity sensor R3 located in one branch.

3. Full-wave rectifier connects to the resistance bridge output, Vbridge

4. Capacitance C1 (with bleeder resistance R5) filters the diode-bridge output to dc.

5. Op amp circuit amplifies the input Vin (capacitor voltage) to an output Vout to be monitored.

Goal of the Design Project

Analyze the circuit operation for a range of relative humidity measurements (50% to 90%) to determine the circuit component requirements.

1. Input transformer

Determine the required transformer turns ratio N1/N2 and the maximum input rms current (in the primary windings) during the operation of measuring 50% to 90% humidity.

2. Resistance bridge

Determine the maximum rms voltage Vbridge (this occurs at 50% humidity measurement).

3. Diode bridge

Determine the maximum peak inverse voltage across each diode and the maximum forward current for each diode. These two values will help specify the what size diodes to use in the diode bridge. Assume 0.6 V drop across each conducting diode.

4. Filter capacitor

Determine the maximum dc voltage that will occur on the filter capacitor (this occurs at 50% humidity measurement). The capacitor used must be rated 50% greater than this maximum occurring voltage.

5. Output op amp voltage

Determine op amp output voltage for each humidity measurement: 50%, 60%, 70%, 80%, and 90%. List your results as sets of numbers; no need to plot the results.

Solutions

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