Spring 2000 gaphw.wp5
ET 438b
Continuous and Digital Control
Differential Gap Controller
Refer to the schematic diagram below for the following questions. This circuit
implements a differential gap temperature control using OP AMPs.
1.) Calculate the setting of R2 so that a sensor, S1, input of 50 F will give a voltage of
5.0 V at TP1.
2.) Determine the setting of R7 so that U3 has a 10 F deadband voltage scaled to be
compatible with the input at TP1 Assume that Vsat = 0.8Vcc.
3.) Determine the voltage at TP2 so that the LTP of the comparator U3 will be 90 F.
4.) Calculate the value of R3 so that the voltage at TP2 is equal to the LTP value.
5.) Make a sketch of the input/output response of U3 and mark the input voltage
values where 65 F, 95 F and 120 F are located. Also identify the voltages of the
upper and lower trip points.
6.) Calculate the value of R7 so that the cooling fan relay will be activated when the
control circuit detects an over temperature. (hFE = 200 for 2N3904) Use the 10
times rule to assure that Q1 goes into saturation.
7.) For what range of temperatures will LED1 be on?