References. [1] K.B. MacAdam, A. Steinback and C. Wieman. A narrow-band tunable diode laser system with grating feedback, and a saturated absorption spectrometer for Cs and Rb. Am. J. Phys. 60, 1098 (1992). Parts List. 1 - (Marlow Industries, RC3-6), Al block, heat sink sandwich. 1- Type GE NTC thermistor (embedded in Al block). ~100kΩ @ 20 c. 1 - Power transistor (Toshiba 2sk3767) attached to heat sink. 1 - TL074 quad opamp. 1-10 turn 50kΩ trimpot. 1 - BK Precision 50V, 2A power supply (current limited to 1A). Overview. Figure 1 shows a simplified version of a PID servolock circuit taken from [1]. The original circuit was designed to allow the user to maintain the temperature of a diode laser at a stable value, either above or below ambient. The circuit in Figure 1 contains only those elements necessary to investigate the properties of the Proportional and Integral parameters of a PID servolock. BK Precision 1711 power supply +5V 1A Voltage Divider A _ TL074 + Opamp #1 Thermal Monitor White Lead 1.5kΩ +15V Thermistor 100kΩ @ 20C c Resistive Bridge 150kΩ R3 PI Feedback C1 _ TL074 + R4 Voltage Divider B 1kΩ 2SK3767 Black Lead Power Transistor 1.5kΩ 130kΩ 150kΩ Opamp #3 1.5kΩ 50kΩ d 2Ω 25W _ TL074 + Set Point Monitor Opamp #2 Figure 1. PI temperature control servolock circuit. The servolock circuit will be used to control the current flowing through a Thermo Electric Cooler () so as to maintain the temperature of an Al block at about
15 C. The uses the Peltier effect to transfer heat from the cold side to the hot side. The rate of heat transfer is governed by the magnitude of the current passing through the. The polarity of the current flow through the is important. To avoid damaging the the White Lead must have a positive voltage relative to the Black Lead. As shown in Figure 2, the hot side of the is attached, with thermal paste, to a heat sink while the cold side is attached to the Al block we wish to cool. The thermal paste is sticky enough that it will hold the parts in contact with one another so long as you do not apply too much force to them. Make sure that the hot side of the does not separate from the heat sink as this could result in destruction of the. Thermistor imbedded in hold drilled through Al block. Small Al block. - Cold side. - Hot side. Heat sink. Figure 2. Thermo Electric Controller. A thermistor has been imbedded, with thermal paste, in the Al block to allow its temperature to be monitored. The thermistor used has a resistance of ~100kΩ at 20 C and approximately 150kΩ at 15 C. Eq. 1 shows the relationship between the thermistor resistance and temperature from 20 C to 0 C. T = 51.9160 0.4376R +1.8229 10 3 R 2 4.3102 10 6 R 3 + 4.1807 10 9 R 4 (1) T is temperature in C and R is resistance in kω. Circuit functionality. The circuit shown in Figure 1 can be broken down into 7 parts based on function.
Resistive Bridge. Compares the resistance of the thermistor imbedded in the cold block with a set point resistance that is the sum of a fixed resistor (130kΩ) and a variable resistor (50kΩ). The voltages at points C and D in the bridge are determined by the thermistor and set point resistances respectively. Voltage Divider A. Determines the voltage at the input to the Resistive Bridge. Thermal Monitor. Allows user to monitor the bridge voltage proportional to the thermistor resistance which in turn is proportional to the temperature of the Al block. Set Point Monitor. Allows user to monitor the bridge voltage proportional to the set point resistance. The difference between the Thermal and Set Point Monitor voltages is called the Error Signal. PI Feedback. Op amp 3 compares the thermal and set point voltages using negative feedback to generate an output proportional to their difference. The Proportional parameter of the feedback is determined by the values of resistors R3 and R4. The Integral parameter is determined by the value of the capacitor C1 used in conjunction with R3 and R4. Voltage Divider B. Limits the output voltage from the PI Feedback that controls the gate of the power transistor. Power Transistor. Controls the current passing through the. The circuit essentially compares the voltage drop across the thermistor (Thermal Monitor) to the voltage drop across the set point resistance (Set Point Monitor) and generates an output voltage proportional to the difference between the two. This output voltage is used to control the current passing through the Power Transistor and the. How this output voltage scales with the changing difference between the Thermal and Set Point voltages is determined by the values of R3, R4, and C1. Some questions regarding the the design of the circuit. Q1) What is the purpose of Op amp's #1 and #2 in the Monitors? Why not just monitor the bridge voltages directly at points C and D in the bridge itself. Q2) Why can the + input of Op amp #3 be connected directly to the bridge at D but the - input has to be connected to the output of Op amp #1? Q3) What would be the effect on the temperature control function of the circuit if you were to swap the inputs to Op amp #3? Q4) You will record the Thermal and Set Point Monitor voltages on the lab computers using the ULI interface so that you can see how the circuit performs over time. The ULI interface can digitize voltages in the range of 0V to +5V. So we must ensure that the two monitor voltages are always in that range. Explain how Voltage Divider A and the Bridge together accomplish this.
Q5) Both the and the Power Transistor need to be able to dissipate heat at they operate, up to 25W. If either device draws too much current without adequate heat sinking they can be destroyed. Although both devices are attached to heat sinks, additional protection against overheating can be had by limiting the current that can be drawn. Both devices should operate fine at currents up to 1.5A. Using the information in the data sheet for the 2SK3767 power transistor, explain how Op amp #3 and Voltage Divider B combine to limit the current draw through the to less than 1.5A. Procedure. Open Loop Response. Before attempting to control the temperature of the Al block using the feedback circuit, investigate the behavior of the as a function of the voltage applied to the gate terminal of the power transistor. Build the circuit shown in Figure 1 on the protoboard. Here are some tips. To start with, omit the capacitor C1 and make R3 =1.5MΩ and R4 = 15MΩ. Before connecting the output of Voltage Divider B to the gate of the Power Transistor, verify that the maximum voltage is in the appropriate range so as to not draw too much current from the transistor. Note that the ground of the BK Precision power supply, which supplies the current to the, should NOT be connected to the ground of the protoboard as it can cause the voltages from the protoboard to fluctuate when the current draw is heavy. Also note that for extra protection to the and the Power Transistor the BK Precision power supply has been current limited to about 1A. To observe the behavior of the circuit, record both the Thermal Monitor and Set Point Monitor voltages with the computer. For instructions on how to record data with the computer see appendix A. Open Loop Response. Before attempting to control the temperature of the Al block using the feedback circuit, investigate the behavior of the as a function of the voltage applied to the gate terminal of the power transistor. Disconnect the output of the PI Feedback Op amp from the Power Transistor gate. To control the Power Transistor gate voltage, build a voltage divider using the 1kΩ variable resistor on the protoboard as shown in Fig. 2. Measure the Thermal Monitor voltage with a DMM. Knowing the voltage drop across the whole bridge, the resistance of the fixed resistor in the Thermal Monitor leg of
the bridge, and the Thermal Monitor voltage you can find the thermistor resitance. Using Eq. 1 you can convert the thermistor resistance to temperature. For transistor gate voltages from 0V to 7V measure the equilibrium temperature of the Al block. At each voltage you will have to wait a few minutes for the temperature of the block to equilibrium. Make a plot of the transistor gate voltage vs. temperature. BK Precision 1711 power supply +5V 1A White Lead 15V 1kΩ Black Lead 2SK3767 Power Transistor 1kΩ 2Ω 25W Figure 2. Voltage divider supply for open loop measurement of temperature vs. transistor gate voltage. Now that you know how the temperature of the Al block varies with the voltage applied to the transistor gate, set the voltage to bring the block to about 5 C below room temperature. Connect the Thermal Monitor to the ULI so that you can observe how the temperature of the block changes over time. Estimate how much you have to change the transistor gate voltage to produce about a 1 C temperature change in the block. While recording the Thermal Monitor voltage on the computer, quickly change the transistor gate voltage to by the amount determined above. Observe how the Al block approaches its new equilibrium temperature.
What is the characteristic time for the Al block to reach the new equilibrium temperature with no feedback control? Draw a sketch of the Thermal Monitor voltage vs. time in your notebook. Include relevant information such as the starting and ending voltages and times. Proportional Feedback. Now that you have some feel for how the behaves in open loop mode, add proportional feedback. Remove the 15V voltage divider input to the Power Transistor gate. Reconnect the output of the PI Feedback Op amp to the Power Transistor gate. With only resistors R3 and R4, no capacitor (C1), in the feedback of Op amp #3 the control circuit will provide proportional feedback. Connect the Set Point Monitor to the ULI so that it can be recorded along with the Thermal Monitor. With the BK Precision power supply turned off, adjust the set point resistance to produce about a 5 C drop in temperature of the Al block. Start the data collection on the computer and then turn on the BK Precision power supply. Observe how the temperature of the block approaches the setpoint temperature. Sketch the Thermal and Set Point Monitor voltages in your notebook. Does the Al block reach the set point temperature? What is the characteristic time with which the block reaches equilibrium? Repeat the measurement for different values of the proportional gain. How does the characteristic response time of the change as a function of gain. Find the gain at which the temperature oscillates about the equilibrium value. Integral Feedback. Now add integral feedback and observe the effect on the oscillation induced by the proportional feedback. Choose a capacitor which, when combined with R4, produces an RC time constant about a factor of 3 shorter than the response time you measured using only proportional feedback. Use the computer to record the Monitor voltages. Does the addition of the capacitor stabilize the circuit?
Appendix A Collecting data with the computer. The Universal Laboratory Interface (ULI) and Logger Pro software can be used to record voltages as a function of time. We will record the voltages from the clips which are plugged into the DIN1 and DIN2 channels on the ULI. Note that the ULI can digitize voltages in the range 0V to +5V. Make sure you hook the voltage clips up with the correct polarity. To collect data on the computer make sure that the ULI is turned on (the on/off switch is on the back of the unit), then double click on the Temp_Controller icon. It may take the application 45 seconds to start up. If a warning message comes up indicating that the ULI could not be found, restart the computer and cycle the power on the ULI. The software is setup to collect data for 120 seconds. You can change this run length, if necessary, from the menu Experiment -> Sampling -> Experiment Length. To begin collecting data, click on the Collect button which is located under the menu bar at the right side of the screen. If there is no Collect button then the software failed to detect the ULI on startup and it is necessary to reboot the computer and cycle the power on the ULI. By selecting Analyze -> Examine the cursor can be used to read data values from the graph. The Meters window in the bottom right part of the screen shows, in real time, the voltages being read by the two probes. This function is only active then the computer is not collecting data.