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EET 425/4 – Industrial Electronic Control Laboratory Module EXPERIMENT 3 THERMISTOR 1. OBJECTIVE: 1.1 1.2 1.3 To balance a bridge network Determine which type of temperature coefficient a thermistor exhibits To activate an over-temperature alarm using a Thermistor. 2. INTRODUCTION Temperature sensing circuits are widely used in industry as system control elements, or to detect unwanted operating conditions and activate an alarm. These circuits use sensors that detect temperatures to which they are exposed. The thermistor is one type of sensor used for temperature detection applications. The thermistor is classified as a temperature sensing resistor. It is constructed of a nickel, manganese, and cobalt oxide mixture formed into a semiconductor material in the shape of a disk. Changes of temperature to which it is exposed cause the resistance of the thermistor to change. Thermistor have a negative temperature coefficient, which means that an increase in temperature cause a decrease in resistance. Thermistor has an operating range from –230 degrees to 650 degrees centigrade. When used in a bridge circuit with high amplification, it can detect temperature changes as low as 0.001 degrees centigrade. In this laboratory exercise, the thermistor will be used in a bridge circuit. The output of the bridge is applied to the input of an operational amplifier that produces a high gain to provide increased sensitivity. The output of the amplifier is connected to the gate of an SCR. The thermistor is placed in the bridge network to cause an imbalance that produces a negative-going signal at the inverting input of the op amp when the temperature to which if is exposed increases. The result is that the operational amplifier’s output produces a positive voltage and causes the SCR to turn on. As current flows through the SCR, it turns on a lamp that indicates a certain temperature has been reached. The SCR turns off when the reset switch is opened. It will remain off only if the temperature being measured drops to a point below the level that caused the SCR to turn on. A circuit of this type can be used for an alarm circuit to indicate that temperature has risen to an undesirable level. 1 EET 425/4 – Industrial Electronic Control Laboratory Module 3. COMPONENTS AND EQUIPMENTS : 3.1 3.2 3.3 3.4 3.5 3.6 3.7 3.8 3.9 3.10 3.11 3.12 3.13 3.14 3.15 Dual DC power supply – 1 unit Voltmeter – 1 unit Ammeter - 1 unit Breadboard – 1 unit Resistor 47Ω – 1 unit Resistor 1kΩ – 1 unit Resistor 2.7kΩ – 1 unit Resistor 10 kΩ - 2 unit Resistor 220kΩ - 1 unit Potentiometer 100kΩ – 1 unit Silicon Control Rectifier(SCR) – 1 unit Thermistor – 1 unit 741 Op- Amp – 1 unit SPST switch – 1 unit Wires 4. PROCEDURE: Step 1 Measure the resistance of the thermistor. Avoid touching the glass part of the thermistor when attaching the test leads. With the ohmmeter connected, wait a short time for the resistance to stabilize. Record the stabilized resistance: __________ ohms While observing the ohmmeter, grasp the glass bead between your index finger and thumb. The resistance of the thermistor __________ as the temperature to which it is exposed increases. A. increases B. decreases This observation shows that the thermistor has a ___________ temperature coefficient. A. positive B. negative Step 2 Using an ohmmeter, set the decade resistance box to the thermistor resistance value recorded in Step 1. Step 3 Assemble the circuit in Figure 18-1. Adjust the dual voltage DC power supply for –9 volts and +9 volts. Place the reset switch in the open position. 2 EET 425/4 – Industrial Electronic Control Laboratory Module Step 4 Turn on the dual voltage power supply to energize the circuit. Balance the bridge by adjusting the decade resistance box to produce a zero indication on the voltmeter connected to the output of the op amp. Note: A positive voltage produced by the op amp is required to fire the SCR. To insure that the SCR does not fire at ambient temperature, set the decade resistance box so that a slight negative voltage is produced at lead 6 of the op amp. Step 5 To test the sensitivity of the circuit, place your finger on the glass bead of the thermistor while observing the output of the op amp with a voltmeter. If the circuit is operating properly, the voltmeter should indicate a positive potential when heat is applied by your finger, and a negative potential when the thermistor is re-exposed to ambient temperature. Step 6 Observe the voltmeter while trying other forms of heat, such as blowing on it or placing other heat sources near the thermistor’s body. Avoid temperatures over 150 degrees centigrade. Note: The op amp produces a positive potential when the voltage at its noninverting input is greater than the voltage applied to its inverting input lead. 3 EET 425/4 – Industrial Electronic Control Laboratory Module Step 7 Observe and record the voltages at the inputs and output of the op amp before and after heat is applied to the thermistor. Before Inverting input Non-inverting input Output _________ volts _________ volts _________ volts After Inverting input Non-inverting input Output _________ volts _________ volts _________ volts Step 8 Close the reset switch. Place your finger near the thermistor while a voltmeter is connected to the gate of the SCR. Observe the voltage as the SCR is fired into conduction. After the SCR is turned on, remove your finger and wait several seconds. Observe the voltage at the gate of the SCR. Does it remain at a positive voltage? __________ (yes, no) Does the SCR turn off? __________ (yes, no) Once the SCR is on, it continues to conduct even when the voltage at its gate is no longer a positive potential. The only way it can turn off is to reduce the SCR’s holding current. To prove this, try disconnecting the connection to the gate of the SCR labeled X in the circuit. Observe what happen to the current? Does the current remain the same(the SCR still turned ON)?Why? Step 9 Turn the SCR off by opening the reset switch and reducing its holding current to zero. Step 10 Disassemble the circuit and return the parts and equipment to their proper storage locations. 4 EET 425/4 – Industrial Electronic Control Laboratory Module Name : ______________________________ Matrix No : ______________________________ Date : ______________ 5. RESULTS: Step 1 __________ ohms While observing the ohmmeter, grasp the glass bead between your index finger and thumb. The resistance of the thermistor __________ as the temperature to which it is exposed increases. A. increases B. decreases This observation shows that the thermistor has a ___________ temperature coefficient. A. positive B. negative Step 7 Before Inverting input Non-inverting input Output _________ volts _________ volts _________ volts After Inverting input Non-inverting input Output _________ volts _________ volts _________ volts Step 8 Observe the voltage at the gate of the SCR. Does it remain at a positive voltage? __________ (yes, no) Does the SCR turn off? __________ (yes, no) 5 EET 425/4 – Industrial Electronic Control Laboratory Module Name : ______________________________ Matrix No : ______________________________ Date : ______________ What happen to the current when the connection to the gate of the SCR labeled X in the circuit is disconnected? Does the current remain the same(the SCR still turned ON)? _________________________________________________________________________ Why? _________________________________________________________________________ _________________________________________________________________________ QUESTIONS: EXPERIMENT QUESTIONS: 1. A thermistor has a __________ temperature coefficient. A. positive B. negative 2. When the temperature to which the thermistor in this bridge circuit rises, the voltage at the non-inverting input of the op-amp _________, and the voltage at the inverting input _________. A. increases B. decreases C. stays the same 3. The op amp output goes positive when the voltage applied to its _________ input is greater than its _________ input. A. inverting B. non-inverting 4. The SCR will remain on after the positive gate voltage is removed because it has a _____________ current. 5. If the resistance of the thermistor is __________ than the resistance of the decade box, the voltage at the SCR’s gate will be negative. A. less B. greater 6