by Roger Desrosiers
In electronics # 2 we discussed how thermisters work, and some of their uses. Now we will use one as a temperature sensor with an op amp as in the circuit below, and see how it will affect the voltage to the negative input to the amplifier pulling in the DPDT relay.
You will remember how a thermostat’s resistance changes with temperature. If an increase in temperature results in an increase in resistance, the thermister has a Positive Temperature Coefficient and is called a PTC thermister. These are not common.
If an increase in temperature results in a decrease in resistance, the thermister
has a Negative Temperature Coefficient and is called a NTC thermister. These are the normal types usually used in our work.
Thermisters have their value given in ohms based on a temperature of 25°C, the
NTC1 that is used in this circuit is 20k ohms @ 25°C (77*F).
The Operational Amplifier
The operational amplifier (op-amp) is used as a voltage comparator.
When the voltage on pin 3 is more positive than the voltage on pin 2 the output will be high (9V). When the voltage on pin 3 is less positive than the voltage on pin 2 the output will be low (0V). This op-amp can be used to switch something on or off such as a Refrigeration Compressor or a Supply Fan, a Boiler, etc.
The voltage on pin 3 will increase with a rise in temperature. When this voltage is greater than the voltage on pin 2, the output voltage on pin 1 will quickly rise to 9V. With pin 1 at 9V base, current to the transistor will flow and the transistor will be switched on, the LED will be on and the relay energized. When pin 3 voltage falls below that of pin 2, then the output of pin 1 will fall to 0V, the LED will be off and the relay de-energized. Changing the voltage at pin 2 by changing VR1 or R1 will result in the circuit responding to different temperatures. In other words,
VR1 could be called the thermostat set point.
The diac is used primarily as a triggering device, so when a positive or negative reaches its break over voltage, the diac rapidly switches from a high resistance state to a low resistance state. Since the diac is a bi-directional device, it is ideal for controlling triacs, which are also bi-directional. Following is the symbol for the diac and break over voltage graph.
Now let’s look at a simple circuit where a diac is controlling a triac and driving a universal motor.
In this circuit, capacitor C1 charges up to the firing voltage of the diac in either direction (positive or negative). Once fired, the diac will apply voltage to the gate of the triac. The triac will conduct and apply power to the load. The speed of the load (universal motor) may be changed by varying the resistance of the potentiometer R1, which in turn varies the time of the firing voltage on the gate. Notice that since the universal motor is basically a series D.C. motor current flowing in either direction, and will cause rotation in only one direction. You will see this type of circuit in many electronic controls.
Here is a circuit for controlling an ordinary DC motor using two pairs of transistors (1 NPN and 1 PNP for each pair). Can you tell which transistors is NPN?
This DC motor runs in one direction if the required voltage is applied across its winding, and runs in the opposite direction if the polarity of the applied voltage is reversed. This function can easily be achieved by the circuit above.
In this circuit, a "logic 1" (on) voltage at Control 1 and a "logic 0" (off) voltage at Control 2 will turn on Q1 and Q4 and turn off Q2 and Q3, causing the motor to turn in one direction. Reversing the voltage levels at Control 1 and Control 2 will reverse the pairs of transistors that are "on" and "off", causing the motor to turn in the opposite direction. Putting the same logic input at Control 1 and Control 2 (both "1" and both "0") will cause the motor to stop turning.
The values of the base resistors of the transistors (or even the transistors themselves) required by the circuit may be different from those shown in Figure 1, depending on the motor being driven. Experimentation may therefore be required on the part of the hobbyist to make this circuit work.
In Electronics # 4, I will go into fundamental of electronic controls such as features of electronic control systems, sensors and output devices.
Roger J. Desrosiers
About the Author: Roger is a contributing faculty member of HVACReducation.net He has over 40 years experience in Air Conditioning and Refrigeration. He is also a member of R.S.E.S., CM, The Association of Energy Engineers, Certified Energy Manager, ASHRAE, Certified Pipe Fitter United Association and is 608 Universal Certified.