Experiment (1) Principles of Switching

Transcription

1 Experiment (1) Principles of Switching Introduction When you use microcontrollers, sometimes you need to control devices that requires more electrical current than a microcontroller can supply; for this, electrical relays and transistors are used. In this experiment we will investigate two types of switching; electromechanical switching and solid state switching. Objectives This experiment aims to: 1- Understand the basic principles of switching devices. 2- Introduce the students to electromechanical switching as well as solid state switching. 3- Understand the advantages and disadvantages of each type. 4- Study the relative speed of different type of switches. Theory Electromechanically Relays Relays are electromechanical devices that use an electromagnet to operate a pair of movable contacts from an open position to a closed position. The relay is a switch that is controlled by a small electric current. It can be used to control motors, heaters or lamps circuits which themselves can draw a lot more electrical power. Figure 1 shows the circuit symbol for a relay. Figure 1: Circuit symbol for a relay In figure 2, the relay s coil is energized by the low-voltage (12 VDC) source, while the singlepole, single-throw (SPST) contact interrupts the high-voltage (480 VAC) circuit. It is quite likely that the current required to energize the relay coil will be hundreds of times less than the current rating of the contact. Typical relay coil currents are well below 1 amp, while typical contact ratings for industrial relays are at least 10 amps. Page 1

2 Figure 2: Relay drive an AC load One relay coil/armature assembly may be used to actuate more than one set of contacts. Those contacts may be normally-open, normally-closed, or any combination of the two. As with switches, the normal state of a relay s contacts is that state when the coil is de-energized and not connected to any circuit. Figure 3 shows different relay s contacts arrangement. Figure 3: Different Relay's Contacts Arrangement Relay contacts may be open-air pads of metal alloy, mercury tubes, or even magnetic reeds. Consider these factors when choosing a relay for use in industrial controls: 1. Voltages driving loads are the first concern. The voltage rating of a relay must be greater than or equal to the voltage driving the load. The frequency of the switched voltage is also critical. Because ac current fluctuates from positive to negative crossing through zero, the switched voltage will vary between the maximum voltage and zero. Dc voltage, on the other hand, is always at the maximum value, causing maximum wear on the contacts with every switch. 2. The current required depends on the type of load. Most loads don't draw a constant current. In fact, the current demand of most loads varies somewhat predictably. It is also important to avoid switching currents that are too small for the relay to operate reliably. Proper operation of a switch relies, to some extent, on the switching of some minimum current. This current is often referred to as a wiping current because it will burn off traces of contaminants that may build up on the relay contacts. The lower limit of current that can be reliably switched is a function of several factors such as contact material, contact geometry, and mechanical sliding of the contact surfaces. Page 2

3 Open-air contacts are the best for high-current applications, but their tendency to corrode and spark may cause problems in some industrial environments. Mercury and reed contacts are spark-less and will not corrode, but they tend to be limited in current-carrying capacity. Aside from the ability to allow a relatively small electric signal to switch a relatively large electric signal, relays also offer electrical isolation between coil and contact circuits. This means that the coil circuit and contact circuit(s) are electrically insulated from one another. One circuit may be DC and the other AC (such as in the example circuit shown earlier in figure 2), and/or they may be at completely different voltage levels, across the connections or from connections to ground. While relays are essentially binary devices, either being completely on or completely off, there are operating conditions where their state may be indeterminate, just as with semiconductor logic gates. In order for a relay to positively pull in the armature to actuate the contact(s), there must be a certain minimum amount of current through the coil. This minimum amount is called the pull-in current, and it is analogous to the minimum input voltage that a logic gate requires to guarantee a high state (typically 2 Volts for TTL, 3.5 Volts for CMOS). Once the armature is pulled closer to the coil s center, however, it takes less magnetic field flux (less coil current) to hold it there. Therefore, the coil current must drop below a value significantly lower than the pull-in current before the armature drops out to its spring-loaded position and the contacts resume their normal state. This current level is called the drop-out current, and it is analogous to the maximum input voltage that a logic gate input will allow to guarantee a low state (typically 0.8 Volts for TTL, 1.5 Volts for CMOS). The hysteresis, or difference between pull-in and drop-out currents, results in operation that is similar to a Schmitt trigger logic gate. Pull-in and drop-out currents (and voltages) vary widely from relay to relay, and are specified by the manufacturer. Solid State Relays While the electromechanical relay is inexpensive, easy to use and allow the switching of a load circuit controlled by a low power, electrically isolated input signal, one of the main disadvantages of an electromechanical relay is that it is a mechanical device, that is it has moving parts so their switching speed (response time) due to physically movement of the metal contacts using a magnetic field is slow. Over a period of time these moving parts will wear out and fail, or that the contact resistance through the constant arcing and erosion may make the relay unusable and shortens its life. Also, they are electrically noisy with the contacts suffering from contact bounce which may affect any electronic circuits to which they are connected. To overcome these disadvantages of the electrical relay, another type of relay called a Solid State Relay was developed which is a solid state contactless, pure electronic relay. The solid state relay being a purely electronic device has no moving parts within its design as the mechanical contacts have been replaced by power transistors, thyristors or triac s. The electrical Page 3

4 separation between the input control signal and the output load voltage is accomplished with the aid of an opto-coupler type Light Sensor. The Solid State Relay provides a high degree of reliability, long life and reduced electromagnetic interference, (no arcing contacts or magnetic fields), together with a much faster almost instant response time, as compared to the conventional electromechanical relay. Also the input control power requirements of the solid state relay are generally low enough to make them compatible with most IC logic families without the need for additional buffers, drivers or amplifiers. However, being a semiconductor device they must be mounted onto suitable heatsinks to prevent the output switching semiconductor device from overheating. Figure 4 shows an AC type Solid State Relay. It turns ON at the zero crossing point of the AC sinusoidal waveform, prevents high inrush currents when switching inductive or capacitive loads while the inherent turn OFF feature of Thyristors and Triacs provides an improvement over the arcing contacts of the electromechanical relays. A Resistor-Capacitor (RC) snubber network is generally required across the output terminals of the solid state relay to protect the semiconductor output switching device from noise and voltage transient spikes when used to switch highly inductive or capacitive loads. In most modern solid state relays this RC snubber network is built as standard into the relay itself reducing the need for additional external components. Figure 4: Solid State Relay Non-zero crossing detection switching (instant ON ) type solid state relays are also available for phase controlled applications such as the dimming or fading of lights at concerts and shows for motor speed control type applications. As the output switching device of a solid state relay is a semiconductor device (Transistor for DC switching applications, or a Triac/Thyristor combination for AC switching), the voltage-drop across the output terminals of a solid state relay when ON is much higher than that of the electromechanical relay, typically volts. If switching large currents for long periods of time an additional heat sink will be required. The main disadvantages of solid state relays compared to that of an equivalent wattage electromechanical relay is their higher costs, the fact that only single pole single throw (SPST) Page 4

5 types are available, OFF -state leakage currents flow through the switching device, and a high ON -state voltage drop and power dissipation resulting in additional heat sinking requirements. Transistors The design and types of relay switching circuits is huge, but many small electronic projects use transistors as their main switching device as the transistor can provide fast switching (ON-OFF) control. There are different types of transistor: 1. The Bipolar Junction Transistor (BJT) is a three-layer device constructed form two semiconductor diode junctions joined together, one forward biased and one reverse biased. There are two main types of bipolar junction transistors (BJT) the NPN and the PNP transistor, see figure 5. These transistors are Current Operated Devices where a much smaller base current causes a larger emitter to collector current, which themselves are nearly equal, to flow. Figure 5: The two types of BJT Bipolar Junction Transistors (BJT s) can also be used as an electronic switch between their saturation and cut-off regions to control devices such as lamps, motors and solenoids etc. If Inductive loads such as DC motors, relays and solenoids are used, they require a freewheeling diode placed across the load. This helps prevent any induced back emf s generated when the load is switched OFF from damaging the transistor. Let us consider npn bipolar junction transistor for understanding the cut-off and saturation regions: Cut-off region: Here the operating conditions of the transistor are zero input base current (IB), zero output collector current (IC) and maximum collector voltage (VCE) which results in a large Page 5

6 depletion layer and no current flowing through the device. Therefore, the transistor is switched Fully-OFF, see figure 6. Figure 6: Cut-off region Then we can define the cut-off region or OFF mode when using a bipolar transistor as a switch as being, both junctions reverse biased, VB < 0.7v and IC = 0. Saturation Region: Here the transistor will be biased so that the maximum amount of base current is applied, resulting in maximum collector current resulting in the minimum collector emitter voltage drop which results in the depletion layer being as small as possible and maximum current flowing through the transistor. Therefore, the transistor is switched Fully-ON, see figure 7. Figure 7: Saturation region Then we can define the saturation region or ON mode when using a bipolar transistor as a switch as being, both junctions forward biased, VB > 0.7v and IC = Maximum. The transistor operates as a single-pole single-throw (SPST) solid state switch. With a zero signal applied to the Base of the transistor it turns OFF acting like an open switch and zero collector current flows. With a positive signal applied to the Base of the transistor it turns ON acting like a closed switch and maximum circuit current flows through the device. The simplest way to switch moderate to high amounts of power is to use the transistor with an open-collector output and the transistors Emitter terminal connected directly to ground. When used in this way, the transistors open collector output can thus sink an externally supplied voltage to ground thereby controlling any connected load. Page 6

7 2. Field Effect Transistors, or FET s are Voltage Operated Devices ; by controlling the voltage between gate and source, the output current gets varied. They can be divided into two main types: Junction-gate devices called JFET s and Insulated-gate devices called IGFET s or more commonly known as MOSFETs, see figure 8. Figure 8: FET types FET s have very high input resistances so very little or no current (MOSFET types) flows into the input terminal making them ideal for use as electronic switches. Let us consider N channel JFET for understanding the cut-off and saturation regions: Cutoff Region: This is the region in which the drain current ID is zero and the device is OFF. In this the gate source voltage VGS is less than pinch off voltage VP. This means the value of VGS is more negative than VP. Therefore, the channel closes and doesn t allow any current flow through the device. Saturation Region: This region starts from the point where VDS is greater than VGS minus VP (the pinch-off voltage). In this region, the drain current ID entirely depends on the VGS and not a function of VDS. The FET operates in this region to amplify the signals as well as for switching operations. When VGS is zero, the maximum current ID flows. Note: Pinch off voltage is the drain to source voltage after which the drain to source current becomes almost constant and JFET enters into saturation region and is defined only when gate to source voltage is zero. Page 7

8 From the above discussion, it is clear that the FET can be used as a switch by operating it in two regions, they are cutoff and saturation region. When the V GS is zero the FET operates in saturation region and maximum current flows through it. Hence it is like a fully switched ON condition. Similarly, when the VGS applied is more negative than the pinch off voltage, FET operates in cutoff region and doesn t allow any current flow through the device. Hence FET is in fully OFF condition. Another type of FET is a MOSFET which is also a voltage controlled device. The level of VGS at which the drain current will increase or starts flowing is called threshold voltage VT. Therefore, if we increase the VGS, the drain current also increases. And if we increase the VGS by keeping the VDS constant, then the drain current will reach to a saturation level as in the case of JFET. MOSFET operates in the cutoff mode when VGS is below the threshold level. Therefore, no drain current flows in this mode. Hence acts as OPEN switch. For a better understanding consider figure 9 where N-channel enhancement type MOSFET is switched for different voltages at the gate terminal. Figure 9: N-channel MOSFET in cut-off and saturation regions In the previous figure, MOSFET gate terminal is connected to VDD, so that the voltage applied at gate terminal is maximum. This makes the channel resistance becomes so small and allows maximum drain current to flow. This is called as saturation mode and in this mode the MOSFET is completely turned ON as a closed switch. For P-channel enhancement MOSFET, for turning ON, gate potential must be more negative with respect to source. Page 8

9 In cutoff region, VGS applied is less than the threshold voltage level so the drain current is zero. Hence, the MOSFET is in OFF mode just as open switch as shown in figure 9. Table 1 shows a simple comparison between FET s and BJT a: Table 1: Comparison between FET s and BJT s Field Effect Transistor (FET) Bipolar Junction Transistor (BJT) Low voltage gain High current gain Very high input impedance High output impedance Fast switching time Some require an input to turn it OFF Voltage controlled device More expensive than BJT Difficult to bias High voltage gain Low current gain Low input impedance Low output impedance Medium switching time Requires zero input to turn it OFF Current controlled device Cheap Easy to bias Equipment DC power supply. Function generator. 5 V DC relay. Breadboard. Digital multi-meter. Transistor TIP 120. Resistors (various). Procedures Part 1: Relay Switching a- To examine the relay provided, do the following steps: 1) Measure the resistance of the relay coil using the multimeter. Record the value of its resistance. 2) Connect the variable voltage power supply to the relay coil (ensure that the voltage power supply is set to zero). 3) Turn the voltage up very slowly until you hear the relay turn on. Record the voltage of the power source using the multi-meter. 4) Turn the voltage down very slowly until you hear the relay switch off. Record the voltage at which the relay drops. Page 9

10 5) Plot these results on the curve below (figure 10). Figure 10: Relationship between coil voltage and relay status b- To examine the relay as a switch to turn on and off a DC motor, do the following steps: 1) Connect the circuit shown in figure 11. Figure 11: Relay switching speed circuit 2) Change the control signal (i.e. the signal to the coil) between 0 V and 5 V and note its 3) Connect a function generator to the coil. 4) Generate a square wave signal with a 5V peak value. 5) Increase the frequency of the square signal and note the behavior of the DC motor. Part 2: Transistor Switching a- To examine TIP 120 transistor as a switch, do the following: 1) Connect the circuit shown in figure 12. 2) Calculate the value the required based resistor (Rb). Page 10

11 Figure 12: Transistor switching a resistive load b- To examine the transistor switching speed, do the following steps: 1) Replace the 5V DC power supply by a function generator to generate a square wave of 5 V peak value. 2) Increase the frequency of the square signal and note the behavior of the DC motor. Discussion and Conclusions 1- How many terminals does the relay have? Draw a schematic diagram for the relay and identify the function of each terminal. 2- What is the shape of the relationship between coil voltage and relay status curve? And what is this phenomenon called? 3- State the main advantages and disadvantages of using a transistor as a switch compared to a relay? Page 11