SCR Characteristics Trainer NV6530 Learning Material Ver 1.1 141-B, Electronic Complex, Pardeshipura, Indore- 452 010 India Tel.: 91-731- 4211500 Toll-free:1800-103-5050 Email: info@nvistech.com Website: www.nvistech.com
SCR Characteristics Trainer NV6530 Table of Contents 1. Introduction 3 2. Features 4 3. Technical Specification 5 4. Theory 6 5. Experiment Experiment 1 14 To Study and plot V-I Characteristic of SCR Experiment 2 19 To study the phenomenon of holding current and latching current. 6. Data Sheet 21 7. Warranty 23 8. List of Accessories 23 Nvis Technologies Pvt. Ltd. 2
Introduction SCR Characteristic Trainer NV6530 is a compact, ready to use experiment board to demonstrate the fundamental firing concept and operating characteristic of SCR. This is useful for students to plot V-I characteristics of SCR. It can be used as stand alone unit with inbuilt DC power supply hence no external components required. It has inbuilt voltmeter and ammeter to measure the SCR parameters of the circuit like voltage and current. Student can observe and plot a graph between voltage and current in a very simple manner. Nvis Technologies Pvt. Ltd. 3
Features Unique and Compact design Stand alone operation Simple representation of SCR characteristic Ammeter and voltmeter to measure parameters Designed by considering all the safety standards Learning Material 2 Year Warranty Nvis Technologies Pvt. Ltd. 4
Technical Specification Mains Supply : 230 V AC ±10%, 50 Hz DC Power Supply : +35V, +15V, 250mA Voltmeter : 0-35V (max.) Ammeter : 0-50 ma (max.) Weight : 2.15Kg Dimensions (mm) : 345 240 110 Nvis Technologies Pvt. Ltd. 5
Power Electronics: Introduction: Theory Power electronic converters can be found wherever there is a need to modify the electrical energy form (i.e. m odify its voltage, current or frequency).therefore their power range from some milliwatts (as in a mobile phone) to hundreds of megawatts (e.g. in a HVDC transmission system). With "classical" electronics, electrical currents and voltage are used to carry information, whereas with power electronics, they carry power. The first very high power electronic devices were mercury arc valves. In modern systems the conversion is performed with semiconductor switching devices such as diodes, thyristors and transistors. In contrast to electronic systems concerned with transmission and processing of signals and data, in power electronics substantial amounts of electrical energy are processed. An AC/DC converter (rectifier) is the most typical power electronics device found in many consumer electronic devices, e.g., television sets, personal computers, battery chargers, etc. The power range is typically from tens of watts to several hundred watts. In industry the most common application is the variable speed drive (VSD) that is used to control an induction motor. The power range of VSDs starts from a few hundred watts and end at tens of megawatts. The power conversion systems can be classified according to the type of the input and output power Principle: AC to DC (rectification) DC to AC (inversion) DC to DC (chopping) AC to AC As efficiency is at a premium in a power electronic converter, the losses that a power electronic device generates should be as low as possible. The instantaneous dissipated power of a device is equal to the product of the voltage across the device and the current through it (P = VI). From this, one can see that the losses of a power device are at a minimum when the voltage across it is zero (the device is in the On-State) or when no current flows through it (Off-State). Therefore, a power electronic converter is built around one (or more) device operating in switching mode (either On or Off). With such a structure, the energy is transferred from the input of the converter to its output by bursts. Applications: Power electronic systems are virtually in every electronic device. For example, around us: DC/DC converters are used in most mobile devices (mobile phone, pda.) to maintain the voltage at a fixed value whatever the charge level of the battery is. These converters are also used for electronic isolation and power factor Nvis Technologies Pvt. Ltd. 6
correction. AC/DC converters (rectifiers) are used every time an electronic device is connected to the mains (computer, television,...) AC/AC converters are used to change either the voltage level or the frequency (international power adapters, light dimmer). In power distribution networks AC/AC converters may be used to exchange power between utility frequency 50 Hz and 60 Hz power grids. DC/AC converters (inverters) are use d primarily in UPS or emergency light. During normal electricity condition, the electricity will charge the DC battery. During blackout time, the DC battery will be used to produce AC electricity at its output to power up the appliances. SCR Theory The Silicon Controlled Rectifier (SCR) is a semiconductor device that is a member of a family of control devices known as Thyristors. The SCR has become the work house of the industrial control industry. Its evolution over the year has yielded a device that is less expensive, more reliable and smaller in size than ever before Typical applications include : DC motor control, generator field regulation,variable frequency drive (VFD) DC bus voltage control solid state Relays and lighting system control. The SCR is a three lead device with an anode and a cathode ( us with a standard diode) plus a third control lead or gate. As the name implies, it is a rectifier which can be controlled or more correctly one hat can be triggered to the ON state by applying a small positive voltage (VTM) to the gate lead. Once gated ON, the trigger signal may be removed and the scr will remain conducting as long as current flows through the device. The load to be controlled by the SCR is normally placed in the anode circuit. Introduction:- Commutation:- For the SCR to turn OFF current flow through the device must be interrupted, or drop below the maximum holding current(ih), for a short period of time (typically 10-20 microseconds) which is known as the commutated turn off time (tq). When applied to alternating current circuits or pulsating DC system, the device will self commutate at the end of every half cycle when the current goes through zero. When applied to pure DC circuits, in applications such as alarm or trip circuit latching, the SCR can be reset manually by interrupting the current with a push button. When used in VFD's or inverters,scrs are electronically forced OFF using additional Commutating circuitry, Such as smaller SCRs and capacitors, which momentarily apply an opposing reverse bias voltage across the SCR.(This is complicated everything has to be exactly right). Nvis Technologies Pvt. Ltd. 7
SCR is one of the most important types of power semiconductor device. They are operated as bitable switches, operating from non conducting state to conducting state. Figure l shows the schematic symbol of SCR and its internal layer diagram. Basic three modes of operation of SCR are Reverse blocking mode Symbol and Internal Layer Cathode is positive with respect to anode with gate open. Figure 1 SCR is in reverse bias i.e., junction J1 & J3 in reverse bias J2 is in forward bias. The device act as two PN diode connected in series with reverse voltage applied across it. Small leakage current of the order of a few milliampere or microampere flows, this is off state of SCR. If reverse voltage increases, then at critical breakdown level or reverse breakdown voltage (VBR) an avalanche occurs at J1 & J3 & reverse current increase rapidly, so more loss in SCR. This may lead to SCR damage because junction temperature is increasing. Maximum working reverse voltage across SCR does not exceed VBR. If applied reverse voltage across SCR < VBR, then the device offers high impedance in reverse direction. SCR is treated as open switch. Forward blocking mode (off state mode) Anode is positive with respect to cathode with gate open. SCR is forward bias, junction J1 & J3 is forward bias and J2 is reverse bias. Here small forward leakage current flow. If forward voltage increases then J2 junction (rev. bias) w ill have avalanche breakdown called forward break over voltage (VBO) Maximum working forward voltage across SCR does not exceed VBO. If forward voltage < VBO; SCR offers high impedance. Hence SCR is treated as open switch even though it is forward blocking mode. Nvis Technologies Pvt. Ltd. 8
3. Forward conduction mode (on state mode) If we want to bring SCR from forward blocking mode to forward conduction mode there are two modes: By exceeding the forward break over voltage (VBO). By applying gate pulse between gate and cathode. Then SCR is in on state and behave as closed switch. Terminal Characteristic of Thyristor: V-1 Characteristics Figure 2 Thyristor is a four layers, three junction, p-n-p-n semiconductor switching device. It has three terminals; anode, cathode and gate. Fig. 3 gives constructional details of a typical Thyristor. Basically, a Thyristor consists of four layer of alternate p-type and n-type silicon semiconductors forming three junctions j1, j2, j3 as shown in Fig. 3.The threaded portion is for the purpose of tightening the Thyristor to the frame or heat sink with the help of a nut.gate terminal is usually kept near the cathode terminal, Fig. 1.Schematic diagram and circuit symbol for a Thyristor are shown respectively in Fig. 4 and Fig. 5.The terminal connected to outer p region is called anode(a),the terminal connected to outer n region is called cathode and that connected to inner better p region is called the gate (G). For large current application, Thyristor need better cooling; this is achieved to a great extent by mounting them onto heat sinks. SCR rating has improved considerably since its introduction in 1957. Now SCR of voltage rating 10kV and an rams current rating of 3000 A with corresponding power-handling capacity of 30MW are available. Such a high power Thyristor can be switched amplification capability (=3*10power 6) of this device. As SCR are solid state devices, they are compact, posses high reliability and have low loss. Because of these useful features, SCR is almost universally employed these days for all high power-controlled devices. Nvis Technologies Pvt. Ltd. 9
Th re a d e d s tu d Anode (Aluminium) P J 1 N J 2 P N G a te te rm in a l w e ld e d to P re g io n J 3 C a th o d e Constructional details Figure 3 Schematic diagram Figure 4 Circuit symbol of a thyristor Figure 5 Nvis Technologies Pvt. Ltd. 10
An SCR is so called because silicon is used for its construction and its operation as a rectifier (very low resistance in forward conduction and very high resistance in the reverse direction) can be controlled. Like the diode, an SCR is an unidirectional device that blocks the current flow from cathode to anode. Unlike the diode, a thyristor also blocks the current flow from anode to until it is triggered into conduction by a proper gate signal between gate and cathode terminals.for engineering applications of thyristors, their terminal characteristics must be known. Static I-V Characteristics of a Thyristor: An elementary circuit diagram for obtaining static I-V characteristics of a thyristor is shown in Fig. 6. The anode and cathode are connected to main source through the load.the gate and cathode are fed from a source Es which provides positive gate current from gate to cathode. Fig. 7 shows static I-V characteristics of thyristor. Here Va is the anode voltage across thyristor terminal A, K and Ia is the anode current. Typical SCR, I-V characteristic shown in Fig. 7 reveals that a thyristor has three basic modes of operation ; namely reverse blocking mode, forward blocking (off-state) mode and forward conduction ( on state ) mode. These three modes of operation are now discussed below: Reverse Blocking Mode: When cathode is made positive with respect to anode with switch S open, Fig. 6, thyristor is reverse biased as shown in Fig.8.Junctions j1,j3 are seen to be reverse biased whereas junction j2 is forward biased. The device behaves as if two diodes are connected in series with reverse voltage applied across them. A small leakage current of the order a few milliamperes (or a few microamperes depending upon the SCR rating) flows. This is reverse blocking mode, called the off-state,of the thyristor. In Fig 7, reverse blocking mode is shown by OP. If the reverse voltage is increased, then at a critical breakdown level, called reverse breakdown voltage VBR, an avalanche occurs at J1 and J3 and the reverse current increases rapidly. A large current associated with VBR gives rise to more losses in the SCR. This may lead to thyristor damage as the junction temperature rise. It should, therefore, be ensured that maximum working reverse voltage, across a thyristor does not exceed VBR. In Fig.7, reverse avalanche region is shown by PQ. When reverse voltage applied across a SCR is less than VBR, the device offers high impedance in the reverse direction. The SCR in the reverse blocking mode may therefore be treated as an open switch. + L o a d A S Va E G E s _ Elementary circuit for obtaining thyristor I-V characteristics K Figure 6 Nvis Technologies Pvt. Ltd. 11
VBO = Forward breakover voltage VBR = Reverse Breakdown voltage Ig = Gate Current Static I-V characteristics of a thyristor Figure 7 Note that I-V characteristic after avalanche breakdown during reverse blocking mode is applicable only when load resistance is present, a large anode current associated with avalanche breakdown at VBR would cause substantial voltage drop across load and as a result, I-V characteristic in third quadrant would bend to the of vertical line at VBR. J2 forward biased and J1, J3 reverse biased Figure 8 Nvis Technologies Pvt. Ltd. 12
Forward blocking mode: J2 reverse biased and J1, J3 forward biased Figure 9 When anode is positive with respect to the cathode, with gate circuit open, thyristor is said to be forward biased as shown in Fig.9. It is seen from this figure that junctions J1,J3 are forward biased but junction J2 is reverse biased. In this mode, a small current, called forward leakage current, flows as shown in Figs.7 and 9. In Fig.7, OM represents the forward blocking mode of SCR. AS the forward leakage current is small, SCR offers high impedance. Therefore, a SCR can be treated as an open switch even in the forward blocking mode. Forward conduction mode: When anode to cathode forward voltage is increased with gate circuit open, reverse biased junction J2 will have an avalanche breakdown at a voltage called forward break over voltage VBO. After this breakdown, thyristor gets turned on with point M at once shifting to N and then to a point anywhere between N and K. Here NK represents the forward conduction mode. A SCR can be brought from forward blocking mode to forward conduction mode by turning it on by applying (1) a positive gate pulse between gate and cathode or (2) a forward break over voltage across anode cathode. Forward conduction mode NK shows that voltage drop across thyristor is of the order of 1 to 2 V depending upon the rating of SCR. It may also be seen from NK that voltage drop across SCR increase slightly with an increase in anode as voltage drop across SCR is quit small. This small voltage drop VT across the device is due to ohmic drop in four layers. In forward conduction mode, thyristor is treated as a closed switch. Nvis Technologies Pvt. Ltd. 13
Experiment 1 Objective: Study and plot V-I Characteristic of SCR Equipments Needed: 1. NV6530 SCR Characteristic Trainer 2. 2mm Patch cords Circuit diagram: Circuit used to plot different characteristics of SCR is shown in figure 10. NV6530 SCR Characteristics Trainer Power 5 6 7 +35V 1 4 +35V +15V 12 R1 A P1 SCR K G 9 10 11 R2 P2 Gnd 2 8 13 Voltmeter Ammeter +15V 3 V ma + - + - Procedure: Figure 10 1. Connect terminal1 to terminal 4, terminal 2 to terminal 8 and terminal 3 to terminal 12 as shown in figure 11. Nvis Technologies Pvt. Ltd. 14
NV6530 SCR Characteristics Trainer Power 5 6 7 +35V 1 4 +35V +15V 12 R1 A P1 SCR K G 9 10 11 R2 P2 Gnd 2 8 13 Voltmeter Ammeter +15V 3 V ma + - + - Figure 11 Figure 12 Nvis Technologies Pvt. Ltd. 15
2. Connect Voltmeter across terminal 7 and 8 and Ammeter across terminal 9 and 10 as shown in figure 12. 3. Make short terminals 5 and 6. 4. Rotate the knob P1 and P2 fully in counter clockwise. 5. Switch ON the power supply. 6. Set the value of Anode Voltage at 35V by using the knob P1. 7. Now Increases gate current Ig gradually by varying knob P2 and observe it. 8. At certain value of gate current, voltmeter reading falls down to almost zero. This action indicates the firing of SCR. 9. Note the gate current value at this position (firing of SCR). 10. Keep the gate current constant by shorting terminal 9 with 10 and connect Ammeter to the terminal 5 and 6 (as in figure 13). NV6530 SCR Characteristics Trainer Power 5 6 7 +35V 1 4 +35V +15V 12 R1 A P1 SCR K G 9 10 11 R2 P2 Gnd 2 8 13 Voltmeter Ammeter +15V 3 V ma + - + - 11. Rotate the potentiometer P 1 fully in counter clockwise. Figure 13 12. Rotate knob P1 (from initial position to its maximum limit) gradually and record Anode current for respective value of anode voltages. 13. Plot the graph between anode voltage V a and anode current I a. Nvis Technologies Pvt. Ltd. 16
Observation Table: Result Analysis:- Anode Voltage (Va) Anode current (Ia) Gate Current Ig = 7 ma 0 0 1.3 1.9 3.6 2 5.6 2.1 7.2 2.3 8.9 2.4 11.1 2.6 12.7 2.9 14.6 3.5.8 15.1.8 42.1 Nvis Technologies Pvt. Ltd. 17
Characteristic of SCR Figure 14 Latching Current and Holding Current: Latching Current:-Once the SCR is conducting a forward current, reverse biased junction J2 no longer exists. As such, no gate current is required for the device to remain in on state. Therefore, if the gate current is removed, the conduction of current from anode to cathode remains unaffected. However, if gate current is reduced to zero before the rising anode current attains a value, called the latching current, the thyristor will turn off again. The gate pulse width should therefore be judiciously chosen to ensure that anode current rises above the latching current. Thus latching current may be defined as the minimum value of anode current which it must attain during turn-on process to maintain conduction when gate signal is removed. Holding Current:-Once the thyristor is conducting, gate loses control. The thyristor can be turned-off (or the thyristor can be returned to forward blocking state) only if the forward current falls below a low level current called the Holding current. Thus holding current may be defined as the minimum value of anode current below which it must fall for turning -off the thyristor. The latching current is higher than the holding current. Nvis Technologies Pvt. Ltd. 18
Experiment 2 Objective: To study the phenomenon of holding current and latching current. Apparatus Required: 1. NV6530 SCR Characteristic Trainer 2. 2mm Patch cords Circuit Diagram: NV6530 SCR Characteristics Trainer Power 5 6 7 +35V 1 4 +35V +15V 12 R1 A P1 SCR K G 9 10 11 R2 P2 Gnd 2 8 13 Voltmeter Ammeter +15V 3 V ma + - + - Figure 15 Procedure: 1. Make the connections as shown in the above figure 2. Rotate both the potentiometer at fully anticlockwise direction. 3. Set the value of Anode Voltage at 35V using the potentiometer P1. 4. Increase the value of gate current as slow as you can using the potentiometer P2 and bring the SCR in Conduction band. Nvis Technologies Pvt. Ltd. 19
5. Now remove the gate current by removing the patch cord between the point 9 and point 10. 6. Here you will observe that the SCR is still in conduction band. It shows that, once the SCR is triggered no gate current is required for the device to remain in on state. That value of anode current is known as Latching Current. 7. Now decrease the anode current very slowly using the potentiometer P1, here you will see that after a particular value of anode current the SCR will turn-off. That value of anode current is known as Holding Current. Result Analysis: Latching Current: 42mA (approximately) Holding Current: 31 ma (approximately) Nvis Technologies Pvt. Ltd. 20
Data Sheet Nvis Technologies Pvt. Ltd. 21
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Warranty We guarantee the product against all manufacturing defects for 24 months from the date of sale by us or through our dealers. Consumables like dry cell etc. are not covered under warranty. The guarantee will become void, if The product is not operated as per the instruction given in the learning material. The agreed payment terms and other conditions of sale are not followed. The customer resells the instrument to another party. Any attempt is made to service and modify the instrument. The non-working of the product is to be communicated to us immediately giving full details of the complaints and defects noticed specifically mentioning the type, serial number of the product and date of purchase etc. The repair work will be carried out, provided the product is dispatched securely packed and insured. The transportation charges shall be borne by the customer. List of Accessories 1. 2mm Patch Cord 12...10 Nos. 2. Mains Cord...1 No. 3. Learning Material CD. 1 No. Nvis Technologies Pvt. Ltd. 23