thyristors and sensors

Copyright ©2009 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 • All rights reserved.

Electronic Devices and Circuit Theory, 10/e Robert L. Boylestad and Louis Nashelsky

Department of Electronics and Communication Engineering

Industrial Electronics

Unit 3: Thyristors



Thyristors

-are solid-state semiconductor devices with four layers of alternating N and P-type material.

- comes from the Greek word that means “door”.

Use internal feedback to produce switching action.

They act as bistable switches, conducting when their gate receives a current pulse, and continue to conduct for as long as they are forward biased (that is, as long as the voltage across the device has not reversed).



SCR—Silicon-Controlled Rectifier

The SCR is a switching device for high-voltage and high-current operations. Like an ordinary rectifier, an SCR conducts in one direction

The terminals are: • Anode • Cathode • Gate

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SCR Operation

Once an SCR is switched on, it remains latched on, even when the gate signal is removed.

To switch on an SCR:

• Forward bias the anode-cathode terminals (VF)

AND

• Apply sufficient gate voltage (Vgate) and gate current (IGT)

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• Forward Breakover Voltage V(BR)F* is the voltage above which the SCR enters the conduction region. It is dependent on the condition of the gate terminal as:

O= Open Circuit from G to K S=Short Circuit form G to K R=resistor from G to K V=fixed bias (voltage) from G to K • Holding current (IH) is the minimum required current from anode to cathode • Forward and reverse blocking regions correspond to the open-circuit condition for the

controlled rectifier which block the flow of charge (current) from anode to cathode. • Reverse breakdown voltage is the maximum reverse bias voltage for the SCR

equivalent to the Zener or avalanche region.

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SCR Characteristics



Commutation circuitry is simply a class of switching devices connected in parallel with the SCR. A control signal activates the switching circuitry and provides a low impedance bypass for the anode to cathode current. This momentary loss of current through the SCR turns it off. The switching circuitry can also apply a reverse bias voltage across the SCR, which also will turn off the SCR.

SCR Commutation

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2. Current Interruption is done by either opening the series switch ( Figure A) or closing the parallel switch (Figure B) . In this way, the anode-to-cathode current will drop down below its holding current value and the SCR will switch to its off state.

Switching off the SCR

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1. The anode to cathode current should be reduced to less than its holding current, IH. This can be done by reducing the VCC to a low value.

SW RG

RL

VCC

VIN

SW RG

RL

VCC

VIN

Figure A Figure B



Switching off the SCR

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3. Forced commutation happens when the switch is depressed, a negative VAK voltage is momentarily applied. This reduces the anode-to-cathode current below IH and turns off the SCR.



Basic SCR Circuit

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Required Input Voltage

GGTGTin RIVV +=

Supply Voltage for SCR Turnoff

LH RIVVcc += 7.0

where: VGT=gate trigger voltage IGT=gate trigger current IH= holding current



Example

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The SCR shown below has a trigger voltage of 0.75V and a trigger current of 7mA. a. What is the input voltage that turns the SCR on? b. If the holding current is 6mA, what is the supply voltage that turns it off?

+15V

Vin

100Ω

1kΩ VGT = 0.75V IGT = 7mA IH = 6mA



Example

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0.2µF

900Ω

Vout

VGT = 1V IGT = 200µA

110V

1kΩ

a) What is the peak output voltage? Assume that the gate voltage is one-tenth of the output voltage.

b) What is the frequency of the sawtooth wave if its period is approximately 20 percent of the time constant?

100Ω



SCR Applications

In these applications the SCR gate circuit is used to monitor a situation and trigger the SCR to turn on a portion of the circuit.

• Battery-charging regulator • Temperature controller circuit • Emergency-lighting system

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SCS—Silicon-Controlled Switch

An SCS is like an SCR, except that it has two gates: a cathode gate and an anode gate.

Either gate can fire the SCS

• A positive pulse or voltage on the cathode gate

• A negative pulse or voltage on the anode gate

Either gate can switch off the SCS

• A negative pulse or voltage on the Cathode gate

• A positive pulse or voltage on the anode gate

Note: The anode gate requires higher voltages than the cathode gate.

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SCS

Comparison of the SCR and SCS:

• The SCS has a much lower power capability compared to the SCR

• The SCS has faster switching times than the SCR • The SCS can be switched off by gate control

Applications

• Pulse generator • Voltage sensor • Alarm circuits

Pin Identification

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GTO—Gate Turn-Off Switch

GTOs are similar to SCRs, except that the gate can turn the GTO on and off. It conducts only in one direction.

Applications

• Counters • Pulse generators • Oscillators • Voltage regulators

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Anode

Gate

Cathode



GTO Comparison of the GTO and SCS:

• GTO is a low power device • The gate signal necessary to fire the GTO is larger than the SCR gate signal. • The gate signal necessary to turn the GTO off is similar to that of SCS • The switching rate for turning the GTO off is much faster than the SCR

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A simple sawtooth generator using GTO

Time Constant:

1GTO

13CR

CR=τ=τ

where: RGTO= internal resistance of GTO



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The Phototransistor

The phototransistor is a light-controlled transistor. The current through the collector and emitter circuits is controlled by the light input at the base.

The collector current is the product of the transistor current gain (hfe) and the light induced base current (Iλ).

λfeC IhI =



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Phototransistor IC Package



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Opto-Isolators

Photodiode

Photo-Darlington Photo-SCR

It is a package that contains both an infrared LED and a photodetector such as a silicon diode, transistor Darlington pair or SCR.

They are designed with response times so small that they can be used to transmit data in the MHz range.



A potentiometer is a variable resistor that can be used as a voltage divider.

The popular usage of the term potentiometer describes an electrical device which has a user-adjustable resistance.

Usually, this is a three-terminal resistor with a sliding contact in the center.

If all three terminals are used, it can act as a variable voltage divider.

If only two terminals are used (one side and the wiper), it acts as a variable resistor.

Its shortcoming is that of corrosion or wearing of the sliding contact, especially if it is kept in one position.

Potentiometer

Unit 6: Input Transducers and Measuring Devices



Potentiometer as Measuring Instrument

Potentiometers are sometimes provided with one or more switches mounted on the same shaft.

For instance, when attached to a volume control, the knob can also function as an on/off switch at the lowest volume.

Linear Potentiometer

PCB mount trimmer potentiometers, or "trimpots", intended for infrequent adjustment

A typical single-turn potentiometer

1 is normally connected to ground, the input is applied to 3, and the output taken from 2 (wiper) allowing the output to be varied from ground (no signal) to input (maximum signal).

Terminals for Single Turn Pot:

http://en.wikipedia.org/wiki/File:PCB_variable_resistors.jpg

http://en.wikipedia.org/wiki/File:Faders.jpg



Transducer is a device that converts one type of energy to another.

The conversion can be to/from electrical, electro-mechanical, electromagnetic, photonic, photovoltaic, or any other form of energy.

While the term transducer commonly implies use as a sensor/detector, any device which converts energy can be considered a transducer.

Types of Transducers A sensor is used to detect a parameter in one form and report it in another form of energy (usually an electrical and/or digital signal). For example, a pressure sensor might detect pressure (a mechanical form of energy) and convert it to electricity for display at a remote gauge. An actuator accepts energy and produces movement (action). The energy supplied to an actuator might be electrical or mechanical (pneumatic, hydraulic, etc.). An electric motor and a loudspeaker are both transducers, converting electrical energy into motion for different purposes. ultrasonic waves, and acting as a sensor to detect ultrasonic waves. Combination transducers have both functions -- they both detect and create action. For example, a typical ultrasonic transducer switches back and forth many times a second between acting as an actuator to produce ultrasonic waves, and acting as a sensor to detect ultrasonic waves.



The Linear Variable Differential Transformer is a transducer that can convert the rectilinear motion of an object to which it is coupled mechanically into a corresponding electrical signal.

LVDT

The LVDT operation does not require electrical contact between the moving part (probe or core rod assembly) and the transformer, but rather relies on electromagnetic coupling; this and the fact that they operate without any built-in electronic circuitry are the primary reasons why LVDTs have been widely used in applications where long life and high reliability under severe environments are required, such as Military/Aerospace applications.



The LVDT's primary winding, P, is energized by a constant amplitude AC source. The magnetic flux thus developed is coupled by the core to the adjacent secondary windings, S1 and S2.

If the core is located midway between S1 and S2, equal flux is coupled to each secondary so the voltages, E1 and E2, induced in windings S1 and S2 respectively, are equal.

At this reference midway core position, known as the null point, the differential voltage output, (E1 - E2), is essentially zero.

If the core is moved closer to S1 than to S2, more flux is coupled to S1 and less to S2, so the induced voltage E1 is increased while E2 is decreased, resulting in the differential voltage (E1 - E2).

Conversely, if the core is moved closer to S2, more flux is coupled to S2 and less to S1, so E2 is increased as E1 is decreased, resulting in the differential voltage (E2 – E1).

Principles of Operation



Features of LVDT

One of the most important features of an LVDT is its friction-free operation. In normal use, there is no mechanical contact between the LVDT's core and coil assembly, so there is no rubbing, dragging or other source of friction. This feature is particularly useful in materials testing, vibration displacement measurements, and high resolution dimensional gagging systems.

Friction-Free Operation

Since an LVDT operates on electromagnetic coupling principles in a friction-free structure, it can measure infinitesimally small changes in core position. This infinite resolution capability is limited only by the noise in an LVDT signal conditioner and the output display's resolution. These same factors also give an LVDT its outstanding repeatability.

Infinite Resolution



Features of LVDT

Unlimited Mechanical Life Because there is normally no contact between the LVDT's core and coil structure, no parts can rub together or wear out. This means that an LVDT features unlimited mechanical life. This factor is especially important in high reliability applications such as aircraft, satellites and space vehicles, and nuclear installations. It is also highly desirable in many industrial process control and factory automation systems. Separable Coil And Core Because the only interaction between an LVDT's core and coil is magnetic coupling, the coil assembly can be isolated from the core by inserting a non-magnetic tube between the core and the bore. By doing so, a pressurized fluid can be contained within the tube, in which the core is free to move, while the coil assembly is unpressurized. This feature is often utilized in LVDTs used for spool position feedback in hydraulic proportional and/or servo valves.



is a junction between two different metals that produces a voltage related to a temperature difference.

is widely used type of temperature sensor for measurement and control and can also be used to convert heat into electric power.

is inexpensive and interchangeable, is supplied fitted with standard connectors, and can measure a wide range of temperatures.

Thermocouple



Properties of Thermocouple

Wire Size of Thermocouple: Selecting the wire size used in the thermocouple sensor depends upon the application. Generally, when longer life is required for the higher temperatures, the larger size wires should be chosen. When sensitivity is the prime concern, the smaller sizes should be used.

Length of Thermocouple Probe: Since the effect of conduction of heat from the hot end of the thermocouple must be minimized, the thermocouple probe must have sufficient length. Unless there is sufficient immersion, readings will be low. It is suggested the thermocouple be immersed for a minimum distance equivalent to four times the outside diameter of a protection tube or well.



Thermocouple Reference Table

Thermocouple Type Names of Materials Useful Application Range

B Platinum 30% Rhodium (+) Platinum 6% Rhodium (-)

2500 -3100F 1370-1700C

C W5Re Tungsten 5% Rhenium (+) W26Re Tungsten 26% Rhenium (-

)

3000-4200F 1650-2315C

E Chromel (+)

Constantan (-) 200-1650F 95-900C

J Iron (+)

Constantan (-) 200-1400F 95-760C

K Chromel (+) Alumel (-)

200-2300F 95-1260C

N Nicrosil (+)

Nisil (-) 1200-2300F 650-1260C

R Platinum 13% Rhodium (+)

Platinum (-) 1600-2640F 870-1450C

S Platinum 10% Rhodium (+)

Platinum (-) 1800-2640F 980-1450C

T Copper (+)

Constantan (-) -330-660F -200-350C



Thermistors

are temperature-sensitive resistor; that is, its terminal resistance is related to its body temperature.

are resistors whose value changes with temperature.

Operation Thermistors are negative-coefficient devices—their resistance decreases as the temperature increases.

Schematic Symbol

Applications

Sensors in instrumentation circuits Temperature correction circuitry



Example:

0.2V 2mA, 0V

Rvariable Thermistor

10Ω

If the current through the sensitive movement is 2 mA and the voltage drop across the movement is 0V, what is the resistance of the thermistor?

A simple change in current through the device will result in an internal change in temperature. By changing the temperature of the surrounding medium or immersing the thermistor in a hot or cold solution can change the temperature of the device.

Two fundamental ways to change the temperature of thermistor

Ω=Ω−Ω=−=

+=

Ω==

=

9010100)Thermistor(RRR)Thermistor(R

)Thermistor(RRR

100mA2

V2.0R

IVR

iablevarT

iablevarT

T

T



Example: Consider the circuit given below to be used for temperature measurement. The used thermistor in this circuit is a type. The meter is a 50mA meter with a resistance of , Rc is set to a resistance of and supply Vt is 15V. Using the given graph, calculate the meter reading at 77oF and 150oF



From the graph of temperature versus resistance, the resistance at 77oF is Ωk4 . Therefore the current at 77oF is:

At 150oF, the graph of temperature versus resistance shows that the thermistor resistance is approximately Ω950 , the meter reading them will be:

mARVI

t

t 73.33174000

15=

++==

mARVI

t

t 5.15317950

15=

++==

thyristors and sensors

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