1. Introduction Thyristors SCR TRIAC DIAC Stepping Motors Summary
Final Control 1
Final control refers to the device that is the final link in the control chain. To power these motors, valves, etc. high power switching devices are required This presentation will focus on how some of the basic theory of thyristors and their specifications and applications. Then there will be a discussion of the fundamentals of how stepping motors work.
Transition:
First is a look at the fundamental definition of a Thyristor.

2. Thyristor SCR-Silicon Controlled Rectifier Triac Diac etc
A thyristor is a four-layer semiconductor device, consisting of alternating P type and N type materials (PNPN). A thyristor usually has three electrodes: an anode, a cathode, and a gate (control electrode). The most common type of thyristor is the silicon-controlled rectifier (SCR).
Thyristors are used in motor speed controls, light dimmers, pressure-control systems, and liquid-level regulators.
Thyristor SCR-Silicon Controlled Rectifier Triac Diac etc 2
You are familiar with diodes which have a single PN junction. You have also studied BJT’s and MOSFET’s which have a PNP or NPN configuration with two PN junctions. Thyristors have alternating PNPN junctions and are a three junction device. Thyristors are generally used for controlling high currents and the Hockey Puck SCR, shown on the right of the slide from the web site provided can control over 2000 amps of current. The thyristor shown on the left is of the same type and I use it as a paperweight (since it is burned out). It weights over a pound and the top and bottom are metal and there is a gate control on the side. You can see the size of the one in my office from the image on the left of the slide.
The symbol for an SCR is also shown in the middle of the slide.
Transition:
Lets first look at when SCR’s were invented.
SCR Symbol
Hockey Puck SCR

3. A commercially practical solid state rectifier has been operated by engineers at General Electric's Clyde, New York Rectifier Engineering Laboratory. This silicon-controlled rectifier operates in the same manner as a thyratron, and is capable of switching 1000 watts. A power rating of this magnitude is sufficient for most military and commercial applications. The device is expected to be first used in missiles.
The size of the silicon controlled rectifier is approximately twice that of a signal type transistor and 1/100 the size of a thyratron. The predicted life of the device is over 300,000 hours as compared to the 1000 hour life of a thyratron.
(Electronic Design, Jan. 8, 1958, p. 7)
The first SCRs were compared to thyratrons because that's what vacuum-tube era engineers understood. For today's engineer, we'd probably have to reverse the analogy--a thyratron, a gas-filled grid-controlled rectifier, was like an SCR.--Steve Scrupski 3
The article in red is from the January 8, 1958 Electronic Design Magazine and discusses General Electric’s invention of the SCR. The SCR is compared with the electron tube device called a Thyratron. The solid-state SCR is much smaller and more efficient, and has a much longer life than the tube device it replaced.
Thyratrons are still manufactured and a picture of one is shown on the right courtesy of the website shown below the image. Modern applications of the Thyraton are in high energy physics, high energy medical devices, and lasers and all must put out kVolt pulses with an amp or more of current. This is apparently still outside the range of solid state electronics.
Transition:
Next is a look at how an SCR works.
Technical Papers
Thyratron Power Supply at Fermilab
SCR Origins

4. Operation of an SCR http://www.tpub.com/neets/book7/26c.htm 4
A good explanation of how an SCR works is at the website shown. The 4 images with a green background are also from this website. The image on the upper left is of a power SCR that can switch hundreds of amperes of current on and off. The upper right image is the circuit symbol for an SCR.
The lower left image shows the structure of an SCR with three PN junctions. The lower middle image is the two BJT analogy to help explain the operation of an SCR. The lower right image is also the 2-BJT representation of an SCR, using standard transistor circuit symbols.
Consider how an SCR is switched on. First assume that the SCR is in the OFF state. If the Anode voltage (A) is greater than the Cathode voltage (K), the main current from the anode to the cathode is blocked until there is an input current into the Gate (G). When there is a voltage between the Gate and the Cathode large enough to turn on the bottom PN junction, this turns on the lower transistor. When this lower, NPN, transistor is turned on, it pulls current out of base of the upper PNP transistor, which turns it on. This causes the entire Device to rapidly conduct large amounts of current from the Anode to the Cathode. This is the ON condition of the SCR switch.
Once the device has been turned on, the only way to turn it off is for the voltage/current from the Anode to the Cathode to become very small. This turns off the upper transistor which turns off the device. Remember that BJT’s are current devices. They show a current gain from the base to the output in the collector or emitter circuit. Also remember that BJT’s are controlled by the base current. When it is high enough, the transistor CE junction is turned on. Here, the gate current turns on the lower transistor, which draws current from the base of the upper transistor and turns on the upper transistor. A LARGE flow of current through the entire device is then activated. From the diagram on the right you can see that the current into the anode flows through the CE junction of the upper transistor, through the base of the upper transistor, through the base of the lower transistor and through the CE junction of the lower transistor. These junctions are made to handle large amounts of current, but operate in the same manner as any PN junction.
The SCR is widely used in industry and you should look at the website shown to become familiar with its operation.
Transition:
Next is a further look at how an SCR works and some characteristics of SCR’s.

5. Gate Controlled turn-on time
Igate
(turn-on)
Imax
(Main)
Gate Controlled turn-on time mA
.8-7 Amps
<2 usec 5
SCR’s come in many sizes and package shapes. The ones shown near the bottom of the slide are from the website provided and have a gate turn-on current from .8 to 25 mA to control a maximum current through the SCR of .8 to 7 amps. The gate-controlled turn-on time is less than 2 microseconds for all of the devices shown.
Remember that SCR’s can control currents of several thousand amps and, of course the gate current for turn-on is generally larger when a larger current is being controlled because PN junctions increase in size to handle the greater anode to cathode current and therefore require more base current to turn on. The turn-on time is also greater when the SCR junctions are physically larger, which is necessary to handle a higher current. The forward resistance of an SCR that is turned on is low, on the order of 1 volt or less for many SCR’s.
Transition:
Next is a look at the Volt-Amp curve for an SCR.
SCR Characteristics

6. SCR V-I curve I V VAK= 1-1.5 volts Very High Very High Reverse voltage
to break it down 6
The V-I curve for an SCR is shown in general terms on the slide, from the website at the bottom of the slide. You must choose an SCR so that the forward and reverse breakdown voltages are higher than the peak AC voltage controlled. The forward breakover voltage is the voltage that would cause the device to conduct without gate current. This is not desirable and is not used.
Essentially, in the reverse direction, only a small leakage current flows through the device. In the forward direction, you can turn the device on using the gate so that at the time desired, the device goes from the blue, blocking line, to the red, high conduction line.
Transition:
Next a look at a PSpice SCR circuit.
SCR V-I curve

7. SCR Circuit Characteristics of 2N15957
The slide shows a simple SCR circuit to deliver power to the load during the positive cycle of the sinusoidal input. The 5 volt DC voltage at the gate combined with the 1k resistor provides the turn-on current for the SCR gate. The characteristics of the 2N1595 are at the website shown near the top of the slide.
Some of the key specifications for this low current, inexpensive, SCR are shown near the bottom of the slide. The 2N1595 can handle a peak reverse voltage of 50 volts without breaking down, but the peak voltage for the input is only 20 volts, so this is not a problem.
In this circuit the gate is set in a turned-on state at all times since it has 5 volts across it. Remember that the SCR is a unidirectional device so that it can only turn on in the forward direction, like a diode. When the voltage is negative, the SCR turns off, no matter what the gate is set at. So, this circuit should produce a current through the load only when the input voltage source is greater than 0.
The SCR in this circuit is performing like a diode and passing the current through in one direction only.
Transition:
Next is a look at the current across the load.
Peak Reverse Blocking Voltage = 50 volts
RMS Forward Current (max) = 1.6 amps
Typical Gate Trigger Current= 2 mA
Typical Gate Trigger Voltage= .7 volts
Turn-on time = .8 usec Turn-off time=10 usec
SCR Circuit

8. 8
The current output of the SCR circuit is shown here. Note that without the SCR, the peak current would be exactly 1 Amp so there is some voltage across the SCR. This voltage is from the anode to the cathode and is shown as VAK on the circuit diagram. Only the first two cycles of the output current are shown on the slide. The PSpice circuit is available for you to download and run. You should look at the various voltages and currents in the circuit so you understand how an SCR works.
Notice that this circuit simply rectifies the current from the source, so an inexpensive diode, without a trigger circuit would work just as well
Transition
Next is a look at a simple, useful SCR circuit.
SCR Output

9. SCR for Overvoltage Protection
Computer
Vin 9
This simple circuit is quite useful for protecting a circuit from voltage spikes or other overvoltage conditions. As an example, the input could be to a computer and should be less than 7 volts. The zener diode could be a standard 6.2 volt zener so that the zener diode goes into breakdown mode at 6.2 volts and this turns on the gate of the SCR, which then shorts all current to ground. Of course, you would also want a circuit breaker or fuze in case this is more than a short term condition. However, since an SCR can generally turn on in a few microseconds, this prevents high voltage spikes from getting to the device being protected.
Transition:
Next is a look at phase control using, which is one of the major uses of SCR’s.
SCR for Overvoltage Protection

10. Phase Control with an SCR10
A simple phase control circuit using an SCR is shown on the left. The output of the circuit, from PSpice, is shown on the right. You can also download this circuit and run it to see how it works.
Essentially, The input voltage increases until the voltage and current at the gate are large enough to trigger the SCR. With R5 at .18Kohms, the SCR is only triggered near its peak and you only get half the power of the half-wave rectified waveform. Generally, you would make the resistor R5 a potentiometer so you could control the point at which the SCR is triggered.
This circuit, with a 5k potentiometer could trigger the SCR on for almost the entire half-cycle with R5 set at 5kohms. It could also trigger the SCR on for only about a quarter of a cycle with R5 set at a value of about .18kohms.
This is called phase control because it controls when the phase when the SCR is triggered. With a high value of R5, the SCR is triggered near 0 degrees in phase and conducts for the entire half-cycle. With a small value of R5, such as .18kohms, the SCR is only triggered on for about ¼ of a cycle. This means that you can control the power to the load from a value of half of the maximum to a value of about ¼ of the maximum power. Maximum power to the load would be for the full sine wave across the load.
Transition:
Next is a look at another phase control circuit.
Characteristics of 2N1595
Phase Control with an SCR

11. 11
The circuit shown on the left is similar to the example in the text and is another method of controlling the phase of an SCR. This time a zener diode is used to maintain a constant voltage and the 10 kohm potentiometer is used to control the phase at which the SCR is turned on. The capacitor charges and discharges and the 50 ohm resistor protects the gate of the SCR from damage when the capacitor discharges.
The current across the load resistor, which could be a motor or light bulb, etc is shown on the right of the slide. You can see that at 2.02 kohms, the minimum power for this type of configuration is used. The maximum power would be when the potentiometer is at its maximum value. At a lower value than that shown, no current is through the load. You can download this PSpice circuit and see how it works.
Transition:
Next is a look at Diacs and Triacs.
SCR Phase Control

12. Diac
Triac
Diacs and Triacs
The Diac and Triac are bilateral thyristors, meaning that they conduct current in both directions
The Diac is designed to conduct when breakdown occurs in both directions The triac is like two parallel SCR’s, with one in each direction.
Triac’s have less current carrying ability than SCR’s 12
Diacs are often used to trigger SCR’s and Triacs and are also PNPN devices. The Diac is designed to break down in both directions without harming the device. Diac’s are often used to turn SCR’s and Triacs on. Triac’s are handy devices for low to medium currents levels because they act like two parallel SCR’s built into a single package. Triac’s have a gate that can turn the device on in BOTH directions so it becomes easier to control the full power to a load with an AC circuit.
Transition:
Next is a look at a Triac circuit.

13. A Triac Full-Wave Circuit
Τau = RC = .12 msec 13
The circuit on the left is similar to the previous circuit except no zener diode is used and the Triac in the PSpice Evaluation model is used. This 2n5444 Triac has a 200 volt breakdown voltage so 120 volt rms power is used as if you wired it from a normal household wall circuit. The load is still 20 ohms and without the other circuitry, there would be approximately 8.5 amps peak, sinusoidal current across the load.
The idea of this circuit is to be able to control the full sinusoidal power that is available. You can see that for a small variable resistance of 1 kohm, almost all of the possible power is being delivered to the load. You can also see that there is only a small power loss in the Triac. The voltage across the capacitor builds up to the voltage needed to turn on the Triac. The time constant of the capacitor is 0.12 msec
Transition:
Next is a look at the same circuit with a higher variable resistance.
A Triac Full-Wave Circuit

14. A Triac Full-Wave Circuit
Τau = RC = 3.8 msec 14
The same Triac circuit is shown here with the variable resistance set at 11.8 kohms. Now the time constant, tau, is longer at 3.8 msec and thus, it takes longer for the voltage at the top of the capacitor to build up to the voltage required to turn the Triac on. Remember that this is a device, like BJT’s that is turned on by current, but of course, voltage divided by resistance is current, so the two are directly related. The capacitor charges slower with this circuit and therefore, the Triac is turned on later in the cycle. So, once again, we have a form of phase control, since here only about half of the maximum power is available compared to the circuit of the previous slide. Notice, from the Probe display, that the Triac is turned on at a bit past the 3.8 msec time constant. The actual time is dependent on the current/voltage needed for turn on. The capacitor voltage builds up to this voltage and then the device turns on, which discharges the capacitor down to about .7 volts, which is the normal forward biased silicon PN junction voltage.
Transition:
Next is a look at a circuit that can control the power to the load over a full range of values.
A Triac Full-Wave Circuit

15. Diac-Triac Phase Control15
The two figures shown are from the website near the bottom of the slide. This website contains fundamental information about real-world SCR, Diac, and Triac circuits. As you can see, the Diac-Triac circuit on the left is similar to the previous PSpice circuit except there is a Diac in it. The evaluation version of PSpice that I use does not have a Diac model.
The waveforms due to this circuit are shown on the right. I recommend that you go to this website and read the material to become more familiar with Thyristor circuits.
Transition:
Next is a look at a Triac lamp dimmer.
Power Control with Thyristors and Triacs
Diac-Triac Phase Control

16. Diac-Triac Light Dimmer 16
The circuit shown is from the website provided on the slide and shows a wiring diagram for a light dimmer that might be drawn for an electrician.
The MAINS refers to the 120 volt, AC, main power and the LAMP is the light that will be controlled by the dimmer, which is the 470 kohm potentiometer shown.
This circuit is essentially the same one shown on the previous slide. The capacitor is charged through the 470 kohm potentiometer and when the voltage across the capacitor is high enough to trigger the Diac, the Triac is turned on. This occurs for both a positive and a negative voltage.
Transition:
Next is a look at stepper motors.
Diac-Triac Light Dimmer

17. A stepper motor system is an electro-mechanical rotary actuator that converts electrical pulses into unique shaft rotations. This rotation is directly related to the number of pulses. The speed is synchronous to the rate of pulsing.
Stepper motors feature bi-directional control, built-in braking, variable torque, power control, precision accuracy, high resolution, open-loop control, and direct interface to digital systems Stepper Motors - General Description
A step motor converts electrical energy into discrete motions or steps.
The motor consists of multiple electrical windings wrapped in pairs (phases) around the outer stationary portion of the motor (stator).
The inner portion (rotor) consists of iron or magnetic disks mounted on a shaft and suspended on bearings.
The rotor has projecting teeth which align with the magnetic fields of the windings. When the coils are energized in sequence by direct current, the teeth follow the sequence and rotate a discrete distance necessary to re-align with the magnetic field.
The number of coil combinations (phases) and the number of teeth determine the number of steps (resolution) of the motor. For example, a 200 step per rev (spr) motor has 50 rotor teeth times 4 coil combinations to equal 200 spr.
There are no brushes between the rotor and stator assembly; a stepper motor is a multipole (polyphase) brushless DC motor.
These multiple coil pairs can be connected either positive or negative resulting in four unique full steps. When the coils are sequenced correctly, the motor rotates for- ward. When the sequence is reversed, the motor rotates in reverse. 17
A general description of a stepper motor is shown on the slide. The two websites shown near the bottom of the slide have good descriptions of some of the details of stepper motor construction and use. The top website is from a Professor of Computer Science at the University of Iowa and is quite comprehensive.
Stepper motors are precise and inexpensive, but generally require computerized electronics to drive them. They must be pulsed to move forward or backward, but can be used in an open loop system with good reliability.
Transition:
Next is a look at a stepper motor.
Stepper Motors

18. In the KP4M4-001 stepper motor, the permanent magnet lies North - South along the shaft. It is encased in two "stacks" each with 25 teeth round the rim. The teeth on the South stack are out of phase with the teeth on the North stack by half the gap between teeth as can be seen in the photo of the shaft shown above. 18
Some images of the KP4M4-001 stepper motor are shown on the slide from the website near the bottom of the slide.
So for the stepper motor shown, at the same time that one set of teeth are being attracted by and lining up with the teeth on the currently magnetized pole of the stator, the other set of teeth are being repelled and thus lining up with the gaps between the teeth on that pole.
With 25 teeth around the edge of the rotor and 4 coils excited individually in turn, the KP4M4-001 stepper motor takes 100 steps per complete revolution. The spacing between the teeth is 360°  /  25 teeth  =  14.4°. When the teeth on the rotor are aligned with the teeth on the stator pole of the currently excited coil, they are misaligned by a quarter of that angle with the teeth on the next stator pole. So when the coil on that pole becomes energized instead, the rotor is pulled round through one quarter of 14.4° producing a step of 3.6°.
Generally, for this kind of stepper motor: Steps per complete revolution = Number of phases (coils) x Number of teeth on rotor Note that with other types of stepper motor, the number of coils does not equal the number of phase.
Transition:
Next is a different look at how stepper motors work.
A Stepper Motor

19. Simple Stepper Operation
Permanent Magnet or can also be switched
Simple Stepper Operation 19
Shown on the slide is one type of simple stepper motor. Notice that in these simple diagrams, which are from the website shown, the rotor has a single North and a single South Pole. In the top left diagram, the electromagnet is configured so that the North is on the left and the South on the right with a switch that just changes the polarity of the electromagnet. This makes the rotor move to the position shown in the upper left where the North is halfway between the two south poles.
If the switche is moved to its other contact you get the setup on the upper right where, once again, the North pole of the rotor is moved to a position between the two south poles. From the upper left position to the upper right position is a movement of 900. If the simple stepper in the upper right has its electromagnet disconnected, then a 450 movement of the rotor will take place. This will place the South pole of the rotor directly up toward the North pole of the permanent magnet.
This simple example shows the basics of stepper motors. Of course, modern stepper motors are much more complex and must be driven by a computer or microcontroller. A number of manufacturers make stepper motor drivers specifically to take advantage of characteristics of specific stepper motors.
Transition:
The final slide is a summary of what was covered in this presentation.

20. Summary Introduction Thyristors SCR TRIAC DIAC Stepping Motors Summary20
This presentation introduced the most commonly used Thyristors.
The Silicon Controlled Rectifier is used for switching very high currents of up to 3000 amps. It can be turned on by a gate, but conducts in only one direction.
The Diac, meaning Diode Alternating Current switch, is a Bilateral trigger diode that can conduct in both directions and is like two Shockly diodes that are back to back. It only conducts after the breakdown voltage is reached in either direction.
The Triac is a bidirectional gate controlled thyristor and the word probably means Triode Alternating Current Switch although this was not clear in any references.
I recommend that you download the PSpice circuits and run them to become familiar with how the SCR and Triac operate with AC voltages and currents. You should also go to some of the websites shown in this presentation and read additional material that is of interest to you.
Transition:
So long until the next presentation.