# Thyristors

## Discrete Semiconductor Devices and Circuits

• #### Question 1

 Don’t just sit there! Build something!!

Learning to mathematically analyze circuits requires much study and practice. Typically, students practice by working through lots of sample problems and checking their answers against those provided by the textbook or the instructor. While this is good, there is a much better way.

You will learn much more by actually building and analyzing real circuits, letting your test equipment provide the “answers” instead of a book or another person. For successful circuit-building exercises, follow these steps:

1. Carefully measure and record all component values prior to circuit construction, choosing resistor values high enough to make damage to any active components unlikely.
2. Draw the schematic diagram for the circuit to be analyzed.
3. Carefully build this circuit on a breadboard or other convenient medium.
4. Check the accuracy of the circuit’s construction, following each wire to each connection point, and verifying these elements one-by-one on the diagram.
5. Mathematically analyze the circuit, solving for all voltage and current values.
6. Carefully measure all voltages and currents, to verify the accuracy of your analysis.
7. If there are any substantial errors (greater than a few percent), carefully check your circuit’s construction against the diagram, then carefully re-calculate the values and re-measure.

When students are first learning about semiconductor devices, and are most likely to damage them by making improper connections in their circuits, I recommend they experiment with large, high-wattage components (1N4001 rectifying diodes, TO-220 or TO-3 case power transistors, etc.), and using dry-cell battery power sources rather than a benchtop power supply. This decreases the likelihood of component damage.

As usual, avoid very high and very low resistor values, to avoid measurement errors caused by meter “loading” (on the high end) and to avoid transistor burnout (on the low end). I recommend resistors between 1 kΩ and 100 kΩ.

One way you can save time and reduce the possibility of error is to begin with a very simple circuit and incrementally add components to increase its complexity after each analysis, rather than building a whole new circuit for each practice problem. Another time-saving technique is to re-use the same components in a variety of different circuit configurations. This way, you won’t have to measure any component’s value more than once.

• #### Question 2

All thyristor devices exhibit the property of hysteresis. From an electrical perspective, what is “hysteresis”? How does this behavior differ from that of “normal” active semiconductor components such as bipolar or field-effect transistors?

• #### Question 3

What is required to make a Shockley diode or DIAC begin conducting current? What condition(s) have to be met in order for electrical conduction to occur through one of these devices?

Also, explain what must be done to stop the flow of electric current through a Shockley diode or a DIAC.

• #### Question 4

Silicon-controlled rectifiers (SCRs) may be modeled by the following transistor circuit. Explain how this circuit functions, in the presence of and absence of a “triggering” voltage pulse at the gate terminal:

• #### Question 5

Shown here is an illustration of a large “stud mount” type of SCR, where the body is threaded so as to be fastened to a metal base like a bolt threads into a nut:

With no test instrument other than a simple continuity tester (battery and light bulb connected in series, with two test leads), how could you determine the identities of the three terminals on this SCR?

Hint: The threaded metal base of the SCR constitutes one of the three terminals.

• #### Question 6

Explain what happens in each of these circuits when the pushbutton switch is actuated and then released:

• #### Question 7

Explain what must be done to the SCR to make it turn on and send power to the light bulb:

Next, explain what must be done to turn the SCR off so that the light bulb de-energizes.

• #### Question 8

When an SCR latches “on,” it drops very little voltage between anode and cathode. Explain why this is, and what advantage this gives SCRs over transistors when conducting heavy load currents.

• #### Question 9

Explain what a TRIAC is, and how it is at once similar and different from an SCR. What applications might a TRIAC be used in that an SCR would be inappropriate for?

• #### Question 10

Label the terminals on a TRIAC with their proper designations:

• #### Question 11

Explain how Silicon-Controlled Rectifiers (SCRs) differ from TRIACs, in terms of their respective behavior.

• #### Question 12

Some SCRs and TRIACs are advertised as sensitive-gate devices. What does this mean? What is the difference between a “sensitive gate” SCR and an SCR with a “non-sensitive gate”?

• #### Question 13

Explain what a crowbar circuit is, and how it employs an SCR to protect a circuit from excessive voltage.

• #### Question 14

What is a quadrac, and how does it differ from a regular TRIAC?

• #### Question 15

A student of electronics has just recently learned how to build audio amplifier circuits, and this inspires dreams of designing a super-powerful amplifier for a home entertainment system. One day, this student comes across a donation of electronic components from a local business, and in this donation are several industrial SCRs, rated at 20 amps each.

“Wow,” says the student, “these components look like really big transistors, but they’re rated for a lot of current. I could build a huge amplifier with these!”

The student approaches you for advice, because you’ve just recently learned how SCRs function in your electronics class. What do you tell the student, concerning the use of SCRs as audio amplification devices? How do you explain to this excited student that these devices will not work in an amplifier circuit?

• #### Question 16

One way that SCRs may be triggered into their “on” state is by a transient voltage applied between the anode and cathode terminals. Normally, this method of triggering is considered a flaw of the device, as it opens the possibility of unwanted triggering resulting from disturbances in the power supply voltage.

Explain why a high $$\frac{dv}{dt}$$ present on the power supply rail is able to trigger an SCR, with reference to the SCR’s equivalent circuit. Also suggest what means might be employed to prevent false triggering from power supply transients.

• #### Question 17

Identify three different ways that an SCR or a TRIAC may be triggered into its “on” (conducting) state:

1.
2.
3.
• #### Question 18

Identify two different ways that an SCR or a TRIAC may be forced into its “off” (non-conducting) state:

1.
2.
• #### Question 19

The unijunction transistor, or UJT, is an interesting device, exhibiting hysteresis just like SCRs and TRIACs. Its schematic symbol is as follows:

One equivalent circuit diagram for the UJT uses a pair of transistors and a pair of resistors:

When the two base terminals of a UJT are connected across a source of DC voltage, the two base resistances (RB1 and RB2) form a voltage divider, splitting the applied voltage into lesser portions:

How much voltage, and of what polarity, must be applied to the emitter terminal of the UJT to turn it on? Write an equation solving for the magnitude of this triggering voltage (symbolized as VP), given RB1, RB2, and VBB.

• #### Question 20

A unijunction transistor with an intrinsic standoff ratio (η) of 0.8 is powered by a 15 volt DC source. Calculate the emitter voltage needed to “trigger” this UJT into its conductive state.

• #### Question 21

Describe what happens to the UJT as the potentiometer is slowly adjusted upward to provide a variable voltage at point A in this circuit, starting from 0 volts and ending at the trigger voltage VP:

Now describe what must be done to the potentiometer to cause the UJT to turn back off again.

• #### Question 22

This circuit uses a unijunction transistor (UJT) to latch an LED in the “on” state with a positive pulse at the input terminal. A negative voltage pulse at the input terminal turns the LED off:

Explain how the unijunction transistor functions in this circuit.

• #### Question 23

Predict how the operation of this UJT latch circuit will be affected as a result of the following faults. Consider each fault independently (i.e. one at a time, no multiple faults):

Capacitor C1 fails open:
Capacitor C1 fails shorted:
Resistor R1 fails open:
Solder bridge (short) past resistor R1:
Resistor R2 fails open:
Solder bridge (short) past resistor R2:

For each of these conditions, explain why the resulting effects will occur.

• #### Question 24

Identify at least three different types of thyristors (besides SCRs):

1.
2.
3.
• #### Question 25

Find one or two silicon-controlled rectifiers and bring them with you to class for discussion. Identify as much information as you can about your SCRs prior to discussion:

Terminal identification (which terminal is gate, anode, and cathode)
Continuous voltage rating
Continuous current rating
Continuous power rating
Whether or not it is a ßensitive gate” device