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Thyristors
25 questions By Tony R. Kuphaldt
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Question 22 of 25
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.
Reveal answerUnijunction transistors are hysteretic, like all thyristors. A positive pulse to the emitter terminal latches the UJT, and a negative pulse makes it “drop out”.
Challenge question: what is the purpose of resistor R3 in this circuit?
Notes:Ask your students to identify the terminals on the UJT. The designations for each terminal may be surprising to your students, given the names of bipolar transistor terminals!
The challenge question may only be answered if one carefully considers the characteristics of an LED. Resistor R3 helps overcome problems that might potentially arise due to the nonlinearities of the diode in its off state.
I got this circuit from the October 2003 issue of
Electronics World magazine, in their regular “Circuit Ideas” section. The design is attributed to André de Guérin.
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Question 23 of 25
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.
Reveal answer- Capacitor C1 fails open: Neither pushbutton switch has any effect on the LED.
- Capacitor C1 fails shorted: Circuit behaves normally.
- Resistor R1 fails open: LED always off, refuses to turn on.
- Solder bridge (short) past resistor R1: LED always on, refuses to turn off.
- Resistor R2 fails open: LED always on, refuses to turn off.
- Solder bridge (short) past resistor R2: LED always off, refuses to turn on.
Notes:The purpose of this question is to approach the domain of circuit troubleshooting from a perspective of knowing what the fault is, rather than only knowing what the symptoms are. Although this is not necessarily a realistic perspective, it helps students build the foundational knowledge necessary to diagnose a faulted circuit from empirical data. Questions such as this should be followed (eventually) by other questions asking students to identify likely faults based on measurements.
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Question 24 of 25
Identify at least three different types of thyristors (besides SCRs):
- 1.
- 2.
- 3.
Reveal answer- 1. DIAC
- 2. TRIAC
- 3. Quadrac (TRIAC DIAC)
- 4. Shockley diode
- 5. GTO
- 6. UJT
- 7. SCS
Notes:Challenge your students to identify even more types of thyristors, if they can!
Can someone please explain why do we need C1 capacitor in question 22 ? Circuit behaves exactly the same without it and I am confused about it’s purpose.