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Thyristor Application Circuits
16 questions By Tony R. Kuphaldt
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Question 13 of 16
Explain how this battery charger circuit uses a TRIAC to control DC power to the battery:
Also, identify some component failures in this circuit that could prevent DC power from getting to the battery.
Reveal answerThe TRIAC controls power to the primary winding of the step-down transformer. Afterward, that AC power is rectified to DC for charging purposes.
Notes:The interesting point of this circuit is that by controlling AC power with the TRIAC, DC power to the battery is subsequently controlled.
Ask your students to explain the purpose of each component in the circuit, and pose some troubleshooting questions for them to analyze. There are many possibilities for component failures stopping DC power from getting to the battery. Discuss the examples your students think of, and determine the relatively likelihood of each.
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Question 14 of 16
Commutation is an important issue in any kind of thyristor circuit, due to the “latching” nature of these devices. Explain what “commutation” means, and how it may be achieved for various thyristors.
Reveal answerCommutation is nothing more than a fancy word for “switching” (think of the commutator in a DC electric motor - its purpose being to switch polarity of voltage applied to the armature windings). In the context of thyristors, “commutation” refers to the issue of how to turn the device(s) off after they have been triggered on.
Follow-up question: in some circuits, commutation occurs naturally. In other circuits, special provisions must be made to force the thyristor(s) to turn off. Identify at least one example of a thyristor circuit with natural commutation and at least one example of a thyristor circuit using forced commutation.
Notes:An important feature of all thyristors is that they latch in the “on” state once having been triggered. This point needs to be emphasized multiple times for some students to grasp it, as they are accustomed to thinking in terms of transistors which do not latch.
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Question 15 of 16
The following circuit exhibits very interesting behavior:
When the power is first turned on, neither lamp will energize. If either pushbutton switch is then momentarily actuated, the lamp controlled by that SCR will energize. If, after one of the lamps has been energized, the other pushbutton switch is then actuated, its lamp will energize and the other lamp will de-energize.
Stated simply, each pushbutton switch not only serves to energize its respective lamp, but it also serves to de-energize the other lamp as well. Explain how this is possible. It should be no mystery to you why each switch turns on its respective lamp, but how is the other switch able to exert control over the other SCR, to turn it off?
Hint: the secret is in the capacitor, connected between the two SCRs’ anode terminals.
Reveal answerThis circuit is an example of a parallel capacitor, forced commutation circuit. When one SCR fires, the capacitor is effectively connected in parallel with the other SCR, causing it to drop out due to low current.
Notes:This method of switching load current between two thyristors is a common technique in power control circuits using SCRs as the switching devices. If students are confused about this circuit’s operation, it will help them greatly to analyze the capacitor’s voltage drop when SCR1 is conducting, versus when SCR2 is conducting.
