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    • #11086

        Hi all,

        You may have noticed that in our last experiments with a sawtooth oscillator, we made use of 2 transistors, a PNP and a NPN transistor.
        Let’s study this 2 transistor circuit more in detail.

        When you look at the diagram, you see that the Base of T1 is connected to the Collector of T2.
        And so is the Base of T2 connected to the Collector of T1.
        This forms a so called Thyristor.

        The Base of the NPN transistor T2 is shunted via R2 (100K) resistor to GND.
        The Base of the PNP transistor T1 is connected via R3 (100K) to VCC.
        These resistors are very important, they turn off the transistors when you connect the battery.
        When finishing the circuit, the lamp should be OFF.

        When you press S1, what do you think will happen ?
        Now, when you press S2, what happens now ?

        By a single press on S1, the thyristor gets activated and the lamp turns on. Pressing S2 turns the lamp back off.

        But how does this work ?
        In rest condition (lamp is OFF), no current flows through T1 and T2 (Remember R2 and R3 ?).
        R2 connects T1 to GND and turning it OFF, R3 connects T2 to VCC and turning it OFF.
        By pressing S1, a small positive current flows via R1 and S1 to the Base of T2 and turning it ON.
        Now, the Base of T1 is connected via the conductive T2 and the 47Ω resistor to GND, thus turning it ON.
        And as the Collector of T2 is connected to the Base of T1, it will also conduct and providing T2 a positive Base voltage.
        This will form a closed loop and the current will keep on flowing making our lamp stay lit.

        When you follow the current path of the lamp, you’ll see that the battery voltage VCC flows through S2 – lamp – Emitter Base path of T1 – Collector Emitter path of T2 – R4 of 47Ω to GND.

        Now when you press S2, you disconnect this current path and the transistors T1 & T2 block, returning the situation to normal. The lamp will go OFF and will stay OFF.

        Some experiment tips :

        • Swap S2 with R4, what will happen then ?
        • Remove R1 or R3 and try the circuit.
        • Swap the lamp with R4, do you think it will still work ?

        A little background of the working principle.
        There exists a fantom capacitor between the Collector and Base of a transistor. Although the value is very small, it is large enough to falsely trigger the thyristor.
        R2 and R3 keep this “capacitor” charged so our circuit will work as intended.
        Try this once to observe …

        • Remove R3
        • Turn power on and press S1, the lamp will go on and stays on (it might be already on).
        • Press S2, the lamp turns off but turns back on when you release S2 !
        • Swap R4 with S2 and repeat the 2 steps above.
        • Now the lamp stays off when you release S2 !

        This comes from the fact that the fantom capacitor in the 1st situation is discharged. When powering on, the small capacitor charge will trigger the Base of T2.
        In the other situation, this small capacitor is charged and will not influence the thyristor.

        Here’s the diagram :


      • #11093

          A really thorough and great presentation of the Thyristor!  I especially like the what if scenarios Frank 🙂

          What is the practical application of the Thyristor in electronics?

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