In a sawtooth generator(circuit below) , the capacitor charges then when the thyristor turns on due to the bias voltage of R2 and R3, capacitor discharges to the thyristor like a short. Then after discharging the capacitor, the thyristor turns off and the capacitor charges again. This will form a sawtooth generator.

enter image description here Question : "The thyristor turns off after discharging the capacitor", why would the thyristor reach its "holding current" and turn off when there is still a constant current coming from R1?

This current from R1 is approximately 100V/1kohm=100mA. Even if capacitor is fully discharged, this 100mA is enough to prevent the thyristor from turning off.

Since the thyristor act like a short(with small forward voltage), I thought current from R1 will short through thyristor thus the thyristor will never reach its holding current and will forever be on regardless how small the charge from the capacitor is.

  • \$\begingroup\$ "Open" means open (block) circuit and "close" means short circuit in electronics - I think you have this the wrong way round. If you were dealing with hydraulics then that would be fine but this is an EE site so please use the correct terminology. \$\endgroup\$ – Andy aka Jun 1 at 10:37
  • \$\begingroup\$ Sorry I just edited that now \$\endgroup\$ – Iwatani Naofumi Jun 1 at 10:37
  • \$\begingroup\$ OK, it seems that your logic is correct so, where did the schematic come from? \$\endgroup\$ – Andy aka Jun 1 at 10:41
  • \$\begingroup\$ From Albert Malvino's book: "Electronic Principles" \$\endgroup\$ – Iwatani Naofumi Jun 1 at 10:41
  • \$\begingroup\$ Well, personally I think it's wrong because if the holding current is below 100 mA then it's going to stay on. \$\endgroup\$ – Andy aka Jun 1 at 10:51

Here's my circuit: -

enter image description here

If I choose a generic SCR and set the hold current to be 110 mA (i.e. 10 mA above what V1 and R3 can deliver) I get this: -

enter image description here

Looking good!

If I set the hold current to be 70 mA, I get this: -

enter image description here

I think there is a great deal of justification in saying that the circuit in Albert Malvino's book: "Electronic Principles" (as portrayed in your question) is faulty. Of course I don't have that book so I can't tell you if there is some small print somewhere that says the holding current has to be above 100 mA for this circuit to work.

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