I have the following circuit implemented in the control of a thyristor. I know the basic functionality of the circuit, but i need a complete description of every component.

Thank you!

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  • 5
    \$\begingroup\$ Is this a homework question? In any case, if you know the basic functionality then include that in your question so we don't waste our time writing something you already know. "i (sic) need a complete description of every component. You might want to edit this bit as it sounds rather demanding. \$\endgroup\$
    – Transistor
    Jul 2, 2018 at 18:21
  • \$\begingroup\$ I know that RB2 protects the base of the Q2 transistor against big current. D8 and Rosc2 in theory should help the primary of the transformer to desaturate faster. Dr2 should protect the gate of the thyristor against negative voltage. Finaly, Dz2 and Rp3 should protect the gate against over-voltage spikes. \$\endgroup\$
    – user16634
    Jul 2, 2018 at 18:26
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    \$\begingroup\$ Good. Add that into your question and explain where you are stuck. It seems that you understand most of it. \$\endgroup\$
    – Transistor
    Jul 2, 2018 at 18:35
  • \$\begingroup\$ The capacitor's role i don't know too well. \$\endgroup\$
    – user16634
    Jul 2, 2018 at 18:48
  • \$\begingroup\$ @user16634 try to remember what a capacitor try to do when there is a change in voltage. It will help you figure out what is the capacitor's role \$\endgroup\$ Jul 2, 2018 at 18:51

1 Answer 1


This circuit is used to provide an isolated power supply, its used for protecting a load from undesired rush current.

PWM is used for switching purposes, fast switching rate result in increase and decrease of electrical current through the primary winding of the transformer making a magnetic field. this magnetic field is transferred back to electrical current through the secondary winding of the transformer.

RB2 A base resistor provides the necessary resistance to bias the base junction of a bipolar junction transistor (BJT)

Q2 a NPN bipolar junction transistor (BJT), is a semiconductor device which can be used for switching or amplification. You can read a book about BJTs and their applications!

D8 is called a Schottky diode and its used due to its fast switching response and low forward voltage drop. A normal diode will have a voltage drop between 0.6 to 1.7 volts, while a Schottky diode voltage drop is usually between 0.15 and 0.45 volts.

Rosc2 current limiting resistor and osc refers to the oscillation of PWM

78253/55 its a type of transformer that transfers electrical power from a power source to some equipment or device, and is used for isolation. This type of isolation is called galvanic isolation.

DR2 Is a diode and used to allow current to pass current in a one way direction.

C is a capacitor and is used to smooth the output voltage and eliminate voltage ripples.

Dz2 is Zener Diode and is used as a voltage regulator.

Rp3 A resistor is connected in parallel to a load in a circuit for both safety and to have maximum power delivered to the load.

  • \$\begingroup\$ Actually the purpose depends if the circuit is a leading edge narrow pulse voltage clamp pulse or a saturating flyback as shown which is sub-obptimal since transformer has avg DC voltage saturating the core. ( hence incorrect useage) \$\endgroup\$ Jul 2, 2018 at 19:30
  • \$\begingroup\$ Can you clarify more @TonyStewartolderthandirt, I have always used to learn from you :) \$\endgroup\$
    – ahm_zahran
    Jul 2, 2018 at 19:39
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    \$\begingroup\$ At 50% PWM the Vdc avg across coil input = Vcc/2 Current will create Remanence and saturate down to 0 inductance. So circuit is wrong for PWM . Diodes clamp voltage but not avg DC current \$\endgroup\$ Jul 2, 2018 at 19:41
  • \$\begingroup\$ This is a very specific clarification of the circuit, thank you @Tony for that. I still hope to learn from you more. \$\endgroup\$
    – ahm_zahran
    Jul 2, 2018 at 19:47

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