Power Electronics: SCR Switch Circuit Simulation / Theory

[wfg42438]

Hello,

At the moment i have an assignment dealing with a SCR switch


We are asked to build and simulate the circuit shown below. Unfortunately we have not covered the material needed in class at the moment. So i figured id ask around to get a head start on this assignment.

From my understanding the SCR switch acts as a controlled diode. The gate terminal when excited allows current flow and when the anode has a voltage more negative than the one across the cathode then the SCR turns off. The SCR will only begin to conduct until it is once again excited by some specific gate current.

Now unfortunately that's all i know based on some internet searches i conducted. However its not enough to get started.

For this exercise we need the following:


Need to calculate the gate current I_G and what is being referred to as current I_A

Im assuming I_A is the DC current that splits between the two 1k resistors current

Can someone please give me some pointers on where to start??

Also there's a component shown in the bottom center of the circuit diagram is this a switch? If so what kind?

If someone can also point out why V_supply=-7V that would be great!

Thank you in advance!!!

 

[CataM]

The SCR will only begin to conduct until it is once again excited by some specific gate current.

This is what they say here: https://en.wikipedia.org/wiki/Silicon_controlled_rectifier

SCR can be brought from blocking mode to conduction mode in two ways: either by increasing the voltage across anode to cathode beyond breakover voltage or by applying positive pulse at gate.

If someone can also point out why V_supply=-7V that would be great!

I think it is a DC voltage source that can vary between 5 V and 12 V.

 

[wfg42438]

Say we apply Vs=5V , how would I go about finding the gate current supplied by the left hand 1k resistor to the SCR?
Under the assumption the LEDS are considered ideal:

VSupply-V_R1 (Left Side)-V_Gate-V_R2 (Right side) = 0

Where V_R1=V_R2 so... Vsupply -2*(V_R1)-V_Gate=0


how would I proceed from here to find I_gate ?


Also the switch symbol shown in the center is this a simple manual switch to allow or block flow current flow?

 

[BradtheRad]

Two transistors can be connected so that they mimic an scr. (Similar to fabricating an scr from several PN junctions.) By experimenting with such a model, you might discover what volt levels and Ampere levels exist within an scr.

 

[flaxwalter]

I G= (Vcc - 2.2 - 0.6)/1000
If Vcc= 5V, then I G = 2.2 mA
If Vcc= 12V, then I G = 9.2 mA
It depends on the Vcc value.
Another thing: V_supply must not be negative, so -7V does not turn on the SCR.

 

[CataM]

What said in wikipedia on post #2 is wrong. Watching its doping profile shows that only way to turn on is when applied V gate positive.
To analyze the circuit, when switch is opened, there is no light on the LEDS. When button is pressed, then applies what flaxwalter has done.