In industries, controlled devices are used. At this point, the question arises in your mind that what is control device?
Control means to utilize something according to our own choice.
A simplest control device is a switch (i.e. a switch will make and break the circuit). For e.g. in order to control the fan speed, we use a control device i.e. dimmer to slow or fast the speed of the fan.
What is an SCR?
SCR belongs to the group of thyristor family, a thyristor is a semiconductor device consisting of 4 layers of alternate P-N-P-N material and have three terminals (i.e. electrodes).
SCR is a three-terminal switching device. It consists of three terminals namely anode, cathode, and gate. It is a unidirectional device (i.e. it allow current to flow only in one direction).
It has three junctions (i.e. J1, J2, J3 ). If we apply a positive voltage to the anode with respect to the cathode, then the junction J1 and J3 will be forward biased and J2 will be reversed biased (i.e. depletion region will start to increase ). Now the reversed biased junction J2 will make the SCR OFF and the voltage drop across SCR will be maximum (i.e. open switch). Now to make SCR ON, the method known as break over method, if we increase the anode-cathode voltage to a value known as break over voltage (Vbo), then breakdown will occur at junction J2 (i.e. minority carrier from then-region starts to inject to p-region and the minority carrier from p-region will start to inject to n-region) and the voltage across J3 will start to reduce. When the switch is open then the maximum voltages is across the switch and zero voltage is across the load when the switch is closed when the voltage across the switch is ideally zero and the maximum voltage is across the load.
Break over means when the breakdown will occur at the region then the voltage across the switch (i.e. SCR ) will start to decrease and current will start to increase. Now every time to make the switch (i.e. SCR ) ON through the break over we have another option to make SCR ON, known as gate triggering method because to ON the SCR using break over method may damage the SCR. In gate triggering method the triggering pulse is applied at the gate with respect to a cathode of SCR due to which the junction J3 will become forward bias and the voltage across the SCR will start to decrease and current will increase as shown in the characteristic curve of SCR. Now in SCR two terms are introduced
- Latching current.
- Holding current.
LATCH CURRENT: is the amount of current needed to make SCR ON.
HOLDING CURRENT: is the current when the current fall below this value then the SCR will be turned OFF.
Now to understand the concept of latching two transistor analog model is to use.
In order to understand the latching phenomena, the P-N-P-N material is divided into two transistors. When the gate pulse is applied IG will start to flow which makes the transistor Q2 to conduct and the emitter current of Q2 will start to flow since the emitter current of the transistor is equal to the collector current, then the Ic2 which is equal to the base current Ib1 of Q1 will start to flow as a result IA flow and IC1 will flow. When the gate pulse is removed (i.e. IG =0) then the transistors will still conduct due to the flow of current which makes both the transistors ON (i.e. SCR will). Now to make SCR off we have to interrupt the main supply of SCR.
S2 is normally closed switch , when S1 open then no gate current will flow and SCR will remain off and maximum voltage is across SCR and led will remain Off . when S1 is closed then gate current will flow which makes SCR on and the voltage across SCR will be ideally zero (but in practically 0.7v ) and LED will glow.
In my next article, I’ll be discussing the applications of SCR and how to test SCR.