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Operation of SCR ELECTRONICS II LAB REPORT

Muhammad Mushtaq Muhammad Zawar

150701004 150701016

Abstract This Lab Report describe the experiment to examine the structure and operation of the Silicon Controlled Rectifier or SCR. SCRs are mainly used in devices where the control of high power, possibly at high voltage, is needed. The ability to switch large currents on and off makes the SCR suitable for use in medium to high-voltage AC power control applications, such as lamp dimming, regulators and motor control.

Theory A silicon controlled rectifier (SCR) is a four-layer solid state current controlling device with 3 terminals. They have anode and cathode terminals like a conventional diode and a third control terminal, referred to as the Gate. SCRs are unidirectional devices, i.e. they conduct current only in one direction like a diode or rectifier. SCRs are triggered only by currents going into the gate. The SCR combines the rectifying features of diodes and the On - Off control features of transistors. SCRs are generally used in power switching applications. In the normal OFF state, the device restricts current flow to the leakage current. When the gate-to-cathode current exceeds a certain threshold, the device turns ON and conducts current. The SCR will remain in the ON state even after gate current is removed so long as the current through the device exceeds the holding current. Once the current falls below the holding current for a period of time, the device will switch OFF. If the gate is pulsed and the current through the device is below the latching current, the device will remain in the OFF state. Looking at figure 1(a), the four layer structure of the SCR, we see the three terminals, one from the outer p-type layer called the anode A, the second from the outer n-type layer called the cathode K and the third from the base of the lower NPN transistor section and is called gate G.

Understanding Working with Transistor Analogy The SCR, as shown in figure 1(b), can be visualized as separated into two transistors. The equivalent circuit of an SCR is composed of a PNP transistor and an NPN transistor interconnected as shown in figure 1(c). We see that the collector of each transistor is connected to the base of the other, forming a positive feedback loop. The SCR has two stable states. The first is the non-conducting OFF state. With the Gate terminal open let us first assume that no current is flowing into the base terminal of NPN transistor Q 2. Given zero base current the collector current of Q2 will also be zero. Given zero collector for Q2 we infer that there should be zero current flowing out of the base of PNP transistor Q 1. Given zero base current in Q1 we infer that there should be zero collector current in Q1. This is consistent with our original assumption of zero current in the base of Q2. With zero collector current (and zero base current) in both Q1 and Q2 we can infer that there should be no emitter current in either transistor as well. This zero current OFF state is stable so long as any leakage current through Q1 or Q2 from emitter to collector is very small. The second stable state is the conducting ON state. We can transition or switch the SCR from the OFF state to the ON state by injecting a small current into the Gate terminal. Going through the same procedure around the loop we just did for the off state we can see that as soon as a base current is supplied to Q2, a larger collector current ( ßNPN times the base current ) will start to flow. This Q2 collector current becomes base current for Q1. This base current in Q1 produces again a larger collector current (ßPNP times the base current) in Q1. The collector current of Q1 feeds back into the base of Q2 increasing its base current even more. Once this feedback loop of current is established the initial gate current can be removed and the SCR will remain in the conducting ON state for as long as the external circuit around the SCR supplies current through the SCR. The only way to turn off the SCR is for the current to drop below a critical “holding” current level. An observation to note about this positive feedback loop is that it will hold the SCR ON and remain in this latched state as long as the following is true: ßPNP * ßNPN ⇒ 1 The voltage drop across the SCR from terminal A to K when the SCR is conducting is the sum of Q1-VBE and Q2-VCE-SAT in parallel with the sum of Q2-VBE and Q1VCE-SAT. We know that the ß of BJT devices falls as the collector base junction is forward biased into the saturation region i.e VCE less than VBE. The VCE of the two transistors will drop until the above positive feedback gain equation is satisfied and ßPNP * ßNPN is equal to 1. It is also important to note that the ß of BJT transistors is very low for very small values of collector current and from the above equation, the SCR will remain in the OFF state so long as the leakage current is so small that ßPNP * ßNPN is less than 1 at this low leakage current level.

Equipment BT151

LED

Resistor 470Ω Power supply

Bread Board

Jumper Wires

Procedure 

A rudimentary test of SCR function, or at least terminal identification, may be performed with an ohmmeter. Because the internal connection between gate and cathode is a single PN junction, a meter should indicate continuity between these terminals with the red test lead on the gate and the black test lead on the cathode like this



All other continuity measurements performed on an SCR will show “open” (“OL” on some digital multimeter displays). It must be understood that this test is very basic and does not establish a complete assessment of the SCR. It is possible for an SCR to give good ohmmeter indications and still be defective. Ultimately, the only way to test an SCR is to subject it to a load current. Make the circuit according to the manual. Turn On the Power Supply The LED will remain OFF Now, apply a +ve voltage greater than the junction voltage of the Gate-Cathode (p-n) junction approx. 1v at the gate. The SCR turned ON (because of Holding current) and start conducting, so that the circuit completes and hence the LED will start to glow. To stop the conduction of the SCR the current is to be minimized below the holding current, this could be done by either decreasing the voltage at the anode or simply grounding the anode terminal.

     

Measurements 𝐿𝑎𝑡𝑐ℎ𝑖𝑛𝑔 𝐶𝑢𝑟𝑟𝑒𝑛𝑡 = 3.18 𝑚𝐴 𝐻𝑜𝑙𝑑𝑖𝑛𝑔 𝐶𝑢𝑟𝑟𝑒𝑛𝑡 = 4.41 µ𝐴

@

1𝑣

Review Questions 1. Before applying trigger, observe the glow of the LED. Why the LED is not glowing? The LED is not glowing as there is no current flowing through it i.e. the circuit is open because the SCR is OFF and behaving as an open switch.

2. After positive trigger at gate, observe the glow of the LED. Why LED turned ON? After positive trigger, the G-K junction becomes forward biased and due to this the A-K junction also becomes forward biased i.e. the SCR turned ON (switch is closed) the circuit is completes so the current will flow and the LED will start to glow.

3. Reduce the Holding Current, what happened to the LED? Holding current is the minimum current which is necessary for the SCR to be stay in ON state. If we reduce the current below the Holding Current the SRC will stop conducting (OFF state) and the LED will stop glowing.

4. Ground the anode, what happened to the LED? Explain the reason. By grounding the anode or removing the supply will remove the IH which is necessary for the SCR to be stay in ON state and hence the LED will stop glowing.

5. Ground the cathode, what happened to the LED? Explain the reason. As cathode is already grounded so there will be no effect on the previous state of the LED.

Conclusion In conclusion, we can say that the students have understood the operation of the Silicon Controlled Rectifier, its Bi-stable states (ON & OFF), how to trigger it to the ON state, and then shut it OFF i.e. it’s biasing. In a nutshell, the basic of the four layer device has completely been absorbed.

References http://en.wikipedia.org/wiki/Silicon-controlled_rectifier

https://www.allaboutcircuits.com/textbook/semiconductors/chpt-7/silicon-controlled-rectifier-scr/