SCR unit II.pptx

all about scr construction and working detailoed for second year electrical subject FPE

1. SCR Symbol The SCR symbol  is very similar to the diode symbol, but it has an additional gate terminal. As SCR only allows current in one  direction like a diode , the symbol is very similar to the  diode symbol . As the anode current is triggered by the gate, therefore the gate terminal is denoted in contrast to the diode symbol .

2. The three terminals of the SCR are named  anode, cathode, and gate . For proper working, connect the anode of the SCR with positive and cathode with negative of the battery. A positive pulse for a short duration is required at the gate to kick start the conduction.

3. SCR Construction SCR has four layers of extrinsic semiconductor materials. These four-layer form three PN junctions named J1, J2, and J3. The layers are either NPNP or PNPN . The anode and cathode terminals are placed at the end layers and where the gate terminal is placed with the third layer. The outer layers are heavily dopped and the inner two layers are lightly dopped . The   SCR Construction  is illustrated here below . 4 Layers of SCR Construction

4. SCR Working SCR working  depends upon the  battery polarity  and the gate input . The SCR can operate in three different modes. Forward Blocking Mode Forward Conduction Mode Reverse Blocking Mode

5. Forward Blocking Mode of SCR When anode of SCR connects to the positive and cathode of SCR with the negative of the battery terminal. And no pulse is applied at the gate terminal. The SCR work in the forward blocking mode. This means that SCR will not conduct even though the polarity of SCR is forward bias. In forward blocking mode, the J1 and J3 PN junctions are forward biased. But the middle junction J2 is reverse biased, therefore, the SCR will not conduct in the forward blocking mode .

6. Forward Conduction Mode of SCR Forward conduction mode is the only mode of SCR for conduction. The SCR can be set into the forward conduction mode in two ways. First by providing the gate pulse to forward bias the J2 junction . Second by increasing the anode to cathode voltage to break down the J2 junction. The gate pulse method is preferred and suitable for many applications. The  breakdown method  reduces the SCR lifetime. The SCR will remain in conduction mode even after the removal of the gate pulse or reducing the applied voltage. If the anode current of the SCR drops below the holding current the SCR will stop falling back to forward blocking mode.

7. Reverse Blocking Mode in SCR If the anode terminal of the SCR connects to the negative and cathode terminal of SCR connects to the positive of battery terminals . The SCR is in reverse blocking mode. In this mode, J1 and J3 junctions are reverse biased. Where the middle junction J2 is forward bias . As two junctions are reverse bias, so there is no current flowing through it but only a small leakage current due to the drift charge carrier.

8. VI Characteristics of SCR

9. VI Characteristics of SCR The curve of  VI characteristics of SCR  is obtained by changing the voltage across the SCR and noticing the current through SCR. When connecting in reverse polarity, the SCR will conduct a small current, leakage current up to breakdown voltage, VBR. After that point, the SCR will break down and start to act like a short circuit. Below the breakdown point, the region is called  reverse blocking mode . When the voltage reaches the V BO  point, the SCR starts the current flow. Alternatively , the SCR can be put into the forward conduction mode by applying the gate that will increase the anode current above the latching current. A higher gate current can put SCR faster in the forward conduction mode as in the graph Ig3>Ig2>Ig1. The SCR will remain in the  forward conduction mode  if the anode current is above the  holding current .

10. SCR Applications The most common SCR application is the DC motor speed control. The DC motor has two windings, where the  armature winding  is connected to the AC supply by two SCR. The SCR controls the amount of current to the motor and ultimately the speed of the DC motor .

11. SCR Clarifying Questions 1.What current need to trigger SCR? To fire or trigger SCR, a minimum anode current greater than the latching current is required. The latching current is the property of the SCR and can be found on the datasheet. The latching current can be spotted on VI characteristics of SCR as I L . 2.Is the heat sink of SCR the cathode or anode or trigger? The heat sink is normally connecting to the anode terminal of the stud base SCR. To completely identify the terminals, use a multimeter on ohms. The trigger or gate terminal and cathode will show connectivity.

12. SCR Clarifying Questions 3. How to connect SCRs to get a higher working voltage? SCRs are  connected in series  for working in higher voltage and improving the blocking capacity. The voltage distribution is between the SCRs relates to the leakage current. Therefore, appropriate adjustments are made for equal voltage distribution. 4. What is the difference between breakdown and the breakover voltage in SCR ? The breakdown voltage is the voltage limit for reverse blocking mode, and it can be found on V BR  on VI characteristics of SCR. Similarly, the breakover voltage is the limit of the forward blocking mode and it can be found as V BO  on the VI characteristics of SCR.

13. 5. How to turn off SCR ? The SCR will turn off automaticity in AC supply due to  zero crossing  called neutral commutation. Where  DC supply , anode current must be reduced below the holding current to turn of the SCR.

14. Mounting of SCR When the  current  passes through  SCR  is greater than the rated value, the thermal stress produced in it which generates mechanical force.  If this mechanical force does not control, the SCR may damaged.  The protection of the  SCR  in such a condition is done by proper mounting of it. The mounting method depends upon the rating of the SCR.

15. 1.Lead mounting 2. Bolt mounting 3. Stud Mounting 1a. This method is used when load current is of small value.  1b.The   SCR  does not require cooling device or  heat sink  in this method because most of the heat is dissipated by  radiation and convection . 2a.There is one hole is provided in the SCR.  2b.The heat sink and  SCR  are joined by the bolt.  2c.The mica or fibre insulation is kept in between the  heat sink  and SCR. This type of mounting is used in the small and medium rating SCR . 3a.There are two molybdenum plates kept on both sides of  SCR .  3b.The anode is soldered with  aluminium  resulting one stud is created. The  SCR  is joined to  heat sink  by this stud.  3c .If there is not necessary electrical isolation, mica or fibre type washers are used.  3d.The conduction of heat is done easily though mica or fibre type washers. It will also works as electrical  insulator .