Switching characteristics of SCR and determination of holding current and latching current.
Aim:
1) To study I-V characteristics of SCR
2) To determine holding current and latching current of SCR
Apparatus required:
Trainer board, chords, SCR (2P4M), LED, Resistors (100Ω, 330Ω and1KΩ), wires.
Experimental Setup:
Fig. 1: Set up for measuring holding and latching currents of SCR
Theory:
A thyristor is a solid state devices with 4-layers of pnpn structure with 3pn junctions-anode, cathode and gate. Structure and symbol of SCR are shown in Fig. 2.
Fig. 2: SCR structure and symbol
SCRs are mainly used in devices where the control of high power, possibly coupled with high voltage, is demanded. Their operation (it can switch large current on and off) makes them suitable for use in medium to high-voltage AC power control applications, such as lamp dimming, regulators and motor control.
Fig. 3: I-V characteristics of SCR
The SCR blocks voltage in both the forward and reverse directions; so that is a symmetric blocking. In the SCR characteristic curve or SCR volt-ampere characteristic or SCR static characteristic (figure 3), the SCR blocking voltage are: (i) forward break over voltage (VBO ), (ii) reverse break down voltage (V BD ).
A several techniques can be used to turn-on SCR. Here, normal method to turn-on SCR is used. If the anode (A) is positive, the SCR can be triggered into conduction by a short positive gate current pulse. But once SCR is conducting, the gate loses its control to turn off the device. The i-v characteristics of SCR indicate that at gate current I G = 0, if 5
forward voltage is applied on the device (mean that the anode is positive respect to the cathode), there will be a leakage current due to blocking of the middle junction. If the voltage exceeds its critical limit or equal to forward breakover voltage, V BO ), then SCR is ON-STATE. By increasing I G , V BO is reduced, and if I G = I G3 , SCR behaves like a diode with the entire forward blocking region removed. SCR will turn on successfully if a minimum current, called a latching current (I L ), is maintained. During ON-STATE, if the I G =0 and the anode current (I A ) falls below a critical limit, called the holding current (IH), SCR reach its forward blocking state. With reverse voltage, the end P-N junctions of the device become reverse-biased and the i-v curve becomes similar to diode. In summary, SCR is turn-on by the positive gate current, and turn of naturally when the anode voltage drop to zero. And, each SCR has its own i-v characteristics. So, in practice we must consider the datasheet of SCR to get the important parameters of this device.
Holding Current: Holding current may be defined as the minimum value of anode current in which the device stops conduction and return to its OFF state. The value of this current is very small in mA.
Latching Current: Latching current of a device is defined as the minimum ON state current required to keep the device in the ON state after the triggering pulse has been removed. Generally, latching current is two or three times larger than the holding current.
Procedures:
1. Connect the circuit as shown in figure 1. Press the push button and observe the condition of the LED. Then release the button and again observe the condition of the LED.
2. Press the push button. Increase the variable dc power supply gradually from 3Vand monitor when LED turns on. Take the current reading. This current is called latching current of SCR.
3. Release the push button and again observe the condition of the LED.
4. Slowly decrease the DC power supply and monitor the decrease in current through SCR. At a particular value of current, LED will turn off indicating the off-state of SCR. Record that current. This current is called holding current of SCR.
Report:
1. Record latching current and holding current of SCR.
2. Draw the I-V characteristics of SCR and describe its different modes of operation.
3. Differentiate between latching current and holding current.
4. Write down some advantages and disadvantages of SCR.
5. Write down some applications of SCR.
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