Aim
To study the Input V-I characteristics of Silicon control Rectifier (SCR)with positive biasing and plot the curve between V and I
Apparatus:
- NV 6530 SCR Characteristics Trainer
- 2 mm Patch cords
Theory
An SCR is an acronym for silicon-controlled rectifier and is commonly referred to as a thyristor. It is a unidirectional semiconductor similar to a diode, which allows for current to flow in only one direction. It differs from a standard diode by having a third connection called a gate that controls when the SCR turns on. The gate signal is typically a voltage pulse
that will turn on the SCR and allow for it to conduct the current. The SCR will continue to stay on as long as it is forward biased, and the voltage across it is the proper polarity. To shut off the SCR, the voltage across it has to be reversed. They are typically found on AC circuits where the energy is continuously switching polarities. In an AC circuit, it is possible to control what portion of each half cycle is passed through the SCR by controlling the time of the gate pulse relative to zero crossings. Since current can only flow in one direction,
SCRs are also used in rectifying circuits where the AC voltage is converted to DC, such as in DC drive. These devices are typically used in high power applications.
that will turn on the SCR and allow for it to conduct the current. The SCR will continue to stay on as long as it is forward biased, and the voltage across it is the proper polarity. To shut off the SCR, the voltage across it has to be reversed. They are typically found on AC circuits where the energy is continuously switching polarities. In an AC circuit, it is possible to control what portion of each half cycle is passed through the SCR by controlling the time of the gate pulse relative to zero crossings. Since current can only flow in one direction,
SCRs are also used in rectifying circuits where the AC voltage is converted to DC, such as in DC drive. These devices are typically used in high power applications.
Basic three modes of operation of SCR are
Reverse blocking mode
A cathode is positive concerning anode with the gate open. SCR is in reverse bias, i.e., junction J1 and J3 in reverse bias J2 are in forwarding bias. The device act as two PN diode connected in series with the reverse voltage applied across it. Leakage current of the order of a milliampere or
microampere flows, this is off the state of SCR. If reverse voltage increases, then at critical breakdown level or reverse breakdown voltage (VBR), an avalanche occurs at J1 and J3 and reverse current growth rapidly, so more loss in SCR. This may lead to SCR damage because the junction temperature is increasing. The maximum working reverse voltage across SCR does not exceed VBR. If the applied reverse voltage across SCR < VBR, then the device offers high impedance in the opposite direction. SCR is treated as an open switch.
Forward blocking mode (off-state mode)
The anode is positive concerning cathode with the gate open.
SCR is forward bias, junction J1 and J3 is forward bias, and J2 is reverse bias.
Here small forward leakage current flow.
If forward voltage increases then J2 junction (rev. bias) will have avalanche
breakdown called forward break over voltage (VBO)
The maximum working forward voltage across SCR does not exceed VBO. If
forward voltage < VBO; SCR offers high impedance. Hence SCR is treated as
an open switch even though it is forward blocking mode.
Forward conduction mode (on-state mode)
If we want to bring SCR from forwarding blocking mode to forward conduction
mode there are two modes:
By exceeding the forward break over voltage (VBO).
By applying gate pulse between gate and cathode.
Then SCR is in on state and behaves like a closed switch.
Circuit Diagram
1. Connect terminal-1 to terminal-4,terminal-2 to terminal-8 and terminal 3 to terminal 12 as shown in figure
2. Connect a voltmeter across terminal- 7 and 8 and Ammeter across terminal-9 and 10 as shown figure-2
3. Make short terminal 5 and 6
4. Rotate the knob `P_1` and `P_2` fully in counterclockwise
5. Switch ON the power supply.
6. Set the value of Anode current `I_g` gradually by varying knob `P_2` and observe it
7. Now Increases gate current `I_g` gradually by changing knob `P_2` and keep it.
8. At a particular value of gate current, voltmeter reading falls down to almost zero. This action indicates the firing of SCR.
9. Note the gate current value at this position
10. Keep the gate current constant by shorting terminal 9 with 10 and connect Ammeter to the 5 and 6 ( figure 3)
11. Rotate the potentiometer `P_1` fully counterclockwise.
12. Rotate knob `P_1` (from the initial position to its maximum limit )gradually and record Anode current for the respective value of anode voltages
13.Plot the graph between anode voltage `V_a` and anode current `I_a`
Observation Table
Gate current `I_g` = 7.5 mA
you can do the virtual experiment by this link
Result
Hence, Verify the Silicon Control Rectifier Voltage versus Current Characteristic