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Thyristor Rectifier



Thyristors, also called silicon controlled rectifiers, or SCRs, are electronic devices used in light dimmers, motor speed controllers, and voltage regulators. Like a diode, a thyristor has an anode and a cathode, and conducts in only one direction. In fact, the schematic symbol for a thyristor resembles a diode, but with a third terminal representing the gate, which controls current flow. Unlike a diode, however, a small current pulse into the gate is required to switch the thyristor on so forward current can flow from anode to cathode. The thyristor switches off if this forward current drops below a latching threshold. In the off state, a thyristor blocks conduction in both directions. The ability to switch on and off allows the thyristor to rectify, that is to pass current of only one polarity and regulate the amount of AC power to allode. This video will demonstrate how to control a thyristor by triggering the gate at various points during an AC cycle.

Thyristors are composed of four alternating layers of P and N-type semiconductors, which form a PNPN structure. The anode lead is connected to the P-type material at one end. The cathode lead is connected to the N-type material at the other end. And the gate lead is connected to the P-type layer next to the cathode. In this simple circuit, with an AC power source in series with the thyristor and a load, the AC input by itself cannot drive the thyristor into forward conduction. Current can flow from anode to cathode only after a current pulse to the gate triggers the on state. This pulse must occur while the source voltage is positive. Otherwise, the thyristor remains off and blocks current. Thyristors are bi-stable, meaning they can rest in two different states. So the forward conducting mode persists as long as the source voltage is positive, and the current is above the latching threshold. If the current falls below this threshold, the thyristor enters the blocking mode and stays in that state until triggered again. The phase difference between the gate pulse and the zero crossing of a sinusoidal AC source is the firing angle. For example, a trigger pulse at the same time as the initial zero crossing has a firing angle of zero degrees, resulting in complete half-wave rectification, like a diode. In this case, the thyristor passes all the energy from the positive portion of the cycle to the load. If the pulse coincides with the peak of the AC voltage, the firing angle is 90 degrees, and the load receives energy from only half of the positive cycle. Finally, a pulse at the same time as the negative zero crossing results in a firing angle of 180 degrees, with no current conducted and no energy transferred at all. The objective of this experiment is to study a thyristor rectifier circuit triggered at different firing angles, and to compare the resulting average output voltages.

Because these experiments use 120 volt AC power, avoid contact with exposed wires, which may cause electrocution and injury or death. Do not touch any part of the circuit while it is energized, and do not ground the VARIAC. For more information about electrical safety, please watch the Jove Science Education video "Safety Precautions and Basic Equipment". First, set up the oscilloscope by connecting the standard scope probe to one channel and the differential probe to a second channel. Configure the differential probe to one over 20 attenuation. Set the amplification on the oscilloscope menu for the differential probe channel. Use 20x if it is available for the differential probe. Otherwise, use 10x and double any oscilloscope measurements. Cancel any oscilloscope offset by clipping the differential probe terminals together and adjusting the traces vertical position to zero volts. During this experiment, the VARIAC provides AC voltage with a line frequency of 60 hertz. Before adjusting the VARIAC, make sure it is turned off and nothing is connected to the output. Then turn the control knob to 15 percent output. Connect the output cable to the VARIAC and connect the differential scope probe terminals to the banana plugs of the cable. Turn on the VARIAC, observe the waveform on the oscilloscope, and adjust the VARIAC so that the amplitude of it's output V0 is 35 volts. Change the time base, that is the time interval per horizontal division of the oscilloscope to display two to five cycles of voltage. Capture and save a copy of this waveform and record this time base and designate it TB0 for later use. Finally, turn off the VARIAC, and do not change its setting.

This first experiment triggers a thyristor rectifier with a firing angle of zero degrees. Assemble the circuit as shown on a proto-board. Use the VARIAC for the input AC source V in. And a wire jumper in place of resistor R2. Connect the standard probe across input voltage V in, then connect the differential probe across load resistor R to observe output voltage V out. Turn on the VARIAC, and set the scope to time base TB0, which was recorded earlier. Because the firing angle is zero degrees, the thyristor acts like a diode, and the output voltage is a half-rectified sine wave. Use the scope's built-in mathematical function to measure the mean output voltage. Adjust the time base to zoom in between the points when the thyristor turns off, then turns on again. Use the scope's cursors to measure this time difference. Turn off the VARIAC, and do not change the voltage setting. Keep all VARIAC and scope connections the same for the next experiment.

To compare the results with two different non-zero firing angles, the next experiment will trigger the thyristor with a small, then a large resistance for R2. The resistances are, in this case, 300 ohms and 620 ohms. Use the smaller resistance to trigger the thyristor at a small firing angle. Remove the jumper that short-circuited R2. Then insert the 300 ohm resistor in its place. Turn on the VARIAC and set the scope to time base TB0. The firing angle is now greater than zero degrees, and, as a result, the thyristor is triggered later in the positive portion of the AC cycle. Measure the average output voltage as described earlier. Then zoom in and measure the time interval between when the thyristor turns off and back on. Turn off the VARIAC. Without changing the VARIAC setting or other connections, replace R2 with the larger resistor, and repeat the test. After the experiments are complete, turn off the VARIAC, set it to zero, and disassemble the circuit.

The output voltage of the thyristor rectifier circuit is zero until a gate pulse triggers the thyristor. After triggering, the output voltage is the remaining portion of a half-rectified wave. As the firing angle increases, the output voltage is more chopped compared to the input, and therefore the mean output voltage decreases. Consequently, the firing angle determines the amount of power a thyristor passes to the load.

Thyristors can control the amount of power transferred to a load, and were common in older adjustable DC power supplies. They are still used in many medium to high-voltage AC power control applications. First, common light dimmers used in homes and offices have a knob or slider than controls a potentiometer, which is a variable resistor. Changing the resistance changes the firing angle of a thyristor, and correspondingly increases or decreases the power that illuminates a light bulb. Anodic arc discharge is a practical and efficient means of synthesizing carbon nanotubes and graphene. Researches have used a magnetic field to enhance the controllability and flexibility of the process. The electrical discharge in this application is similar to that of arc welding. And both use high-voltage thyristors to control the power that creates the arc.

You have just watched Jove's Introduction to Thyristor Rectifiers. You should now understand how thyristors work, and how they enable control of AC power to electrical devices. Thanks for watching.

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