Lab assignment # 1 Lab assignment Title:

Trigging an SCR (silicon controlled rectifier) Objectives:

900 Phase Control of SCRApparatus:

DMM Resistor as load SCR Wires Bread board AC power supply Oscilloscope

Theory:Phase control (PFC), also called phase cutting, is a method of pulse width

modulation (PWM) for power limiting, applied to AC voltagesPhase fired control is often used to control the amount of voltage, current or power that a power supply feeds to its load. It does this in much the same way that a pulse width modulated (PWM) supply would pulse on and off to create an average value at its output. If the supply has a DC output, its time base is of no importance in deciding when to pulse the supply on or off, as the value that will be pulsed on and off is continuous.

PFC differs from PWM in that it addresses supplies that output a modulated waveform, such as the sinusoidal AC waveform that the national grid outputs. Here, it becomes important for the supply to pulse on and off at the correct position in the modulation cycle for a known value to be achieved; for example, the controller could turn on at the peak of a waveform or at its base if the cycle's time base were not taken into consideration.

Phase fired controllers take their name from that fact that they trigger a pulse of output at a certain phase of the input's modulation cycle. In essence, a PFC is a PWM controller that can synchronize itself with the modulation present at the input.

Most phase fired controllers use Thyristor or other solid state switching devices as their control elements. Thyristor based controllers may utilize Gate Turn off (GTO) Thyristor, allowing the controller to not only decide when to pulse the output on but also when to turn it off, rather than having to wait for the waveform to pass within the element's Zero Cross Point.

Procedure a) Consulting the DATASHEET of SCR and noting the triggering current. b) Make the circuit according to the schematic diagram. c) Increasing the Gate voltaged) A time comes when the Gate current is enough to trigger the SCRe) Observing the voltage across Thyristor on oscilloscope f) Observing the voltage across Load on oscilloscope g) Observing the load current on oscilloscope. h) Measured the voltage across the Thyristor when i) Measured the voltage across the load resistorj) Measured the current through the load

Latching current of SCR=74maHolding currene t of SCR-38ma

Conclusion a) When a trigging current passes through the gate the Thyristor turns on and works as

closed switch. b) The small voltage drop occurs across the Thyristor round about 0.1 volts. c) Gate current is of very small value. d) Thyristor remains triggered even if the triggering pulse is removed e) To turn off the Thyristor we have to decrease Thyristor current lower then threshold

value. f) When the Thyristor is turned off all the voltage appears across Thyristor and it will works

as open switch. g) To turn on the Thyristor again we need to apply the gate pulse again.

Lab assignment no 3Lab Assignment Title:

RC firing circuitObjective:

RC firing of SCR and analyzing the wave forms of voltage and current across Thyristor and load. Apparatus

DMM Resistors Resistors as load SCR Wires Bread board AC power supply Oscilloscope Capacitor Diode

Circuit diagram:

LOAD

vO

R

C

VT

v =V sin tS m

D2

VC

+

-D 1

Procedurea) Consulting the DATASHEET of SCR and noting the triggering current. b) Make the circuit according to the schematic diagramc) Increasing the gate voltaged) Observing the voltage across Thyristor on oscilloscope by changing the firing

angle. e) Observing the voltage across Load on oscilloscope by changing the firing angle. f) Observing the load current on oscilloscope by changing the firing angle. g) Measured the voltage across the Thyristor when h) Measured the voltage across the load resistori) Measured the current through the load.

Conclusion a) RC firing is used to fire from angle 90 to 180 b) When a rigging current passes through the gate the Thyristor turns on and works

as closed switch. c) The small voltage drop occurs across the Thyristor round about 0.1 volts. d) Gate current is of very small value. e) Thyristor remains triggered even if the triggering pulse is removed f) To turn off the Thyristor we have to decrease Thyristor current lower then

threshold value. g) When the Thyristor is turned off all the voltage appears across Thyristor and it

works as open switch. h) To turn on the Thyristor again we need to apply the gate pulse again.

Lab assignment # 4

Lab Assignment Title: Resistance Firing Circuit with DC supply

Objective:Resistor firing of SCR with DC supply and analyze the firing of SCR

Apparatus DMM Resistors Resistors as load SCR Wires Bread board DC power supply Capacitor Diode

Circuit Diagram:

Procedure: a) Consulting the DATASHEET of SCR and noting the triggering current b) Make the circuit according to the schematic diagram c) Increasing the Gate voltage. d) Observing the voltage across the Thyristor on DMM changing the supply voltage.e) Observing the voltage across Load on DMM by changing the supply voltage.f) Observing the load current on DMM changing the supply voltage g) Measured the voltage across the load resistor h) Measured the current through the load

Observations Latching voltage is 21 volts. Holding voltage is 10 volts.

Conclusions: Resistor firing is used from angle 0 to 90. When the voltage drop across the resistor is so much higher to trigger the SCR. The small voltage drop occurs across the Thyristor round about 0.1 volts. Gate current is of very small value. Thyristor remains triggered even if the triggering pulse is removed. The triggering pulse is removed to reduce the power losses in the circuit. To turn off the Thyristor we have to decrease Thyristor current lower then threshold

value. When the Thyristor is turned off all the voltage appears across Thyristor and it works as

open switch. To turn on the Thyristor again we need to apply the gate pulse.