FANSHAWE COLLEGE
Lab 5: Phase Shifting an
SCR
ELNC1068
Craig Bettridge
7/12/2009
Observations:
Oscilloscope connected across the SCR only (below)
1.
SCR fired before the waveform reaches the peak (90°). The SCR is triggered roughly at ~1-5°
making the SCR conduct for roughly ~175-179° of the waveform. Therefore, almost maximum
power has been delivered to the load and the bulb is bright as shown in picture.
2. SCR fired after the waveform has reached its peak. The SCR is triggered roughly at ~135° making
the SCR conduct for 45° of the waveform. Therefore a ¼ of the power is delivered and the light
bulb is dim but still on.
3. SCR fired at the end of the waveform after it has reached its peak. The SCR is triggered roughly
at ~179° making the SCR conduct for only 1° of the waveform. Therefore almost no power has
been delivered to the load and the light bulb is off.
Conclusions:
The main application of the SCR in this lab was its use in half-wave power control even though they
possess other characteristics which can be used for different applications. SCR’s can be used to control
AC power for lamp dimmers, electric heaters, and electric motors.
A half-wave, variable-resistance, phase-controlled circuit was utilized to vary the voltage to the load.
We used a standard household light bulb so that we could visually represent the amount of voltage
going to it. The light bulb got brighter as the voltage to the load was increased.
A phase shift is necessary for the SCR to gain complete control of the waveform. To separate the
voltages (voltage applied to gate, voltage applied to anode) a capacitor and potentiometer were used in
the circuit. Since the current in a capacitive circuit leads the voltage, the gate current is shifted out of
phase with the voltage supplied to the anode of the SCR.
The potentiometer is used to determine the amount of phase shifting. The variable resistor will
determine when the gate current reaches the Gate Trigger Current (Igt) thus controlling when the SCR
fires. By adjusting the potentiometer the SCR can be controlled to fire at any point during the cycle. This
is done by a simple voltage divider circuit.
By choosing the correct values of resistance and capacitance, the voltage applied to the load can be
controlled from zero to its full value.
SCR’s can also be used as an ON-OFF control of current and due to their latching capabilities they are
often used in motor control circuits and soft starters.