Thyristors (SCRs, Triacs) are very popular in AC power control applications where they are used to provide an active control of the AC power being fed to the load. The load here could be anything that is operated from AC mains and whose parameter to be controlled happens to depend upon the amount of AC power. Speed of motors, Temperature of heating elements etc are some of the examples. Here, we would build a simple circuit to demonstrate separately the use of SCRs and Triacs to provide AC power control.
SCRs are used for those loads where it is immaterial whether the load current is unidirectional or bidirectional (though the AC input is always bidirectional). Heating element is one such load, an incandescent bulb is another. Remember that amount of heat is always proportional to square of current and square of both positive as well as negative quantities is always positive. Triacs are used to control AC power to the AC loads, AC motors for instance. In the project under construction, SCR has been used to control power and thus vary the light output from an incandescent bulb while the triac has been used to control the speed of an AC motor.
CIRCUIT DESCRIPTION
When the switch SW1 is closed and the switch SW2 is open, AC input is fed only to the SCR circuit. The bridge rectifier comprising of diodes D1 to D4 provides full wave rectification of the applied AC. Since the SCR is initially nonconducting, the full wave rectified waveform does not get applied to the load. The SCR fires (i.e. it switches to the conducting state) at some point on the rectified waveform depending upon the setting of potentiometer P1. Afterthat, the rectified waveform gets applied to the load. As the rectified waveform is on the decline, the SCR turns OFF when the load current falls below the holding current value of the SCR used. It occurs very near to the zero voltage point. When the input is on the rise again, at the same point, the SCR fires again and the process repeats. Fig.4.2 shows the full wave rectified waveform and also the portion of the waveform that is fed to the load for a particular setting of the potentiometer P1. This portion can be altered by changing the setting of the potentiometer P1. The effect of changing input to the load can be clearly seen in the form of a changing light output from the bulb.
The triac circuit also behaves in the same fashion as the SCR circuit except that it is bidirectional. It can conduct in both positive as well as negative half cycles of the AC input (that is why there is no need for rectifying the input). The triac fires when the breakover voltage of the diac is reached. The shaded portion in the waveform of Fig4.3 is the one that is actually applied across the load, an AC motor in this case. The effect of control can be clearly seen in the form of changing speed of the motor as the potentiometer P2 setting is changed.
CONSTRUCTION GUIDELINES
The PCB layout and the components layout are respectively shown in Figs.4.4 and 4.5.
| Parts List |
Specification |
| Resistors |
|
| R1, R2 |
1K, 1/2 W |
| Potentiometers |
|
| P1, P2 |
1M(LIN) |
| Capacitors |
|
| C1 |
0.47μ F, 1000V (Polyester or ceramic) |
| C2 |
0.47 μF, 100V (Polyester or ceramic) |
| Semiconductor Devices |
| SCR |
OE106/SN106 or any 600 volts, 1 Ampere SCR |
| Diodes D1 to D4 |
BY 127/1N 4007 |
| Triac |
KT 206 (Any triac with breakdown voltage greater than 400V and current rating of 1A can be used) |
| Diac |
±18V diac |
| Miscellaneous |
|
| Fuse F1 ,F2 |
1A rating |
| Switch, SW1 and SW2 |
Mains Power ON/OFF switches |
| An electric bulb (40 or 60 watts) and an AC motor as loads, solder metal, |
| multistrand wires, etc. |
|
TESTING GUIDELINES
1. Close switch SW1. The intensity of the light coming from the bulb can be varied with the help of potentiometer PL. The intensity is maximum when P1 introduces minimum resistance and it is minimum when it introduces maximum resistance. The average DC voltage can be observed to vary across the load as the potentiometer is adjusted. But it will be much more informative if you could make arrangement to see the waveform across the load on an oscilloscope. That way, you can clearly see a change in the firing point (or angle) as P1 is being adjusted.
2. Open switch SW1 and close SW2 instead. Adjustment of potentiometer P2 in this case
can be used to vary the speed of the motor. Again observing the waveform across the load on an oscilloscope would tell you much more than what a multimeter would do. The gadget can be tested by connecting a household Juicer/Mixer or a fan and observe
the variation in speed as P2 is adjusted.
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