Th e purpose of the capacitor:
Up to now we it has been indicated that if the thyristor is not triggered by 90° it will never switch on, in this circuit a capacitor has been introduced. Th e time it takes for the capacitor to charge fully depends on the value of the capacitor and the resistor R2. Th e time constant is calculated by t = 5RC. In other words, the smaller R2, the quicker the capacitor will charge up, reaching the required voltage to trigger the DIAC, which in turn will fi re the TRIAC. Th is means the fi ring angle of the TRIAC is small and it will conduct for a longer time, so current can fl ow through the load for a longer time (a light in this case) and it would burn brightly.
Compare this to when the value of R2 is increased. Th e capacitor will take longer to charge and reach the required voltage to trigger the DIAC. (Th is may very well happen aft er 90°.) Th is means the TRIAC will be triggered later (larger fi ring angle) and it will actually conduct for a shorter time. Th is means that current will fl ow for a shorter time, causing the light to burn less brightly.
Th e purpose of the components:
R1 – to prevent a short circuit across the supply if R2 is set to minimum R2 – control the time constant (t = 5RC)
C1 – together with R2 it determines how long it takes for the capacitor to charge to the voltage required to fi re the DIAC. Th is determines the fi ring angle of the TRIAC
DIAC – to allow a specifi c, defi nite signal onto the gate of the TRIAC so that it fi res exactly, and to prevent transient signals on the gate of the TRIAC from triggering it
TRIAC – controls for how long current fl ows through the load, and ultimately the brightness of the lamp, or the speed of the motor
Light – the load that does the work Output waves across the load:
Th e output waves for the lamp dimmer using TRIAC and phase control.
If the TRIAC has a fi ring angle of 130º, the output waves would look as follows.
Remember that current fl ows in both directions through the load, so there will be waves at the top and the bottom of the t-axis.
Figure 8.61:
If the TRIAC has a fi ring angle of 60°, the output waves would look as follows:
A full, complete, operational circuit to perform a lamp-dimming/motor-control function is shown below. Th is particular circuit works really well.
Figure 8.62: The full circuit diagram for a lamp dimmer Food for thought
Why does one make use of thyristors if a lamp can be dimmed by means of a variable resistor? Surely this is a much simpler circuit?
Th is circuit is really simple. With the slider of the variable resistor set to the top, the resistor is bypassed, the resistance is at a minimum, and maximum current can fl ow.
Th is means the lamp would burn brightly.
With the slider set to the bottom, the resistance is obviously maximum, which means current is minimum, so the lamp would be dim.
Th e disadvantage of this circuit is that current (which has to be paid for) fl ows all the time and a lot of unnecessary heat is created across the resistor.
A comparison between variable resistor control and thyristor control, shows that the advantages of using thyristors far outweigh those of using the variable resistor.
VARIABLE RESISTOR CONTROL THYRISTOR CONTROL
On 100% of the time. Current only fl ows for a certain time. For the remainder, zero current fl ows.
Th ere is unnecessary voltage drop across the resistor. Th is creates a voltage divider, which makes the load operate less effi ciently.
Th e full voltage is across the load. Th e load can operate at maximum effi ciency.
Unwanted power is dissipated across resistor. Minimal power is dissipated across the thyristor.
Lots of unwanted heat created across the resistor .
Minimal wasted heat across thyristor.
Moving part of the slider eventually wears out. No moving parts, no wear and tear.
Slider can cause sparking, which is a fi re risk. No moving parts: no fi re risk.
Th icker supply cables required. Th inner supply cables required.
Unwanted running cost. Reduced running cost.
Activity 1
1. Draw neat, fully labelled IEC symbols for each of the following:
a. PN diode
2. Draw neat I/V characteristic curves (or load lines) for the:
a. Zener diode b. TRIAC
3. What is the main function of diodes?
4. How can a diode be tested with a multimeter to check if it is functional?
5. How can a diode be forward biased?
6. What is a Zener diode?
7. What is a transistor?
8. How would you identify the terminals of a transistor?
9. What is the maximum current and voltage rating, case style and the maximum power rating for a BC 108 transistor?
10. Name the three regions of operation for a transistor.
11. What are the Vbe ratings for the transistor as a switch, and as an amplifi er?
12. Name three thyristors.
13. What is special about thyristors?
14. How would you decide on the type of thyristor required for a certain application?
15. Explain the diff erence between a fi ring angle and a conduction angle.
16. What is a LDR?
17. What is a thermistor?
18. Explain the operation of the circuit shown below.
19. For the circuit shown below, we take the supply voltage as 240 V and all the SCRs have a fi ring angle of 80°. Draw the output for two complete cycles of the input.
20. How can we switch an SCR on and off ?
500 Ω + VCC