DIAC TRIAC

SLE 156-163

5.3 Principles and features of Ntaiak (DIAC) and Traiak (TRIAC)
5.3.1 Structure and function of DIAC

Figure 5.3.1 (a) Construction Authority, (b) final structure, (c) Circuit symbol and (d) current-voltage characteristic of a DIAC
   A DIAC is an interactive device. The structure looks like two diodes connected P-N-P-N parallel and vice versa (sch.5.3.1a). The result of such an association (fig 5.3.1v) is a device whose left side, reading from top to bottom, like a diode-N-P N-P (ie a reverse mounted diode-P N-P -N) and right channel with a P-N-P-N. The circuit symbol of the diode DIAC is shown in Fig 5.3.1g. Beyond this there are other symbol.
   The result of this structure is a symmetric behavior with respect to the current-voltage characteristic. In each polarity, the operating principle of the diode DIAC is similar to the diode P-N-P-N. When the voltage across exceed the breakdown voltage VBO does not make sense now to use the term correctly) that part which is under proper polarization process begins so that the locking device to switch to the ON position. When the device is in the ON state and the current, which is flown, reduced below the holding current, then the device switches to
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state OFF. This happens in any polarity, as shown in sch.5.3.1d
   DIAC Diodes are used very often in a phase shift circuit to trigger the provisions TRIAC AC power control.
5.3.2 Structure and function of TRIAC

Figure 5.3.2 (a) Structure and (b) Circuit symbol of a TRIAC
   And the TRIAC is a bidirectional device. The structure is more complex than its predecessors. In sch.5.3.2a shows the structure, which now consists of five P-N contacts. On the left there is a succession of semiconductor P-N-N-P and the right a succession semiconductor P-N-P-N. The two sections are combined with the gate (G), which is connected to an independent part of the N-type semiconductor and the semiconductor type P (P2 in sch.5.3.2a), forming two SCR. These in turn are connected in parallel and in opposite times and have also joined their gates. Depending on the polarity of the voltage applied to the ends of the TRIAC, it is possible to activate one or the other "SCR".
   A TRIAC, as mentioned above, is an interactive device. So do not make sense the concepts of upward and downward, as in SCR, and so the main terminals denoted as A1 and A2. The sole and basic

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cal which is metasy terminal this is that the terminal A2 is the benchmark. In this voltage is applied to the gate to it, measured the voltage at terminal A1. In simple words, the terminal A2 plays a role similar to that of the cathode in SCR. To switch a TRIAC from state OFF to state ON, the special structure of imposing the application of positive gate voltage when Pin A1 is positive and negative when Pin A1 is negative on the A2. A TRIAC returns to OFF when the current which flows through the smaller is the holding current.
5.4 Application of the above to check the validity
   There is a significant number of applications that require control of the current flowing through a circuit. In these applications include welding of metals, lighting control, motor control, and various other industrial applications. O control current, which feeds a burden, can in principle be achieved in two ways:
   • through the supply voltage, the simplest case through the secondary of the transformer power, or
   • by inserting, in series with the circuit, a controlled resistance.

Figure 5.4.1 Power Control circuit with SCR

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   Both solutions are not successful, however, because the first is characterized by high cost and the second by low efficiency, since a significant proportion of the resistance force is spent. The use of SCR and TRIAC significantly reduce the cost of the control current.
   Check with SCR
   If a rise in SCR applied a sinusoidal voltage, the device can switch to ON position only on a positive half period. The transition to OFF state will be at the end of this half period. On the negative half the SCR will not work.
   In the circuit shown sch.5.4.1 power control with an SCR. The circuit consists of the SCR, the load resistance (RL), the control circuit (CC) and the switch device (DD), which can be a PNPN diode or a diode DIAC. To control circuit fed by pulses, which are transferred to the gate of the SCR when he exceeds the breakdown voltage of the PNPN diode or diode DIAC. When a pulse is transferred to the SCR gate and go to state ON, then the voltage across the SCR will be null and passing current IL through RL.

Figure 5.4.2 Trend Waveforms (a) power and the edges of the load and (b) cord and the edges of the SCR

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   O! waveforms of the supply voltages at the ends of the burden and the edges of the SCR are presented in sch.5.4.2. We observe that the SCR switches to ON, ie conducts only during the positive half period. During the negative half-does not conduct, so it is in OFF. If the trigger pulse is applied to time, which corresponds to an angle phi, the SCR switches to ON and the voltage and current at the ends of the load increases immediately (sch.5.4.2a) while the voltage across the SCR is reset (Fig .5.4.2 b). The SCR switches to OFF at the end of the semi-

period and the current and the voltage across the load to zero until, during a positive half-, applied to the gate of an SCR trigger pulse.
   As regards the control of load current, the SCR because the current is given time, only the average current will be determined by the angle triggers F. The greater the angle phi so smaller is the average price of electricity, because the SCR will conduct (ON) for less time.

Figure 5.4.3 (a) Circuit and (b) load voltage waveform
V
   The circuit of Fig (5.4.1) at a disadvantage in terms of performance because it can only operate at 50% a year. In the event that must function throughout the period of the circuit used sch.5.4.3 or a TRIAC, which will be discussed in the next section.
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Transistors

   In sch.5.4.3 a bridge rectifier is used to "load" the system: SCR, impedance load and control circuits. In this circuit the SCR applies fully rectified voltage, which according to the sch.5.4.3a has only positive values. This gives the circuit can operate at 100% of the period.
   Check with TRIAC
   O control with TRIAC not differ significantly from control with SCR. The only difference is because the TRIAC is a amfipoliki provision and therefore can act as both positive and negative half period. O control phase of the pulse trigger a TRIAC is usually achieved by using a DIAC, since this is a amfipoliki provision. Due to the operation of amfipolikis TRIAC control is achieved rms value of voltage or current through the load angle triggers F. As in the SCR, the greater the angle phi so the lower the effective value of current because the TRIAC will conduct (ON) for less time.
   A control circuit of the active peak current, which flows through the load resistance RL, is presented in sch.5.4.4

Figure 5.4.4 Circuit configuration active voltage (current) to the load RL
   In the circuit of the delay phase sch.5.4.4 F determined from the data R1, C1, R2 and C2. O capacitor C2 is charging late

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   Example 5.4.1
In the adjacent circuit given R1 = 12 O, R2 = 68KO, R3 = 2 KO, C = 1mF, V: = 80 V, the breakdown voltage of 35 V is DIAC and the trigger voltage and current of the TRIAC 1V and 10mA. • To calculate:
    A) What is the current which flows through the TRIAC when it is in state ON.
    B) What is the minimum voltage of the capacitor C, causing triggered the TRIAC.
   Solution
    A) consider the voltage across the TRIAC is zero when BPI-
situated in a state of ON. Thus:
years when the voltage across exceed the sum of the TRIAC triggering voltage plus the breakdown voltage of the DIAC will C2 discharged through the DIAC and TRIAC thus forcing the state to go to ON. The angle phi delay increases with the resistance R1. Thus the rms voltage across the RL decreases with increasing R1.

    B) O capacitor charges through R2. To make the trigger
TRIAC should the voltage across the DIAC to overcome the tendency of inter-
convulsions. Because the TRIAC triggering voltage is 1V and the voltage
decay DIAC 35 V we have:
VC = VBO + VT = 35 V + 1 V = 36 V
    This is the minimum voltage that causes triggered the TRIAC.

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Transistors

SUMMARY 5
• The diode P-N-P-N is a device which displays internal (positive) feedback is anasyzefxi, thus has two possible states of operation, the ON state and the state OFF.
• The diode P-N-P-N switches from OFF state to ON state when the voltage across the voltage exceeds the proper division.
• The diode P-N-P-N switches from ON state to OFF state when the voltage across the decrease and become lower voltage restraint.
• The SCR is similar to but PNPN diode has an extra electrode, the gate. The portal provides the possibility to change the proper voltage division.
• By applying positive pulses, the current exceeding the current of mischief, to gate the SCR switches to ON. The return to the OFF state occurs when the voltage across the decrease and become lower voltage restraint.
• The DIAC is a amfipoliki provision and is the equivalent of passage PNPN. At DIAC we have the breakdown voltage, since there is no correct polarity, and voltage restraint.
• The TRIAC is a amfipoliki this provision and is the equivalent of the SCR. O! pulses which lead to the TRIAC is ON state when the positive terminal A1 is positive about the A2 and A1 negative when is negative on the A2.
5 Exercises
5.1.Mia diode P-N-P-N can
5.2.Gia to go to state
used as
ON a diode P-N-P-N should be
a resistance
apply
b amplifier
A. A positive pulse trigger
c Switch
B. A small current
d power source
C. A breakdown voltage
N · the right note.
d an inverted pulse trigger

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N · the right note.
5.3. An SCR switches to ON with
a proper breakdown voltage
b the gate pulse
c voltage decay
d holding current
N · the right note.
5.4. In the next circuit the diode is in ON. If
in this situation the voltage drop at the ends is 0,7 V, what should be the voltage to go in state OFF.
1KO
 n \ MM /

VBO = 10V IH = 4mA
5.5. In the next circuit is RC = 56 Oh, RG = 2,7 KO, VCC = 15 V, IT = 2 mA, IH = 2 mA and the voltage drop

the edges of the SCR in ON state and the VG is 0,7 V. What is the output voltage when the SCR is in the OFF; What is the trigger voltage gate of the SCR; If the VCC fall until the SCR switch in position OFF, what is the price at which this happens?
5.6. If in the previous exercise halved the resistance and we have IT = 1,5 mA, what einvi trend which will cause the Triggered SCR;
5.7. In the circuit of the figure given R1 = 12 O, R2 = 39 KO, R3 = 1 KO, V = 60 V, C = 1 Mf, voltage DIAC 32 V breakdown voltage and gate trigger TRIAC 1 V. If the TRIAC is ON state what the current, which flows through the R1;

DIAC
5.8. In exercise 5.7, if the TRIAC is voltage at the ends of 1 V when the conduct and holding current is 50 mA, what should be the voltage to go to state OFF.

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