PREM|R TRADING CORPORATION
VICHARACTERISTICS OF TRIAC
AIM
Toplot the V-I characteristics of aTriac.
APPARATUS REQUIRED
The instrument comprises of the following built in parts:
1. Variable DC source-(0-25V) V1.
2. Variablc DCsource-(0-10V) V2.
3. TRIAC(BT-136).
4. Digital Voltmeter (0-50V DC) -1Nos.
5. Digital Ammeter (0- 50 mADC) -1 Nos.
6. 1Digilal Ammeter (0- 1 mA/0 50 mA DC) -1 Nos.
7. l'use (600mA) for short circuit protection.
8. Patch cords
THEORY
A triac can, conduct in both the directions. A triac is thus a bidirectional thyristor
with tlhree lerminals. It is used extensively for the control of power in ac circuits. Triac
is the word derived by combining the capital letters from the words TRIODE and AC.
When in opcration, atriac is equivalent to two SCRs connected in anti-parallel. The
cireuit symbol and its characteristics are shown in Fig. l' (a) and (b) respectively. As
the triac can conduct in both the directions, the terms anode and cathode are not
upplicable totriac. Itsthree terminals are usually designated as MTI (main terminal 1),
MI? nd the gale by Gas in a thyristor. For understanding the operation of the triac, its
eross-scetional view showing all the layers and junctions is sketched in Fig. 2.The
pate G is ncar terminal MTI. The cross-hatched strip shows that G is connected toN3 as
1
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well as P2. Similarly, terminal MTI is
N4.
connected to P2 and N2; terminal MT2 to PI and
QMT2 MT2 positive
Ig2>lg1>lgo
lg2 lg1 lg0=0
Go -Ve02
OMT1
Veo1
-lgo -Ig1 -Ig2
MT2 negative
(a)
(b)
|-lo
Fig 1': (a) Circuitsymbol and (b) Static V-I
characteristics of atriac.
MTI
G
N P2
N3 P2
P2
Ni
N, P, MT2
Fig '2': Cross-sectionalview of atriac..
CO
With no signal to gate, the triac will block both half cycles of the ac applied
voltage in case pcak valuc of this voltage is less than the break over voltage of VBOl or (i
VB02 of thetriac, Fig. 1'(b). The triaccan, however, be turned on in cach half cvcle
of the applied voltagc by applying a positive or negative voltage to the gate with
2
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respect to lerminal MII. For convenience, terminal MII is taken as the point for
measuring the voltage and current at the gate and MT2 terminals.
The turn-on process of a triac can be explaincd as under:
MT2 is pos1tive and gate current is also positive. When MT2 is positive with
(i)
respect to MTI, junction PI NI, P2 N2 are forward biased but junction NI P2 is reverse
binscd. When gate terminal is positive with respect to MTI, gate current flows mainly
through P2 N2 junction like an ordinary SCR, Fig. 3' (a). When gate current has
just
injeeted suflicicnt charge into P2 layer, reverse biased junction Ni P2 breaks down
N2 layers.
Is in a normal SCR. As a result, triac starts conducting through PI NI P2
triac
This shows that when MT2 and gate terminals are positive with respect to MT1,
operates in the first
turns on like aconventional thyristor. Under this condition, triac
mode. It is recommended
quadrant of Fig. 1 (b). The device is more sensitive in this
method of triggering if the conduction is desired in the first quadrant.
terminal is negative with
(ii) MT2 is positive but gate current is negative. When gate
junction, Fig. 3° (b) and reverse
respcct lo MTI, gate currcnt flows through P2 Ns
thyristor. As a result, triac starts
biascd junction NI P2 is forward biased as in a normal
conduction of PI NI P2 N3,
conducting through Pl NI P2 N3 layers initially. With the
potential of layer between P2 N3 rises
the voltage drop across this path falls but
portion of P2 is clamped at the
lowards the anode potential of MT2. As the right hand
across layer P2, its left hand region
cathode potential of MT1, a potential gradient exists
region. A current shown dotted is thus
being at higher potential than its right hand
is similar to conventional gate
establishcd in layer P2 from left to right. This current
right-hand part of triac consisting of main
current of an SCR. AB a consequence,
device structure PI NI P2 Na may be
Nlructure PI NI P2 N2 begins to conduct. The
pilot SCR and the structure PI NI P2 N2 as the main SCR, It can then be
regarded as
as the gate current for the main SCR. AB
stutcd that anode current of pilot SCR serves
with turn-on processdiscussed in (i) above, the device with MT2 positive but
comparcd
thereforc, more gate current is required.
gute current negative is less sensilive and
positive. Thc gate current Ig forward biases
(iii) MT2 is negative but gate current is
3
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P2 N2junction Fig. 3' (c). Layer N2 ijects electrons intoP2layer as
shown by doted
arrow..S. As a result, reverse biased junction NI PLbrcaks down as in a
conventional
thyristor. Eventually the structure P2 NI PI N4 is completely turned on.
AB usual, the
current after turn-on is limited bythe external Joad. AB the triac is
turned on by remote
gate N2, the device is less sensitive in the third
quadrant with positive gate currcnt.
(iv) Both MT2 and gate current are negative. In this
mode, Na acts as aremote gatc.
Fig. "3° (d). The gate current Ig flows from P2 to Na as in a normal thyristor.
Reverse
biased junction NIP 1is broken and finally. the structure P 2 N 1PIN 4 is turned on
completely. Though the triac is turned on by remote gate Na in third quadrant, yet thc
device is more sensitive under this condition compared with turn-on action with
positive gate current discussed in (ii) above.
G tMTI(-) HMTI(-) G MTI(+)
N2 N3
N2 N3
P2 P2
P2 N1
N Final
Initial Final
N Conduction
N conduction conduction P
(P, NP, N2 P; (P,N,P, N)
(P, N,P,Ng)
P; P N, N
MT2 (+) UMT24) MT2 (-)
Fig '3': Turning-0N process in a traic
Itcan, therefore, be concluded from above that:
() Sensitivityof the triac is greatest in the first quadrant when turned on with
positive gate current and also in the third quadrant when thrncd on with negative gate
current,
(ii) Sensitivity of the triac is lowin the first quadrant when turned on with
negative gate current and also in the third quadrant when turned-on with positive gatc
current.
Thus the triac is rarely opcrated in lirst quadrant with negative gate current and in
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thirdquadrant with positive gate current.
the
the(wO conducting paths from MTI to MT2 or Irom MT2 to MTI interact with
cnchother in the structure of the lriac ; thcir voltagc, current and frcqucncy ratings are
Imuchlower as compared with conventional thyristors. At present, triacs with voltage
Currentratings of 1200 V and 300 A(rms) are available.
Trines are used extensively in residential lamp dimmers, heat control and for the
control of'small single-phase series and induction motors.
A triac may Sometimes operate in the rectifier
mode rather than in the
bidireetional mode. This may happen due to the following reasons :-
may turn on with MT2
(u) For agiven value of positive gate current, atriac
with MT2 negative.
DONitive in lirst quadrant but may fail to turn on
the triac may turn on with MT2
(b) With constant negative gate current,
with MT2 positive.
nepative in third quadrant but may not turn on
current.
rectifier-mode can be overcome by increasing the value of gate
lhe
MT1
25mA/ 1mA
+
5 mA 0-15V
1002 (V) o-1sV
0-5V
MT2
Characteristics with Open Gate
Fig '4': Forward
open gate will take place at higher voltages,
NOTE Break over of Triac with undesirable to apply this voltage
as Triac is
voltage. It is
miNimnmpermissible forward
ever uscd with open gate.
PROCEDURE:
OR FORWARD CHARACTERISTICS 5
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A WITH OPEN GATE: -
1 Connect the circuit as shown in Fig 4'.
2 Keep gate supply control knob to minimum position so
Zero.
that gate current bccomcs
3. Select millimeter range to I MA.
4 Increase MTI-MT2 supply in steps and note down the corrcsponding MII
current. As MTTcurrent is small Triac is in 'OFF' state.
MT1 25mA/ 1mA 5002 2W
-O
1002 +
5mA
A Vo15V 0-15V
0-5V
MT2
Fig '5': Circuit Diagram with Gate Negative
Sr. No.
POSITIVENEGATIVE W.R.T MT2
B WHEN GATES
Fig A'.
Connect the circuit as shown in
circuit).
given above (in casc of open gutc
2. Repeat step (ii) to (iv)as
to 25mA.
3 Select millimctcrs range
(2.5V app.).
4 MTI& MT2 voltagc at samc voltage
Set
turn
steps, at a particular value of lg, Triac will
current (lg)insmall
Increase gate
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MLMIIR
sudden increase in MTI current and decrease in MTI-MT2 voltage.
(ONresulting
Triac at a given VMT-MI2.
Alsonote the gate current lg required for triggering the
triggering gate currents.
Record different break over voltages and
1.
Sr. No. I
REVERSE CHARACTERISTICS
(Connect the circuit as shown in Fig 6'.
characteristics procedure.
case of forward
Repcat allthe steps as in
Record all the possible results. 5002 2W
25mA/lmA
MT1
o-15V
1002 5mA (o-15V
W
0-5V
MT2
Characteristics
Fig '6': Reverse
V
Sr. No.
rn
