C A M O S U N C O L L E G E

ELECTRONICS DEPARTMENT

( Elex 248 )

E248lab08.doc/ Electronics Department, Camosun College

Laboratory 8

THE TRIAC & DIAC IN A PHASE CONTROL POWER CIRCUIT

OBJECTIVES

1. To become familiar with TRIAC and DIAC devices.

2. To study the operation of TRIAC in a phase control power circuit.

DISCUSSION

Thyristors are widely used to control power in numerous applications:

• Phase Control

• SMPS (Switch Mode Power Supplies)

• DC-DC Converters

• Motor Controls etc.

SCR (Silicon Controlled Rectifier): One type of thyristor used where DC power is to be

controlled.

TRIAC: A thyristor used where AC power is to be controlled. A triac is similar to two

SCR's connected in a reverse parallel configuration.

DIAC: A silicon bi-directional trigger device in a glass envelope intended for use in triac

and thyristor trigger circuits.

The TRIAC is normally off but may be triggered on by a low current pulse to an input

called the gate. Once triggered on, it remains on until the current flowing through the

main terminals of the device drops below the minimum holding current. It is usually not

possible (at least not easy) to turn a thyristor off while current is flowing. However, there

are special types of devices called Gate Turnoff Thyristors (GTO) which are capable of

this type of control as well.

References: - Robbins & Miller ch31 pp 1181 (diac), 1190-1193 (triac)

And Example 31-4 p 1197,

- Class notes and handouts & Component Data Sheets

Elex248- Lab8 The Triac and Diac in a Phase Power Control Circuit Page 2 of 4 _____________________________________________________________________________________

E248lab08.doc/ Electronics Department, Camosun College

LAB EQUIPMENT AND MATERIALS

1 Oscilloscope HP 54645A

1 Digital multimeter HP 34401A shared RLC bridge meter SR 715 1 Isolation Transformer 120V/1A

1 Triac Q4006L4 ( 6 Amps ) 1 Diac 1N5761 ( 28V - 36 V )

1 Lamp 60 W/ 120V

1 Resistor 1 kΩ / 0.25W

1 Resistor 1 kΩ / 1W

1 POT 100 kΩ / 2W

1 Capacitor 0.22uF / 50 V

1 Breadboard

Miscellaneous wires and leads

Procedure

1. Using the DMM and RCL meter measure R1a, R1b, R2, and C1, and record their values

below:

R1a = _________ , R1b = _________ , R2 = _________ , C1 = __________

2. Carefully construct the circuit shown in Figure 8-1 below.

CAUTION: 120VAC is a potentially dangerous voltage – treat it with

respect! Be sure to disconnect the power before making any adjustments to

your circuit.

3. Connect CH#1 and CH#2 of the scope to the points A and B. Connect the scope

COMMON to point D.

Figure 8-1

Q4006L4

TRIAC

M1

M2

G

M1 M2 G

Q4006L4

Q4006L4

TRIAC

1N5761

DIAC R2

1 K

R1a

1 K / 1W

R1b

100 K / 2W

0.22 uF / 50V

C1

LAMP 60W / 120V

T1

ISOLATION

TO 120 Vac

TRANSFORMER

A B

M2

G M1C

D

Elex248- Lab8 The Triac and Diac in a Phase Power Control Circuit Page 3 of 4 _____________________________________________________________________________________

E248lab08.doc/ Electronics Department, Camosun College

4. Connect the circuit to a 120V AC source via the isolation transformer.

5. Observe the RANGE of possible firing angles with 100 KΩ potentiometer (POT) set

to zero and then 100 KΩ. Record them in Table 8-1. [Note: R1 is different in each formula below!!]

6. Minimum Phase Delay:

1) Adjust the 100 KΩ POT to provide minimum phase delay of the triac firing as

viewed at the MT2 pin (at firing, voltage from MT2 to MT1 drops);

2) Measure the phase delay (angles) and record it in Table 8-1;

3) Sketch both waveforms on the same time axis (next page);

4) Note lamp brightness (brighter).

7. Maximum Phase Delay:

1) Readjust the 100 KΩ POT to provide maximum phase delay of the triac firing as

viewed at MT2 pin;

2) Measure the phase delay and record it in Table 8-1;

3) Again, sketch both waveform on the same time axis;

4) Note lamp brightness.

8. Diac Firing Voltage: Remove the CH#1 of the scope from point A and connect it to

the point C ( the capacitor side of the diac). Measure the diac firing voltage and

record it in Table 8-1.

9. Explain the differences between the computed values and the measured values for all

data in Table 8-1 on page 4.

Time/Div = _________ Volts/Div = _________ Volts/Div = _________ Time/Div = _________

Minimum Phase Delay Maximum Phase Delay

Elex248- Lab8 The Triac and Diac in a Phase Power Control Circuit Page 4 of 4 _____________________________________________________________________________________

E248lab08.doc/ Electronics Department, Camosun College

Table 8-1. Phase Delay in a Triac-Diac Circuit

Step No. Parameter Computed Measured

3, 5

Min. Phase Delay

n/a

3, 6

Max. Phase Delay

n/a

7

Diac Firing Voltage

(32V±4V)

10. Briefly summarize your conclusion(s) for Lab 8.