Silicon Valley Computer SHUGART 706 User Manual download pdf (Page 59)

TABLE 2- POPULAR TRIAC'S

Device

PIV

Current

rms

VGT

(Max)

IGT

(max)

IH (max)

C206D

2N6073

C226D

SC146D

TIC246D

400V

10A

15A

2.5V

5mA

30mA

50mA

30mA

70mA

60mA

75mA

50mA

VAC

TR1

C2060

iUAD

10011

1000µF

1N44001

25V

: R2

'1.1009

:8:

: 400V

LT1

FIG. 13 -STEP

-DOWN

TRANSFORMER T1

provides

low-

voltage DC signal for triggering the Triac.

The optional

R2 -C2

network can be used to suppress RFI.

FIG. 14 -THE CONTROL CIRCUIT connected

to the secondary of

causes

UJT

01 to oscillate at

about

50 Hz. The

secondary circuit is

isolated from

the primary

circuit here, unlike

the

figure.

can

turned

on by applying

a brief pulse

to the gate

the

Triac.

The

device

takes

only a few microseconds

to turn

on. A

saturation potential

of one or two

volts is

developed

across the

Triac in the

on state.

After the Triac turns

on, it self -latches

and

remains

on for as long

as main- terminal

current continues

flow.

3. After the

Triac

self -latches, it can

only be turned

off again

by reducing its

main

-terminal current

below a minimum

holding

value. When the Triac

is used as

power

switch therefore,

turn

-off

occurs automatically

at the

zero- crossing

point at the end

of each half -cycle, as

main -terminal currents

fall to zero.

4. The Triac can

be turned on by either

a positive

or a negative gate

signal, irre-

spective of the polarities

of the main

-ter-

minal

voltages. The device thus

has four

possible triggering

modes

or "quad-

rants."

With current

flowing through

MT2 in either the positive

or the

negative

direction, gate

current can

be either

positive

or negative. The

differences

among

the four modes

are primarily

that

the device's

gate sensitivity

is greater

when both MT2 current and

gate current

are the same (positive

or negative).

5. Triacs can handle

very

high

surge

currents. Typically,

a device

with

a I0-

amp (rms) rating may be able to

handle a

single

-cycle

non -repetitive

60 -Hz surge

current of 100 amps.

Table 2 shows basic

specifications for

several

popular Triacs. The

information

presented there should be

sufficient to

help you to select a Triac for many ap-

plications.

With

the

basics in mind, let's

move

and

look at practical

ways

using the

Triac.

Basic

Triac

circuits

Figure 13

shows

the circuit

simple

DC- triggered Triac power

switch, in

which the DC supply is derived

via step

down transformer TI.

When

is open.

no current flows to

the

gate

of the Triac,

it remains off.

When

SI is closed.

however,

gate

drive

is applied to the Triac,

and the

load

turn

on. If an inductive

load (a motor. for example)

used

this

circuit. the R2 -C2 snubber network

must

wired

in place as indicated to

prevent

false rate-

effect

triggering.

In that circuit the negative

side of the

DC supply is connected directly to

one

side of the AC power line. so it's live and

therefore

dangerous.

That snag can

overcome in the UJT-

triggered isolated -

input circuit

shown

in Fig. 14.

that

circuit. as

long as SI

closed.

the UJT

oscillates at

several kHz and

thus delivers

roughly

50 trigger

pulses to the gate

of the

Triac

(via isolation

pulse transformer

TI )

during

each half

-cycle of the

power

line

waveform. Consequently.

the Triac is

fired by the first

trigger pulse

occurring in

each half -cycle;

that pulse

occurs

within

few degrees

of the

start of the half- cycle.

The

point

is that

the Triac is

on almost

constantly

while

SI is closed,

virtually

full power is applied

to the

load.

Figure 15 shows

how the Triac can

used as a

simple line

switch

with line -

derived triggering.

With

open, no gate

drive is applied,

so the Triac and the

lamp

are off.

Suppose, however,

that

SI is

closed.

the

start of each

half -cycle the

Triac is off,

so voltage is applied

to the

gate

via

the

lamp and R1. Shortly

after the

start of the half

-cycle, enough

drive

available

to trigger

the Triac,

so it and the

lamp

on. As the Triac

goes on and

self=

latches, it saturates and

thereby

removes

gate drive

until the

start of

the

half=

cycle. thus

minimizing

RI's dissipation.

The circuit

in Fig_.

15 can he modified

LMP1 SI

E,9

VAC

100W

TRI

C206D

27012

FIG. 15- SIMPLEST

TRIAC

CIRCUIT

corn

posed of

switch.

a resistor. and a

Triac.

provide

halfwave operation

by placing a

diode between SI

and RI.

The diode's

polarity

will

determine

whether the Triac

will

trigger

on positive or

negative half

cycles.

Phase

-triggered

power control

The SCR and Triac circuits that

we have

looked at

so far have all been designed

give a simple on/off form

of power con-

trol. The same devices can,

however, easi-

ly be used to give

fully -variable power

control in AC circuits.

In fact, they're

widely

used

in lamp dimmers and elec-

tric-

motor speed controllers. etc.

The

most

widely

used

system of AC

variable

power control

is known as the phase

trig -

gering

system.

In a basic phase -triggering

circuit that

uses a Triac as the power

-control element,

rather than triggering the

Triac

directly

from the

power

line, it is triggered

via

variable

phase -delay network that

connected between the power

line

and

the

gate

of the Triac.

Phase delay

works

like this. If

the Triac

1 2 ... 54 55 56 57 58 59 60 61 62 63 64 ... 107 108

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