Download Figure 15

Figure 15
A typical Triac circuit which can be used to vary the amount of ac power applied to a
load is shown in Figure 15. This circuit is generally referred to as a full wave phase
control circuit and it operates in a manner similar to the SCR circuit shown in Figure 8.
The primary difference is that the Triac is triggered into conduction on both the positive
and negative alternations of each ac input cycle, while the SCR in Figure 8 conducts only
on positive alternations. Also, a special triggering device is generally used to insure that
the Triac turns on at the proper time.
Capacitor C charges through Rl in first one direction and then the other as the positive
and negative alternations of the ac input signal occur. During each alternation, the Triac
is turned on when the voltage across capacitor C rises to the required level. However, this
voltage is not applied directly to the Triac's gate and MT 1 leads. Instead, it is applied
through a special triggering device which has bidirectional switching characteristics. The
triggering device could be any component which would turn on or conduct when
subjected to a specific voltage level and turn off when the voltage is reduced to a lower
level. One of the most widely used triggering devices will be described in detail later in
this unit. However, at the present time we will just assume that the device is a solid-state
component that has the switching characteristics just described.
During each alternation, the voltage across capacitor C rises to a level which turns on the
triggering device. This causes current to momentarily flow through the Triac's gate lead
and switch it to the on state. The gate current only flows for a moment since the capacitor
discharges through the Triac and loses its accumulated voltage which in turn causes the
triggering device to turn off. How soon the Triac turns on during each alternation is
determined by the value of R1 When the resistance of R1 is reduced to zero, the Triac is
triggered immediately at the beginning of each alternation and full ac power is applied to
load resistor RL. As the resistance of R1 is increased, triggering occurs later during each
alternation and the average power applied to the load is reduced. The voltage waveforms
shown in Figure 9 can also represent the voltage across RL in the Triac circuit if the
negative alternations (which are identical but complementary to the positive alternations)
are inserted.
A triggering device is required because the Triac is not equally sensitive to gate currents
flowing in opposite directions as explained earlier. The triggering device helps to
compensate for the Triac's non-symmetrical or non-uniform triggering characteristics.
The voltage required to turn on the triggering device is identical in either direction and
the device is designed to be as insensitive to temperature changes as possible. The
triggering device works in conjuction with resistor Rl and capacitor C to produce
consistently accurate gate current pulses that are high enough to turn on the Triac at the
proper time in either direction. These gate current pulses can be very short in duration
(several microseconds is generally sufficient) and still trigger the Triac.
Although the Triac has the ability to control current in either direction and respond to
gate currents flowing in either direction, the device does have certain disadvantages when
compared to an SCR. In general, Triacs have lower current ratings than SCR's and cannot
compete with SCR's in applications where extremely large currents must be controlled.
Triacs are available that can handle currents (usually measured in rms values) as high as
25 amperes. By comparison, SCRs can be readily obtained with current ratings (usually
expressed as average values for a half cycle) as high as 700 or 800 amperes, and some are
rated even higher. Also, both devices can have peak or surge current ratings that are
much higher than their respective rms or average ratings.
Triacs often have difficulty in switching the power applied to inductive loads. This
problem also occurs with SCRs but to a lesser degree. When Triacs are used to control
the power applied to inductive loads such as motor windings or heater coils, it is always
necessary to use additional components to improve their operation. Also, Triacs are
designed for low frequency (50 to 400 Hz) applications while SCRs can be used at
frequencies up to 30 kHz. Therefore, in certain applications where full control of an ac
signal is required, SCRs may operate more efficiently than Triacs while in other
applications the exact opposite may be true.
As explained earlier, a triggering device is used in conjunction with the Triac because the
Triac does not have symmetrical triggering characteristics. Various types of triggering
devices can be used with the Triac, but one of the most popular devices is known as a
Bidirectional Trigger Diode, commonly referred to as a DIAC.
We will now briefly examine the construction and operation of this important triggering
device. Then we will see how it is used in conjunction with the Triac.
Simplest Dimmer Schematic
The first schematic is of a normal (2-way) inexpensive dimmer - in fact this contains just
about the minimal number of components to work at all!
S1 is part of the control assembly which includes R1.
The rheostat, R1, varies the amount of resistance in the RC trigger circuit. The enables
the firing angle of the triac to be adjusted throughout nearly the entire length of each half
cycle of the power line AC waveform. When fired early in the cycle, the light is bright;
when fired late in the cycle, the light is dimmed. Due to some unavoidable (at least for
these cheap dimmers) interaction between the load and the line, there is some hysteresis
with respect to the dimmest setting: It will be necessary to turn up the control a little
beyond the point where it turns fully off to get the light to come back on again.
Black o--------------------------------+--------+
|
|
| |
|
R1 \ |
|
185 K /<-+
|
\ v CW
|
|
__|__ TH1
|
_\/\_ Q2008LT
+---|>|
/ |
600 V
|
|<|--' |
C1 _|_ Diac
|
.1 uF --- (part of |
S1
|
TH1)
|
Black o------/ ---------------------+-----------+
MTC300A/MFC300A Modules
Features
·Heat transfer through
aluminium oxide ceramic
isolated metal baseplate
·precious metal pressure
contacts
·Thyristor with amplifying
gate Typical Applications
·DC motor control(e.g. for
machine tools)
·AC motor soft starters
·Temperature control(e.g.
for ovens,chemical processes)
·Professional light dimming(studios,theaters)
Remote Heat Controllers
Just plug your heating equipment into one of these controllers and turn the
adjustment knob to set. "On-off" cycling controls the amount of time a heater is on
by proportioning electric power at line voltage. You should monitor your temperature
with a separate thermometer. Controllers have a 6-ft. power cord with three-prong
plug. Units operate on 115 VAC, and provide 115 VAC up to 1500 watts. The solidstate model reduces arcing of the switch contacts.
Solid-State
Controller
Each
Standard Controller
Solid-State Controller
35655K88
35655K89
35655K89
Remote Heat Controller Solid State Model, 115 Volt, 1500 Watts
In stock at $107.43 Each
$94.86
107.43