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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