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2.9.2 Triac Devices
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A triac is a solid-state device designed to control AC current (Fig. 2.69). Triac switches turn ON in less than a microsecond when the gate (control leg) is energized, and shut OFF at the zero crossing of the AC power cycle.
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Triac
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Opto Coupler Triac Driver
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Inductive or Capacitive Control Circuit
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To AC load circuit
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FIGURE 2.69
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Triac circuit.
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C l a s s i f i c a t i o n a n d Ty p e s o f S e n s o r s
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60 Hz 1 cycle 1 cycle = 16.66 ms 0.5 cycle = 8.33 ms
Contact VAC
Time
FIGURE 2.70
AC power cycle.
Because a triac is a solid-state device, it is not subject to the mechanical limitations of a relay such as mechanical bounce, pitting, corrosion of contacts, and shock and vibration. Switching response time is limited only by the time it takes the 60-Hz AC power to go through one half of a cycle (8.33 ms) (Fig. 2.70). As long as a triac is used within its rated maximum current and voltage specifications, life expectancy is virtually infinite. Triac devices used with inductive or capacitive sensors generally are rated at 2-A loads or less. Triac limitations can be summarized as follows: (1) shorting the load will destroy a triac; and (2) directly connected inductive loads or large voltage spikes from other sources can falsetrigger a triac. To reduce the effect of these spikes, a snubber circuit composed of a resistor and capacitor in series is connected across the device. Depending on the maximum switching load, an appropriate snubber network for switch protection is used. The snubber network contributes to the OFF state leakage to the load. The leakage must be considered when loads requiring little current, such as PLCs, are switched. In the ON state, a drop of about 1.7 V rms is common (Fig. 2.71). Good and bad features of triacs are listed in the following table.
Triac Advantages Fast response time (8.33 ms) Tolerant of large inrush currents Can be directly interfaced with programmable controllers Infinite life when operated within rated voltage/current limits
Triac Disadvantages Can be falsely triggered by large inductive current Snubber contributes to OFF state leakage current Can be destroyed by short circuits
Two
Contact VAC
120 VAC
Load
Time RC Snubber
Contact VAC
120 VAC Time
Load
FIGURE 2.71
Snubber circuit.
2.9.3 Transistor DC Switches
Transistors are solid-state DC switching devices. They are most commonly used with low-voltage DC-powered inductive and capacitive sensors as the output switch. Two types are employed, depending on the function (Fig. 2.72). In an NPN transistor, the current source provides a contact closure to the DC positive rail. The NPN current sink provides a contact to the DC common. The transistor can be thought of as a single-pole switch that must be operated within its voltage and maximum current ratings (Fig. 2.73). Any short circuit on the load will immediately destroy a transistor that is not short-circuit protected. Switching inductive loads creates voltage spikes that exceed many times the maximum rating of
+V +V
C Photoelectric circuit B E Output 36 V Common Photoelectric circuit B
36 V
Output
FIGURE 2.72
DC circuit logic.
C l a s s i f i c a t i o n a n d Ty p e s o f S e n s o r s
(+) Load Load
Load
FIGURE 2.73
Transistor switch.
FIGURE 2.74
Voltage clamp.
the transistor. Peak voltage clamps such as zener diodes or transorbs are utilized to protect the output device. Transistor outputs are typically rated to switch loads of 250 mA at 30 V DC maximum (Fig. 2.74). The following outlines the advantages and disadvantages of transistors.
Transistor Advantages Virtually instantaneous response Low OFF state leakage and voltage drop Infinite life when operated within rated current/voltage Not affected by shock and vibration Transistor Disadvantages Low current handling capacity Cannot handle inrush current unless clamped Can be destroyed by short circuit unless protected
Output Configuration
Output configurations are categorized as follows: Single output normally open (NO) Single output normally closed (NC) Programmable output NO or NC Complementary output NO and NC The functions of normally open and normally closed output types are defined in Table 1.8.
Inductive and Capacitive Control/Output Circuits
A single output sensor has either an NO or an NC configuration and cannot be changed to the other configuration (Fig. 2.75). A programmable output sensor has one output, NO or NC, depending on how the output is wired when installed. These sensors are exclusively two-wire AC or DC (Fig. 2.76).
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