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INDUSTRIAL CONTROL HANDBOOK - 3.8 OPTOISOIATORS AND OPTOINTERRUPTERS

As more electronic devices were used to form circuits, the demand to connect circuits with differing voltage potentials and differing impedances together became more prevalent. For example, as computers and programmable controllers became more usable on the factory floor, it became evident that some type of interface would be needed that could isolate the 220 volt AC and 110 volt AC signals that most machinery used from the small DC bus voltages found in computers. Isolation is also a problem when larger AC and DC voltages need to be interfaced to TTL logic circuits.

FIGURE 3-24 A comparison of the power devices. The following abbreviations are used: HVT (high-voltage transistor), J-FET (J-type field effect transistor), MOS (metal-oxide semiconductor), THY (thyristor), GTO (gate turn-off device), and IGBT (insulated gate bipolar transistor). (Courtesy of Philips Semiconductors.)

 

HVT

J-FET

MOS

THY

GTO

IGBT slow

IGBT fast

Unit

V(ON)

1

10

5

1.5

3

2

4

V

Positive Drive Requirement

-

+

+

+

+

+

+

+=simple to implement

Turn-Off Requirement

-

-

+

(--)

-

+

+

+=simple to implement

Drive Circuit Complexity

-

-

+

(-)

-

+

+

-=complex

Technology

Complexity

+

-

+

+

-

-

-

-=complex

Device Protection

-

-

+

+

-

-

-

+=simple to implement

Delay Time (ts, tq)

2

0.1

0.1

5

1

2

0.5

ms

Switching Losses

-

++

++

--

-

-

-

+=good

Current Density

50

12

20

200

100

50

50

A/cm2

Max dv/dt (Vin=0)

3

20

10

0.5

1.5

3

10

V/ns

dl/dt

1

10

10

1

0.3

10

10

A/ns

Vmax

1500

1000

1000

5000

4000

1000

1000

V

Imax

1000

10

100

5000

3000

400

400

A

Over Current Factor

5

3

5

15

10

3

3

FIGURE 3-25 Electrical block diagram and physical layout of a typical opto-coupler. The optocoupler is also called an optoisolator and it is usually packaged as a six-pin 1C chip.FIGURE 3-25 Electrical block diagram and physical layout of a typical opto-coupler. The optocoupler is also called an optoisolator and it is usually packaged as a six-pin 1C chip. (Copyright of Motorola, Used by Permission.)


The simple solution to this problem is to combine an LED with a phototransistor. The new device is totally encapsulated so that the light from the LED is focused directly on the opening in the phototransistor, and no other forms of light could be detected. The input signal is connected to the LED and the output signal is connected to the transistor. The device is called an optocoupler or optoisolator. Fig. 3-25 shows a block diagram of an optocoupler that shows an LED shining light directly on a photodector, which is usually a phototransistor. The second diagram in the figure shows how the LED is located so its light is focused directly on the phototransistor.

Fig. 3-26 shows the six-pin 1C package for an optocoupler and the electronic diagram of its pin outline. The 1C package may also be called an 1C or a chip. From this diagram you can see that the anode of the LED is pin 1 and the cathode is pin 2. The emitter of the phototransistor is pin 4, the collector is pin 5, and the base is pin 6. It is important to note that each type of optocoupler may use different pin assignments, so you must be sure to check the manufacturer's pin outline diagrams.

FIGURE 3-26 Pin outline for an optocoupler for a six-pin 1C. A sketch of a six-pin 1C is also shown.FIGURE 3-26 Pin outline for an optocoupler for a six-pin 1C. A sketch of a six-pin 1C is also shown. (Copyright of Motorola, Used by Permission.)

FIGURE 3-27 Electrical diagram of an optocoupler used to interface an annunciator horn to a computer. The relay coil is connected to the output stage of the optocoupler.FIGURE 3-27 Electrical diagram of an optocoupler used to interface an annunciator horn to a computer. The relay coil is connected to the output stage of the optocoupler.

 

3.8.1 Applications for Optocouplers

Fig. 3-27 shows an optocoupler interfacing a computer output signal to a relay coil and the contacts of the relay are used to energize a 110 volt alarm lamp and annunciator. This circuit allows the small-voltage signal from the computer to safely energize the high-voltage lamp and horn without the fear of allowing any high-voltage spikes to get back into the computer. Optocouplers are also used in programmable logic controller (PLC) 110 volt input and output modules to provide isolation between the 110 volt signals and PLC bus. In industrial applications a limit switch on a machine is wired for 110 volt AC so that it is not bothered by induced electrical noise. The 110 volt AC signal is connected to the programmable controller input module circuit consisting of a bridge rectifier that converts the AC signal to DC, a resistor, and the LED for the optocoupler. The transistor side of the optocoupler is connected to the input bus of the PLC. Since the signal emitted by the LED is transferred by light, the high and low voltages of the circuit are isolated.

FIGURE 3-28 Optocoupler used in a PLC input module. When the switch closes, 110 volts AC is provided to terminal 1. The phototransistor in the Optocoupler is connected to the PLC input bus.FIGURE 3-28 Optocoupler used in a PLC input module. When the switch closes, 110 volts AC is provided to terminal 1. The phototransistor in the Optocoupler is connected to the PLC input bus.

 

3.8.2 Optoisolation Relays (Solid-State Relays)

The industrial applications that require Optoisolation circuits are so prevalent that several companies make plug-in and stand-alone Optoisolation circuits called solid-state relays (SSRs). The SSR provides the Optocoupler circuit in an encapsulated module that has larger terminals available so that it can be used in industrial circuits and requires 3-32 volts dc to turn it on. The LED section of the Optocoupler acts like the coil of a traditional relay. This part of the SSR requires dc voltage because the LED must be forward biased to produce light.

The phototransistor section of the Optocoupler inside the SSR is equivalent to the contacts in a relay.

If a traditional phototransistor is used, the SSR will be rated for dc voltages. If the SSR is rated for ac voltages, it will use photosensitive solid-state devices to trigger other devices such as triacs or two inverse parallel SCRs for switching, or it can trigger the phototriac directly. Fig. 3-29 shows examples of SSRs used in conjunction with several types of transistor circuits to provide interface capabilities with TTL circuits. The internal diagram of the SSR is shown in Fig. 3-29c. In this diagram you can see that the SSR is an Optocoupler that uses an LED and a phototransistor. A 1000 ft resistor is connected internally in series with the LED so that the user does not need to worry about needing additional resistance to prevent excessive current. It should be noted that if the voltage of the input signal is too low, there may be insufficient current to properly illuminate the LED.

FIGURE 3-29 (a-f) Examples of solid-state relays and their electrical diagrams. The diagrams show pnp and npn transistors used in the output stage Figure 6-21c shows a typical load connected to the relay.FIGURE 3-29 (a-f) Examples of solid-state relays and their electrical diagrams. The diagrams show pnp and npn transistors used in the output stage Figure 6-21c shows a typical load connected to the relay. (Courtesy of Opto 22, Remecula, CA.)

FIGURE 3-30 Typical rack with solid-state relays mounted in it. A wide variety of relays is available to provide interfaces to dc, ac, and analog signals.FIGURE 3-30 Typical rack with solid-state relays mounted in it. A wide variety of relays is available to provide interfaces to dc, ac, and analog signals. (Courtesy of Opto 22, Remecula, CA.)

Since the SSRs are available as stand-alone or plug-in devices, they provide the advantage of being removed and replaced very quickly. If they are the plug-in type, they can be removed and replaced by someone with minimal technical knowledge, since the wiring for this type is connected to the socket which is soldered directly to a printed circuit board.

You will find SSRs in a variety of applications, such as in microprocessor controlled systems like the high-speed weighing system and the single-point temperature controllers where they provide a simple interface for alarms and other outputs. Fig. 3-30 shows the relays plugged into a module board.


3.8.3 Optocouplers Used in Input and Output Module Circuits

Optocouplers are also commonly used to provide isolation for input and output modules that are used to interface between programmable logic controllers (PLCs) or for other computer-type systems. The companies that make the SSRs also provide generic input and output circuits so that a designer can interface a wide variety of ac and dc circuits to the inputs and output bus of a common desktop-type computer. This allows the computer to be used to run a variety of software and still have the ability to read inputs and write outputs (turn them on or off) through its parallel or serial port. Fig. 3-31 shows the typical circuit for an input module. This circuit is similar to a typical PLC input module.

FIGURE 3-31 (a) Electrical diagram of typical solid-state relay used to interface input signals. The diagram shows the terminals in the rack allow for either ac or dc signals to be connected. If an ac signal is used, an ac relay must be installed in the rack, and if a dc signal is used, a dc relay must be used. (b) The diagram for an ac relay, (c) The diagram for a dc relay.FIGURE 3-31 (a) Electrical diagram of typical solid-state relay used to interface input signals. The diagram shows the terminals in the rack allow for either ac or dc signals to be connected. If an ac signal is used, an ac relay must be installed in the rack, and if a dc signal is used, a dc relay must be used. (b) The diagram for an ac relay, (c) The diagram for a dc relay. (Courtesy of Opto 22, Remecula, CA.)

FIGURE 3-32 Electrical diagram of the solid-state relay used as an output module. Notice that the output circuit includes an optocoupler and a transistor that is used as an amplifier. (Courtesy of Opto 22, Remecula, CA.)FIGURE 3-32 Electrical diagram of the solid-state relay used as an output module. Notice that the output circuit includes an optocoupler and a transistor that is used as an amplifier. (Courtesy of Opto 22, Remecula, CA.)

 

3.8.3 Specialty Types of Optocouplers

A large variety of Optocouplers has been designed to meet the demands of numerous applications. For example, Optocouplers are available that are specifically designed for high-gain signals that use darlington pairs and for high-speed switching where Schmitt triggers are used. Other conditions such as common-mode rejection, ac/dc voltage to logic-level signal interfaces, low-current applications, TTL applications, high-gain applications, and for multiplexing data applications require special types of Optocouplers.

Fig. 3-33 shows example circuits of the low-input current logic gate optocoupler. Fig. 3-34 shows examples of specialty types of Optocouplers that use transistors with a base terminal where bias can be added, darlington pair transistors that are used for higher gain, and Schmitt triggers that are used for high-speed switching. Fig. 3-35 shows the diagram for a shunt drive circuit for optocoupler interface between TTL and CMOS signals, and Fig. 3-36 shows an optocoupler that allows ac or dc voltage as the input and the output is converted to a logic-level signal.

FIGURE 3-33 Schematic diagram and pin outline for a low-power optocoupler. This type of device is used where the input signal is a low-power signal.FIGURE 3-33 Schematic diagram and pin outline for a low-power optocoupler. This type of device is used where the input signal is a low-power signal. (Copyright of Motorola, Used by Permission.)


In Fig. 3-34 you can see the diagram of the low-input power logic gate optocoupler. This optocoupler combines a GaAsP LED with an integrated high-gain photon detector. The detector portion of the device provides a three-state output stage and has a detector threshold with hysteresis. The need for pull-up resistors is negated by the three-state output. The hysteresis provides differential-mode noise immunity and prevents the possibility of chatter in the output signal. Chatter may occur if the contacts of the input device bounce during closure. The contact bounce may appear as more than one signal transistion, and the hysteresis in the circuit ensures the input signal only represents the intial contact closure. This optocoupler is specifically designed to switch at small current thresholds as low as 1.6-2.2 mA. A truth table is also provided for this circuit.

Fig. 3-35 shows the diagram for a shunt drive circuit that uses an optocoupler to provide an interface between TTL/LSTTL/CMOS logic circuits. The LED in this circuit can be enabled by as little as 0.5 mA at a frequency of 5 megabaud (5 million pulses per second). This makes the circuit usable as a logic-level translator or for microprocessor I/O isolation. This circuit also eliminates several problems and increases common-mode rejection, since the path for leak current in the LED is eliminated.

FIGURE 3-34 Diagrams of optocouplers that use transistors, darlington transistors, and Schmitt triggers for their output stage.FIGURE 3-34 Diagrams of optocouplers that use transistors, darlington transistors, and Schmitt triggers for their output stage. (Courtesy of Hewlett Packard Company.)

FIGURE 3-35 Electrical diagram of a shunt driver circuit that utilizes an optocoupler to provide an interface between TTL and CMOS logic circuits.FIGURE 3-35 Electrical diagram of a shunt driver circuit that utilizes an optocoupler to provide an interface between TTL and CMOS logic circuits. (Courtesy of Hewlett Packard Company.)

SCHOTTKY DIODE (HP 5082 2800, OR EQUIVALENT) AND 20 pF CAPACITOR ARE NOT REQUIRED FOR UNITS WITH OPEN COLLECTOR OUTPUT.

FIGURE 3-36 Electrical diagram of optocouplers specifically designed for AC input signals.FIGURE 3-36 Electrical diagram of optocouplers specifically designed for AC input signals. (Copyright of Motorola, Used by Permission.)


Other types of specialty optocouplers nave been developed 10 handle problems that occur when optocouplers are used in ac circuits. One circuit is shown in the first part of Fig. 3-38 where a phototriac is used instead of a phototransistor. Since the triac is used, ac voltages can be controlled directly. The second diagram in this figure shows a triac connected to a zero-crossing circuit. The zero-crossing circuit is used to ensure that the triac switches ac voltage on and off exactly when the ac sine wave is at 0 volts. This means that the triac is only turned on when the sine wave is at 0° or at 180°, which means that voltage and current are minimal when the triac allows current to flow to the remainder of the circuit. This allows circuit components such as lamp filaments to last much longer, since they are not subjected to high-voltage transients from switching ac voltage and current when the sine wave is at a peak.

FIGURE 3-37 Electrical diagram of an optocoupler specifically designed to accept an AC or DC voltage input.FIGURE 3-37 Electrical diagram of an optocoupler specifically designed to accept an AC or DC voltage input. (Courtesy of Hewlett Packard Company.)

 

3.8.4 Adding Bias to the Phototransistor of the Optocoupler

The optocoupler is also useful in circuits where the bias of the phototransislor is changed. When bias voltage is added to the base of the phototransistor in an optocoupler, it can make the optocoupler more or less sensitive. If the bias voltage to the base of an NPN transistor is slightly positive, the optocoupler will become more sensitive because the LED does not need to produce as much light to make the phototransistor begin to conduct. If the bias voltage is slightly negative, the optocoupler will become less sensitive and the LED must have more current applied before it can produce enough light to overcome the bias on the phototransistor. The optocoupler must have a base terminal for the transistor brought out to a pin so that it is usable to add bias to it. The optocouplers in Fig. 3-36 show transistors with their base brought out to pin 6.

 

3.8.5 Using an Optocoupler to Convert a 4-20 mA Signal to a Variable-Voltage Signal

The optocoupler can also be used to convert a 4-20mA signal to a variable-voltage signal. A 4-20 mA signal is common in process control applications, such as level sensors or flow sensors in food processing. The 4-20 mA signal is used for two reasons. First, the milliamp signal is a current loop-type signal that is more resistant to noise than a voltage signal; and second, the minimum value for the 4-20mA signal is 4 mA which is offset above 0 mA. Since the minimum signal value is offset 4 mA from 0 mA, a broken wire can be detected if the current value falls to 0 mA.

FIGURE 3-38 Electrical diagram of optocouplers specifically designed with triac and zero-crossing triac drivers as the output circuit.FIGURE 3-38 Electrical diagram of optocouplers specifically designed with triac and zero-crossing triac drivers as the output circuit. (Copyright of Motorola, Used by Permission.)

 

3.8.6 Photo IC Interrupter

Another specialty application for the optocoupler technology is an integrated circuit called a photointerrupter that is specifically designed for high-speed oscillation of the LED and detection circuit. The photointerrupter is also called an optointerrupter.

Fig. 3-39 shows a typical optointerrupter and a circuit for the device. The device is designed with a slot so data material such as punch cards and punch tape can be passed through to be read at high speed.

In this device the traditional phototransistor is replaced with a Schmitt trigger circuit, which can turn on and off at much higher frequencies that provide switching at speeds of 3msec. The LED is mounted on one side of the slot, and the Schmitt trigger is mounted on the opposite side. The Schmitt trigger circuits have either an open collector interface or the traditional pull-up resistor interface, which allows the circuit to produce the active HI or active LO output.

FIGURE 3-39 Drawing and electrical diagram of a photo- interrupter used to detect data from punch cards or other similar media as it moves through the slot in the head where the sender and receiver are mounted. (Courtesy of Hewlett Packard Company.)FIGURE 3-39 Drawing and electrical diagram of a photo- interrupter used to detect data from punch cards or other similar media as it moves through the slot in the head where the sender and receiver are mounted. (Courtesy of Hewlett Packard Company.)


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