Optocouplers

                     &

Optoisolators

Applications

 

 

 

OPTOCOUPLER FUNCTIONS

SPECTRAL RESPONSE OF SILICON

CONSTRUCTION

PRINCIPLE OF OPERATION

Photo diode output

Photo transistor output

Photo Darlington output

Photo SCR output

Slotted optocoupler

Applications

SPECIFICATIONS

REINFORCEMENT EXERCISE

OPTOCOUPLERS

 

An optocoupler, also called opto-isolator, is an electronic component that transfers an electrical signal or voltage from one part of a circuit to another, or from one circuit to another, while electrically isolating the two circuits from each other. It consists of an infrared emitting LED chip that is optically in-line with a light-sensitive silicon semiconductor chip, all enclosed in the same package. The silicon chip could be in the form of a photo diode, photo transistor, photo Darlington, or photo SCR.

optocoupler FUNCTIONS

To isolate one section of a circuit from another, each section having different signal voltage levels to ensure compatibility between them.

To prevent electrical noise or other voltage transients that may exist in a section of a circuit from interfering with another section when both sections have a common circuit reference. Noise or voltage transients can be caused by a poor printed circuit board layout.

 

SPECTRAL RESPONSE OF SILICON

Since silicon has a response to light (spectral response) that peaks at infrared wavelengths (between 800 and 950 nanometers), silicon devices are preferred as the photodetector section in optocouplers in conjunction with an infrared LED emitter (Figure 13.1). Matching the infrared LED to the silicon chip provides a maximum transfer of the desired electrical signal.

Different types of optocouplers have specific characteristics that determine suitability for each unique application. The simplest type is the optocoupler with a photo diode output section. The optocoupler output is often connected to an amplifier (or series of amplifiers) to change a low-level input voltage into an appropriate higher signal level.

L146EPY

Spectral Response Curve of Silicon

Figure 13.1

 

CONSTRUCTION

The input section of an optocoupler is an infrared LED chip. It is separated from the output silicon diode chip by a thin, transparent, mylar plate embedded in clear silicone (a derivative of silicon). The assembly is sealed in a package keyed to designate pin #1. The most commonly used optocoupler package is the plastic DIP (dual-in-line package).

 

PRINCIPLE OF OPERATION

When a forward bias voltage is applied to the input terminals of the LED (positive to the anode), an input current, IIN, limited by the series resistor, RS, will flow in the LED circuit. The current produces the infrared light emission at about 900 nanometers that impinges on the photo-sensitive silicon chip.

PHOTO DIODE OUTPUT

With light impinging on the silicon diode in Figure 13.3, its photovoltaic characteristic will create photo current, IL or IOUT, to flow in the silicon diode. With a load resistor, RL, connected to the output terminals of the coupler, the photo current, IOUT, will develop a voltage, VL, across the load. VL = IOUT x RL.

 

optocoupler with Photo Diode Output

Figure 13.3

As the input signal, VIN, varies, it will vary the intensity of the infrared light. The output current, IOUT, will also change, causing the output voltage, VL, to change in the same manner.

As output current increases, output voltage will also increase, and vice-versa. A small change in input current will produce a proportionate change in output current. This characteristic of the optocoupler will act to couple low-level analog signals or small DC voltage variations with little or no distortion.

In the circuit of Figure 13.3, both signal coupling and input-to-output isolation is achieved, however, the current transfer ratio (CTR) of a diode output optocoupler is extremely low about 10% to 15%. The term current transfer ratio (CTR) defines the relationship of output current, IOUT, to input current, IIN.

The output voltage, VL, can be coupled to the input of an amplifier to increase its amplitude to an appropriate level.

The input section of all optocouplers is an infrared LED, however, the output section can be different depending on the required application. The basic principle of operation is the same, regardless of the particular output section selected.

 

PHOTO TRANSISTOR OUTPUT

Since the CTR of an optocoupler with a photo diode output is so low (10 to 15%) a preferred approach is to replace the diode chip with a silicon bipolar photo transistor (Figure 13.4).

The bipolar transistor, with its inherent current gain, b, will provide a considerably higher CTR (between 50% to 100%) depending on the beta of the photo transistor.

The base lead of the transistor can be reverse biased to reduce sensitivity, or forward biased to increase sensitivity, or left "floating" (disconnected).

optocoupler with Photo Transistor Output

Figure 13.4

 

PHOTO DARLINGTON OUTPUT

If a still higher CTR is needed, the bipolar transistor can be replaced with a Darlington transistor configuration to serve as the photo-detector output section.

optocoupler with Photo Darlington Output

Figure 13.5

In the circuits of Figures 13.4 and 13.5, the output current, IC, of the single bipolar photo transistor or the photo Darlington will develop an output voltage, VL, across the load resistance, RL. This voltage is the product of the output current, IC, and the load resistance, RL.

The optocoupler can be operated either as a linear amplifier or as a digital switch, depending on the forward bias voltage applied to the base of the transistor.

 

PHOTO SCR OUTPUT

If the output section of an optocoupler is a photo SCR, the coupler functions to switch the positive half of the AC voltage across the load, operating under the same principle as an ordinary SCR circuit.

optocoupler with Photo SCR Output

Figure 13.6

The SCR gate current is achieved through the photo-voltaic action produced by infrared light impinging on the SCR gate, while optically isolating the input and output circuits of the optocoupler.

Steady-state DC can be used as the supply voltage, causing the output to be latched ON when the SCR is energized. This type of circuit is applicable for security or fire alarm systems.

To turn the system OFF after being triggered ON, a single-pole, single-throw, normally-closed switch for circuit reset can be inserted in the supply voltage path to momentarily remove the supply voltage after the input pulse returns to zero. The momentary opening of the switch turns the SCR OFF and sets the system to its standby condition.

The same techniques for SCR turn-off, discussed in Chapter Five Thyristors, can be implemented with photo SCR optocouplers.

 

SLOTTED optocoupler

The slotted optocoupler is available with photo transistor and photo Darlington photodetectors, with the device package structured to provide an additional element of control.

The package normally has an air gap between its two sections measuring about one-eighth of an inch in width. One section has an infrared LED and the other, a photodetector (Figure 13.7). Slotted optocouplers with wider air gaps are available.

Slotted optocoupler Package and Schematic

Figure 13.7

 

APPLICATIONS

The infrared light beam that links the two sections of an optocoupler can be broken by the mechanical insertion of any thin object into its air gap to block the infrared light. This device lends itself to many control applications.

Slotted optocoupler Application

Figure 13.8

When a thin plastic or metal disk with perforations or slots cut at its outside edge is rotated inside the slot of the optocoupler, the infrared light can be detected where an opening in the slot exists. As a result, current flows at the output of the coupler. When the infrared light is blocked by the opaque section of the disk, there is no output current (Figure 13.8).

As the infrared light beam is interrupted, pulses are generated at the output of the coupler and rotational speed of the disk can be measured, and motor speed control can be achieved. If the disk is rotated by the flow of a liquid, the rate of flow of the liquid can be determined. With a properly calibrated counting system, the exact number of gallons of gasoline being pumped at a gas station can be measured accurately with this device and its associated circuitry.

Other applications of a slotted optocoupler include a punched-card reader, a parts counter, an end of tape sensor on a printer or tape recorder, and as part of an interlock mechanism.

In addition to the types already discussed, optocouplers are manufactured with many variations of their photodetector sections. One type consists of an infrared LED emitting light to an integrated photo diode/bipolar transistor circuit, allowing direct coupling to a digital logic circuit (Figure 13.9a). Another circuit allows a photo diode to be directly coupled to a logic circuit serving as the circuit's output section (Figure 13.9b).

optocouplers are available as single components, as duals (two separate devices in one package), and as quads (four separate devices in one package).

 

 

SPECIFICATIONS

PRODUCT DESCRIPTION

Information on optocoupler type, circuit configuration, linearity, frequency response, switching speed, isolation voltage, and any typical applications is listed in the manufacturer's data sheet.

PACKAGE OUTLINE

The package drawing appears on the data sheet and includes information on package dimensions, mounting details, terminal spacing, terminal designations, and lead material.

 

ABSOLUTE MAXIMUM RATINGS

At 25C ambient temperature unless otherwise noted

ISOLATION VOLTAGE

Maximum voltage differential the device can tolerate between its input and output sections. Isolation voltage depends on the insulation used air, glass, or plastic. Typical values of isolation voltage range from 500 to 6000 volts.

 

TEMPERATURE CONSIDERATIONS

OPERATING AND STORAGE TEMPERATURE

Plastic packages range from -55C to +100C.

Hermetically sealed devices range from -55C to +125C.

LEAD SOLDERING TEMPERATURE

Typical value is 230 for 7 seconds with the soldering spot at least inch from the seal between lead and package.

 

INPUT SECTION (INFRARED LED) RATINGS

These specifications are identical to those defined and discussed in Chapter Ten Light Emitting Diodes and listed under the heading of Absolute Maximum Ratings. They include:

PEAK INVERSE (REVERSE) VOLTAGE (PIV) or (PRV)

REVERSE DC CURRENT (IR)

CONTINUOUS DC FORWARD CURRENT (IFcont)

PEAK FORWARD CURRENT (IFpeak)

DC POWER DISSIPATION (PD)

 

DETECTOR SECTIONS RATINGS

These specifications are identical to those which are defined and discussed in previous chapters and listed under the heading of ABSOLUTE MAXIMUM RATINGS as follows:

PHOTO DIODE

Chapter Three Diode Specifications Ratings include:

REVERSE VOLTAGE (VR)

DC POWER DISSIPATION (PD)

 

PHOTO TRANSISTOR AND PHOTO DARLINGTON

Chapter Six Bipolar Transistors. Ratings include:

COLLECTOR-TO-EMITTER VOLTAGE (VCEmax)

COLLECTOR-TO-BASE VOLTAGE (VCBmax)

EMITTER-TO-BASE VOLTAGE (VEBmax)

DC POWER DISSIPATION (PD)

 

PHOTO SCR

Chapter Five Thyristors. Ratings include:

REPETITIVE PEAK REVERSE (INVERSE) VOLTAGE (PRV) or (PIV)

NON-REPETITIVE REVERSE VOLTAGE (PRVTRANSIENT)

PEAK POSITIVE ANODE VOLTAGE (PFV)

PEAK ONE-CYCLE SURGE CURRENT (ISURGE)

AVERAGE FORWARD CURRENT (IF)

PEAK ONE-CYCLE SURGE CURRENT (ISURGE)

PEAK GATE VOLTAGE (VGM)

PEAK POSITIVE GATE CURRENT (IGM)

TOTAL POWER CAPABILITY (PT)

 

ELECTRICAL CHARACTERISTICS

INPUT SECTION (INFRARED LED)

These specifications are identical to those which are defined and discussed in Chapter Ten Light Emitting Diodes and listed under the heading of Electrical Characteristics. They include:

FORWARD VOLTAGE (VF)

REVERSE LEAKAGE CURRENT (IR)

 

ELECTRICAL CHARACTERISTICS

DETECTOR SECTION

These specifications are identical to those which are defined and discussed in previous chapters and listed under the heading of ELECTRICAL CHARACTERISTICS as follows:

PHOTO DIODE

Chapter Three Diode Specifications

FORWARD VOLTAGE VF

REVERSE OR LEAKAGE CURRENT IL or IR

TURN-ON TIME (TON) AND TURN-OFF TIME (TOFF)

 

PHOTO TRANSISTOR AND PHOTO DARLINGTON

Chapter Six Bipolar Transistors

COLLECTOR CURRENT CUTOFF (LEAKAGE) (ICES)

CURRENT GAIN BETA (b) or (hFE)

COLLECTOR SATURATION RESISTANCE (RCE(sat))

TURN-ON TIME (TON) AND TURN-OFF TIME (TOFF)

FREQUENCY CUTOFF (FCO)

 

PHOTO SCR

Chapter Five Thyristors

GATE TURN-ON VOLTAGE (VGT)

GATE TURN-ON CURRENT (IGT)

FORWARD VOLTAGE (VF)

REVERSE CURRENT (IR)

TURN-ON AND TURN-OFF TIMES (TON) and (TOFF)

The output of an optocoupler can be connected to any other circuit with the entire assembly enclosed in a single package and treated as a single component. This approach lends itself to the creation of many products that include opto-coupling as part of their features.

Regardless of the circuit design, the same operating principle is maintained, that is: optical coupling between circuits while achieving electrical isolation between these circuits.

 

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