Optocoupler is a device that couples an input control signal to output or load, via using light energy, in such a manner that electrical isolation also remains intact between input signals and load (output). The basic function of an Optocoupler is the coupling of input and output circuits through light energy (due to which it is called Optocoupler) and to provide complete isolation between input and output circuits, therefore Optocoupler is also sometimes known as Optoisolator. The objective of providing electrical isolation between input and output circuits is to protect them against high voltage transients or high voltage short waves, surge voltage (i.e. sudden sharp increase in voltage), and low-level noise. Moreover, the interfacing of different voltage level circuits and different grounds by means of an Optocoupler (that’s different voltage circuits and different grounds can also be set apart or isolated by means of its use). Remember that this device provides isolation up to 2500 volts.
The input circuit of an Optocoupler or optical coupler comprises an atypical light-emitting diode (LED) and its output circuit a phototransistor. In other words, an Optocoupler consists of a combination of a light-emitting device (i.e. a device which emits light) and a light-sensitive device (a device that is sensitive to light). The LED fixed in Optocoupler is actually a gallium arsenide (GaAs) infra-red emitter, which produces light of about 0.9×10-6m wavelengths. For the purpose of an excellent coupling medium, an infrared glass and for best electrical isolation an air gap is used. Remember that apart from LED and phototransistor combination, Optocoupler is also available in LED and photo Darlington packages or LED and LASCR packages.
In figure 7.25, an Optocoupler has been illustrated along with its connections. When the input circuit of an Optocoupler is supplied input voltage Vin, the circuit becomes forward biased. Thus, LED irradiates and starts emitting light. As input and output circuits are inter-coupled via light energy, therefore when LED light transmits to phototransistor fixed in the output circuit, it turns on. Thus, the flow of current from the extrinsic load circuit starts. The greatest advantage of this series is that the best type of electrical isolation exists between input and output circuits (i.e. low power control circuit and high-power load circuit).
Figure 7.25 – Optocoupler with external connections
In the figure, a simple optocoupler has been demonstrated for further clarification. The LED fitted in it operates the transistor. A small change in input circuit voltage or VS brings about a change in LED current. As a result, the current passing through the phototransistor also changes. Thus, variable voltages are created parallel to the collector-emitter terminals. Thus, the signal voltage of the input circuit is coupled with the output circuit.
Figure 7.26 – optocoupler with LED and phototransistor
Important Parameters for an Optocoupler
Important parameters of an Optocoupler are as follows:
The maximum voltage which can sustain without any die-electric breakdown between input and output terminals is called isolation voltage Viso. Their specific values are up to 7500 AC volts.
Current Transfer Ratio or CTR
The ratio between output (phototransistor) current and input (LED) current is known as current transfer ratio which is denoted as a percentage. The output CTR values of a phototransistor may be between 50 to 150 percent. While in the case of photo darling ton output its specific values are between 50 to 500 percent.
It is divided between rising time (tr) and toll time (tl). The tr and tl output stage of a phototransistor is near to about 2-5 microseconds.
This parameter is associated with an optically isolated AC linear coupler. The ratio between the output voltage and the input current is called transfer gain. Its defined value is 200mV/mA.
Types of Optocoupler
Optocouplers are normally of the following types
1). Darlington Transistor
2). SCR output coupler
3). Photo Triac Output Coupler
4). Optically Isolated AC Linear Coupler
5). Digital Output Coupler
In figure 7.27 (a), daring ton output or darling ton transistor coupler has been displayed. As compared to the output of an ordinary phototransistor, an extremely high output current is received from a Darlington coupler. Therefore, these couplers are commonly used with high loads. However, it drawback is that its switching speed is less compared to a phototransistor. These types of couplers are used for DC power switching (or such types of couplers are used for supplying DC power on a load).
In figure (b) SCR output coupler has been depicted. As the output stage of this coupler comprises a light-activated SCR, therefore it is also known as the LASCR coupler. This device is used in such a circuit, where a high voltage relay is required to be latched via input voltages. After getting latched, this relay moves onto some other electro-mechanical device.
In figure (c) a photo Triac output coupler has been exemplified. This device is used for isolated Triac triggering e.g. switching a low-level input to 110-volt AC line.
Figure 7.27 common types of optical coupling devices
In figure (d), an optically isolated linear coupler has been exhibited. This device converts changes in input current to variations in output voltage (i.e. in this device, the output voltage can be changed by means of variations in input current). The output circuit consists of an amplifier. A photodiode has been fixed parallel to its input terminals. Photodiode senses variations in light emitted from LED and sends an input signal to the amplifier, whereas an emitter follower stage exists on the amplifiers’ output. Such couplers are used for telephone line coupling and for audio purposes.
In figure (e), a digital output coupler has been exemplified. This device comprises a high-speed detector circuit, after which there is a transistor stage. When light emits from LED, the detector which is light-activated (which works by means of light radiating on it), operates and turns output transistor on. Thus, the collector reaches a low voltage level. When light does not transmit through LED, it does not illuminate. Thus, the collector reaches a high voltage level (i.e. output is at a high voltage level). Such type of device is most commonly used in digital circuits.
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