Optocoupler | Optoisolator
An opto-isolator or sometimes referred to as optical isolator, optical coupling device, optocoupler, photocoupler, or photoMOS, is an electronic device that transfers an electronic signal between components of a circuit by means of a short optical transmission channel, usually a transmitter and a receiver, while keeping them electrically isolated. Electrical connection is not necessary between the source and the destination circuits because the electrical signal is converted into a light ray, transmitted, then converted back to an electrical signal. Isolation between the source and the destination is rated at 7500 Volt peak for one second for a conventional component which costs for about less than one US dollar in small quantities.
The package of an opto-isolator is composed both of an infrared LED (light emitting diode) and a photodetector such as SCR (silicon controlled rectifier), photosensitive silicon diode or transistor Darlington pair. To achieve the highest possible measure of coupling the wave-length responses of the two components are aimed to be the same as possible.
Optocoupler | Optoisolator Configurations
The usual and common configuration of an opto-isolator is an LED and a phototransistor in a light-proof environment to eliminate the penetration of light and without the same electrical connection, placed in a position wherein the light from the LED will strike on the photodetector. When an electrical signal is applied to the input of the opto-isolator the LED lights and radiates through the photodetector, and as a result will produced an equivalent electrical signal in the output side of the circuit. Opto-isolator has a greater advantage than a transformer because it can give any preferred degree of electrical isolation and has a better protection from excessive voltage conditions in one circuit that may affect the other, and also allows DC coupling. Darlington pair can be used instead of a phototransistor to achieve higher transmission ratio but at the expense of higher delay and poor noise immunity.
When photodiode is used as a detector, the output current is a function of the intensity of incident light provided by the source. The diode can operate in two modes, photovoltaic mode or photoconductive mode. In a photovoltaic mode, the diode operates as a current source connected in parallel with forward-biased diode. The output voltage and current are directly proportional on the intensity of light and load impedance. While in a photoconductive mode, the diode is connected to the supply voltage and the amount of the current produced is dependent to the intensity of light. This type of optocoupler is considerably faster than a photo transistor type, although it has a low transmission ratio; thus an output amplifier circuit is usually included the package.
Either Dielectric waveguide or air can serves as the optical channel. An optical conductive shield can be implemented in this channel to achieved high noise immunity. In mounting these components on the circuit board, the transmitting and receiving components of an optical isolator can be enclosed within a single compact module; hence the module is often referred to as optoisolator.
Optocoupler | Optoisolator Application
Opto-isolators can provide good electrical isolation, help break ground loops and oppose voltage spikes
• Optocouplers are used in a switched-mode power supplies for isolation. Because in an environment that is exposed in electrical noise and utilized a very large signal, low transmission optocouplers are preferred.
• When electrical safety is the major concern, optocouplers can completely isolate circuitry from main electricity, which will give way for a safe troubleshooting.
• Optocouplers are employed in medical equipments.
• In MIDI (Musical Instrument Digital Interface) standard, one of the requirements is opto-isolation of all the input connections.
• Opocouplers are used in oscilloscope and digital multimeters when interfaced with computers.
• Another application of optocouplers is for the isolation of signal circuitry or low-current control from transient spikes caused by power supply and high-current control circuits (usually used in motor and machine control devices).
Opto-isolator parameters
If you open a datasheet for an opto-isolator in a separate browser window, we can go through some of the parameters and explain what they mean. Click here to open a datasheet for the Sharp PC123 in another window, because we will be referring to it.
Collector-emitter voltage – This is the maximum voltage in collector to emitter junction of the receiving component (e.g. phototransistor) before it may break down; when there is no presence of light, phototransistor is turned-off.
Creepage distance – It defines the physical distance of light or spark to travel around the package to get from the source to the destination. If solder flux, contaminants or dampness are present in the package, it will result to a lower resistance path for noise signal to travel along.
Forward current – This is the passing current through the source LED. Normally, an opto-isolator will require about 5mA of forward current to turn on the receiving transistor.
Forward voltage – This is the voltage needed to turn on the input transistor (e.g. LED). Typical diodes have a forward voltage of about 0.7 volt, but in LED it is normally 1 up to 2 volts.
Collector dark current – This is the amount of current that can pass through the receiving phototransistor when there is no presence of light (turned off).
Collector-emitter saturation voltage – This is the amount of voltage across collector and emitter junction of the receiving transistor when it is completely turned on (saturated).
Isolation resistance – This is the resistance between the input and the output side of the package; typically very high.
Response time – Rise time and fall time define the time of the output voltage to rise from zero to maximum. Rise time is a function of the load resistor, because it serves as the pull-up device in the output. Hence, this value is always referred with a fixed load resistance.
Cut-off frequency – This is actually the highest frequency of square wave that can be transmitted through the opto-isolator. It is also the frequency at which the output voltage is only swinging half the amplitude than at DC levels (-3dB = half). For that reason, it is usually associated with the rise and fall times.
Current Transfer Ratio (CTR) – This defines the ratio of how much collector current in the receiving transistor you can achieved given a particular amount of forward current in the source transistor LED.
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