What types of photocouplers are available?

A photocoupler is a product consisting of an LED at the input side and a photodetector at the output side combined together inside. A photocoupler product varies depending on the type of photodetector at the output side. Its main varieties include a transistor-output photocoupler, IC-output photocoupler, triac-output photocoupler, and MOSFET-output photocoupler (called a "photorelay"). In addition, the functions of photocouplers include signal isolation, feedback detection, pre-driver, and isolation switch functions.

1. Transistor-output photocoupler/transistor coupler

When a photocoupler first appeared on the market, only this type was available. Owing to its low cost and high general versatility, this type is still used most in the market and its range of applications is wide. Some products are provided with such features as high transfer ratio, high breakdown voltage, and low-input drive. Function-wise, transistor-output photocouplers are widely used for signal isolation, feedback detection, and isolation switch.

2. IC-output photocoupler/IC coupler

An IC-output photocoupler is a photocoupler into which photodetector devices are integrated in order to realize high speed and incorporation of functions. While a transistor-output type photocoupler can transmit at least several kHz to several tens of kHz signals, this photocoupler can transmit approx. 1 to 50MHz high-speed signals. Roughly speaking, there are three product groups: 1) Group of general-purpose products for logic signal transmission, 2) Group of products with functionality as a pre-driver for a power device (IGBT), and 3) Group of products with current/voltage feedback functionality.

3. Triac-output photocoupler/triac coupler

This type of photocoupler is frequently used as an isolation switch for controlling an AC load, such as a motor and a solenoid, directly connected to a 100Vrms or 200Vrms commercial power supply used in such a place as a home, office, and factory. Using a triac with higher breakdown voltage, while applying electric isolation, it can perform ON/OFF control with a low current of approx. a dozen mA. Only an approx. 100mA AC current  can be controlled by a device alone. However, by using this photocoupler as a pre-driver for the main triac, an AC current up to approx. several amps can be controlled.

4. MOSFET-output photocoupler/photorelay

This photocoupler is a device provided with two MOSFETs connected common-source at the output stage and also provided with functionality equal to that of a mechanical relay or lead relay. Since the MOSFETs have linear output characteristics in their voltage-current characteristics, this device is distinctive in that it can not only function as a simple switch, but also switch analog signals.