H11B255SD

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Optoisolators are external electrical components used to electrically isolate two separate circuits. The most common type of optoisolator is the Transistor, Photovoltaic Output (H11B255SD). This component can be used to convert a voltage or current in one circuit into a voltage or current of a similar magnitude, respectively, in the other isolated circuit, without any electrical connection between the two circuits. The H11B255SD, being a transistor photovoltaic output optoisolator, takes advantage of the photovoltaic effect of a semiconductor to provide the isolating element necessary to achieve electrical isolation while also performing the desired signal conversion.

The ideal schematic diagram for the H11B255SD, is shown in Figure 1. The optoisolator uses a pair of FETs, T1 and T2, connected in a push-pull configuration with an LED mounted between their gates. The LED is the photovoltaic element of the optoisolator, and is the isolating element between the two circuits. An input voltage applied to terminal pins A1 and A2 causes the LED to be forward-biased, and the LED begins to conduct current. The current flowing through the LED then causes an alternative current (AC) voltage to appear across the gates of the FETs, which in turn causes both FETs to be switched on. This creates a voltage path between terminals B1 and B2, which is electrically isolated from terminals A1 and A2.

In many applications, the photovoltaic effect of the LED will need to be strengthened using a small photoresistor and associated circuit components. By attaching the photoresistor to the cathode of the LED (the point where the LED is connected to ground), the photoresistor will act as a load, increasing the current flowing through the LED, and thus increasing the AC voltage across the FETs. This increases the current gain of the circuit, meaning more current will be available for the output circuit when it is switched on.

The key functional features of the H11B255SD optoisolator can be summarised as follows:

• One of the few optoisolators designed for linear operation.
• Incorporates a novel transistor-inside-LED structure, allowing less electrical noise in its operation.
• Uses an LED for isolation, resulting in smaller footprint.
• Offers greater speed compared to optocouplers with DC drivers.
• Requires only low-level input current (about 10 µA)
• High output current (up to 40 mA)
• Very low working voltage (0.2 V)

The H11B255SD optoisolator is an ideal solution for applications that require electrically isolated signals with low-level input current. It is widely used in circuits where reliability and electrical noise must be kept to a minimum, such as motor controllers, sensor systems and industrial control systems. The H11B255SD optoisolator is also used in critical medical applications, such as defibrillators and oxygen breathing apparatus, which require the highest level of signal isolation.

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