![]() Resistors R1 and R2 provide additional voltage control while limiting the input current. A Zener diode ensures that the optocoupler receives input current only beyond a certain voltage – this provides a crude threshold control. A full-bridge rectifier would rectify the AC voltage into DC. Traditionally, a circuit such the one shown in Figure 2 was used to detect an AC voltage. Consequently, the detection of the absence or presence of these line and battery voltages, their exact levels, or the status of relays connected to them is an important function in a wide variety of applications, ranging from industrial automation to protection relays. 110V and 220V DC batteries are used as auxiliary power supplies in power distribution and power-station automation. 110V, 220V and 240V DC and AC voltage detectionġ10V AC and 240V AC are popular line voltages for domestic and industrial use. In this article, I’ll discuss a few voltage detection applications where isolated digital input solutions can bring distinct performance benefits over optocouplers. Table 1: Comparison of optocoupler with isolated digital inputs Integrated comparator with Schmitt trigger No “field-side” power needed – powered by input signal Table 1 summarizes the performance benefits of optocouplers and isolated digital inputs in the context of isolated voltage detection. They may also derive power from the signals they are measuring obviating the need for a “field-side” power supply. In doing so, they incorporate signal conditioning features such as a precise high voltage comparator, and an input current limit. Isolated digital inputs are different from digital isolators in that they directly interface to the real world signals they are isolating. Digital isolators have CMOS or TTL inputs and outputs, typically 5-V or 3.3-V. Examples include interfacing an ADC or a DAC to a microcontroller in an isolated analog input or output module, or isolating an RS-485 or CAN bus. ![]() isolated digital inputsĭigital isolators are popularly used as an alternative to optocouplers, in applications where they fit in the middle of the signal path. Figure 1: Traditional optocoupler-based (a) and new isolated digital input-based (b) approaches to voltage detection Digital isolators vs. ![]() These solutions not only overcome the limitations of optocouplers, but also bring performance benefits in 9V-to-300V isolated voltage detection. New isolated digital input solutions (See ISO1211, ISO1212) are now available that emulate the optocoupler in drawing power from the voltage being sensed, while also incorporating a high-voltage comparator and a precise current limit. Systems engineers have had to work around these limitations using external circuit components or by relaxing product specifications. However, optocouplers provide only basic functionality and are limited by high propagation delays, a large current draw, poor reliability and an inability to operate at high ambient temperatures. New isolated digital inputs for voltage detectionįor many years, voltage detection applications have used optocouplers or photocouplers for isolation, as shown in Figure 1a. Isolation manages ground-potential differences provides noise immunity and protects against high voltages. The voltage levels detected range from 9V to 300V. Examples of usage include monitoring power supply and battery levels and receiving status signals from sensors, switches and relays. Isolated voltage detection is used in many industrial applications, including protection relays, power station automation, high-voltage battery management, telecom power supplies, home automation, programmable logic controller (PLC) digital-input modules, motor control and computer numerical control (CNC) machines. Isolation is a means of preventing DC and unwanted AC currents between two parts of a system while allowing signal and power transfer between those two parts.
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