Galvanic Isolation Circuit for Programmable Logic Controllers
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Solution Overview
Problem
Existing Programmable Logic Controllers (PLCs) face challenges in providing galvanic isolation between high voltage and low voltage sections efficiently, leading to increased power consumption, cost, and space requirements due to the need for multiple power supplies for optical isolators, which limits their applicability across a range of input voltages.
Innovation Solution
A digital input detection circuit with a galvanically isolated section incorporating an integrating capacitor, a relaxation oscillator, and an electronic switch, coupled with an optical isolator, where power is derived from the high voltage input, eliminating the need for separate power supplies and allowing operation across a broad range of voltages with minimal current draw.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If optical isolators with PWM are used to provide galvanic isolation, then galvanic isolation between high voltage and low voltage sections is achieved, but power consumption increases due to the need for separate power supplies
Solution Approach 1:
The optical isolator is powered by the high voltage input signal itself through a resistive divider network, eliminating the need for separate power supplies. The high voltage signal directly provides the power needed to drive the LED in the optical isolator, making the isolation circuit self-powered and reducing overall power consumption.
Solution Approach 2:
The high voltage input signal serves dual functions: it provides both the power to drive the optical isolator and the signal to be isolated. This multi-functionality eliminates the need for dedicated power supply circuits, reducing component count and power consumption while maintaining galvanic isolation.
2Reliability
If multiple power supplies are provided for each input channel to maintain galvanic isolation, then galvanic isolation is maintained, but device complexity and board space increase
Solution Approach 1:
Multiple input channels share a common low voltage power supply and common control circuitry while maintaining galvanic isolation through individual optical isolators for each channel. This merging of common resources reduces overall circuit complexity and board space while preserving isolation integrity.
Solution Approach 2:
Each input channel is segmented into isolated high voltage and low voltage sections connected through individual optical isolators, allowing independent processing while sharing common resources. This segmentation maintains isolation where needed while enabling resource sharing to reduce complexity.
3Ease of manufacture
If simple resistive divider is used to power optical isolator, then galvanic isolation is provided inexpensively, but the circuit can only work at known voltage levels and cannot be universal
Solution Approach 1:
The resistive divider network uses variable resistors or digitally controlled resistors that can dynamically adjust their resistance values based on the input voltage level. This dynamic adjustment allows the circuit to adapt to different voltage ranges while maintaining proper power levels for the optical isolator, providing both low cost and versatility.
Solution Approach 2:
The circuit detects the input voltage level and changes the resistance parameters of the divider network accordingly. By adjusting the resistance values based on the detected voltage, the circuit can operate universally across different voltage ranges while still providing cost-effective galvanic isolation through simple resistive components.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This solution reduces circuitry complexity, board space, and power dissipation while enabling galvanic isolation across a wide voltage range with minimal current consumption, providing a self-powered analog-to-digital conversion and approximate voltage measurement.
Implementation Method 1
an optical isolator has an input side and an output side, wherein the input side is coupled across the integrating capacitor by the switch
Implementation Method 2
a relaxation oscillator coupled across the integrating capacitor
Data Source
AI summary
A digital input includes a galvanically isolated section having an integrating capacitor coupled to a high voltage signal input by at least one current-limiting resistor, a relaxation oscillator coupled across the integrating capacitor, and an electronic switch controlled by the relaxation oscillator. An optical isolator has an input side and an output side, wherein the input side is coupled across the integrating capacitor by the switch, and a low voltage section includes a decoder having an input coupled to the output side of the optical isolator and having a low voltage signal output.