Zero-Crossing Detector With Constant LED Current Isolation
Find Innovative SolutionsGenerate Solutions
Solution Overview
Problem
Existing zero-crossing detectors for high precision electrical test instruments suffer from significant errors in determining AC power mains zero crossings due to varying current through the light emitting diode, leading to inaccuracies exceeding 100 microseconds.
Innovation Solution
A zero-crossing detector utilizing a current source powered by the rectified AC input signal to maintain a constant current through the optoelectric coupler's light emitting diode, minimizing the time difference between true AC mains zero crossings and diode turn-off/turn-on voltages, thereby improving accuracy.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If a voltage divider with resistors is used to drive the light emitting diode, then the circuit is simple, but the detection accuracy deteriorates with errors exceeding 100 microseconds
Solution Approach 1:
The patent changes the electrical parameter driving the LED from a voltage-based system (voltage divider) to a current-based system (constant current source). This parameter change ensures that the LED current remains proportional to the AC input signal while maintaining constant operating conditions, thereby achieving accurate zero-crossing detection without excessive circuit complexity.
2Ease of operation
If the light emitting diode current varies continuously, then the circuit operation is simple, but the zero-crossing detection accuracy deteriorates
Solution Approach 1:
The patent implements a dynamic constant current source that automatically adjusts its output to maintain a constant LED current throughout the signal cycle. This dynamic control ensures that the LED turns on and off at the true zero-crossing points of the AC signal, improving measurement precision while maintaining ease of operation through automatic adjustment.
Solution Approach 2:
The constant current source incorporates feedback mechanisms to monitor and adjust the LED current, ensuring it remains constant and proportional to the input signal. This feedback control eliminates the accuracy errors caused by current variation while keeping the circuit operation simple and automatic.
3Reliability
If a rectifier is used to power the current source, then galvanic isolation is achieved, but the current stability deteriorates without constant current control
Solution Approach 1:
The patent changes the power delivery parameter from uncontrolled rectified voltage to a regulated constant current. This parameter change maintains galvanic isolation through the optoelectric coupler while ensuring current stability by actively controlling the LED drive current to remain constant despite variations in the rectified input signal.
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
The solution reduces the error in detecting zero crossings to less than 100 microseconds, providing a more accurate indication of AC mains zero crossings compared to prior art designs.
Implementation Method 1
the light emitting diode 22A of an optoelectric coupler 22. When the diode 22A is illuminated, the phototransistor 22B of the optoelectric coupler 22 is activated
Implementation Method 2
the light emitting diode 22A of an optoelectric coupler 22. When the diode 22A is illuminated, the phototransistor 22B of the optoelectric coupler 22 is activated
Data Source
AI summary
A zero-crossing detector includes a pair of input terminals, the terminals being adapted to receive an AC input signal; a rectifier, the rectifier rectifying the AC input signal; a current source, the current source being powered by the rectified AC input signal; and an optoelectric coupler having a coupler input and a coupler output, the coupler input being driven by the current source and the coupler output providing a zero-crossing signal. The zero-crossing signal is galvanicly isolated from the AC input signal.