Emergency Lighting Battery Current Sensing With Switched Resistor
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Solution Overview
Problem
Existing emergency lighting devices face challenges in accurately sensing battery discharge current with minimal power loss and cost, as traditional methods using sense resistors and amplifiers incur high costs and thermal strain, while closed-loop current sensors are complex and costly due to the need for additional components.
Innovation Solution
An emergency lighting device that selectively switches between closed-loop and open-loop modes of operation based on duty cycles and battery voltage, using a control circuitry to manage the discharge current, which includes a sensing resistor shunted by a switch to reduce power consumption and eliminate the need for amplifiers.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If a resistor with enough accuracy is used to sense battery discharge current, then measurement precision is improved, but power loss increases significantly
Solution Approach 1:
The patent applies periodic action by switching the sense resistor on and off at a high frequency (e.g., 1 MHz). The resistor is only active during a small duty cycle (e.g., 1-10% of each period), allowing accurate current measurement during the on-period while minimizing power dissipation during the off-period. The control circuitry integrates these periodic measurements to determine the average discharge current, achieving both accuracy and low power loss.
2Measurement precision
If an amplification stage is added to achieve reasonable accuracy with a small sense resistor, then measurement precision is improved, but device complexity and cost increase
Solution Approach 1:
The patent uses feedback by measuring the voltage across the sense resistor during its active period and using this measurement to control the switching duty cycle. The control circuitry continuously adjusts the switching parameters based on the measured current, enabling accurate current sensing without requiring external amplifiers. The feedback loop integrates the periodic measurements to provide accurate average current information to the control system.
3Measurement precision
If a closed-loop current sensor is used to eliminate sensitivity error, then measurement precision is improved, but device complexity, size, and power consumption increase
Solution Approach 1:
The patent extracts only the essential function of current sensing from complex closed-loop sensors. By using a simple resistive sense element switched periodically, it eliminates unnecessary components such as ferromagnetic cores, coils, and high-power amplifiers found in closed-loop Hall-effect sensors. The invention achieves adequate accuracy for battery management applications using only the essential voltage measurement function, removing extraneous complexity.
Solution Approach 2:
The patent employs a simple, inexpensive sense resistor that is switched periodically rather than using expensive, complex closed-loop sensors. The resistor serves its purpose during brief active periods and can be a low-cost component, eliminating the need for expensive precision resistors, amplifiers, and sensor ICs required by traditional accurate sensing methods.
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 approach achieves low-power, accurate current sensing with reduced costs and size, improving efficiency and allowing for a smaller, cheaper battery pack by minimizing the use of amplifiers and optimizing resistor usage.
Implementation Method 1
a resistor with enough accuracy will dissipate significant power
Implementation Method 2
a sense resistor Rs for the battery current
Data Source
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AI summary
The invention relates to a emergency lighting device (200), comprising a battery (201) designed for producing a discharge current to be fed to terminals for emergency light means, and a control circuitry (203). The control circuitry (203) controls, in an emergency mode, in at least one first time period of the discharge, the battery discharge current selectively in closed loop mode using a discharge current feedback signal, and open loop mode in at least one second time period of the discharge.