Silicon Photodiode Flame Detection Circuit with Zero Bias Amplifier
Find Innovative SolutionsGenerate Solutions
Solution Overview
Problem
Existing flame detection systems in combustion appliances face challenges with cadmium sulfide (CdS) sensors being phased out due to RoHS restrictions, requiring a replacement that minimizes dark currents to avoid false alarms while maintaining sensitivity to low light levels and being cost-effective.
Innovation Solution
A circuit using a silicon photodiode connected to a differential amplifier with zero voltage drop, ensuring the photodiode operates in a non-reverse biased mode, reducing dark current and employing a low quiescent current amplifier to prevent false positives, with sensitivity adjustment and a two-wire connection for power and output signal transmission.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If CdS sensors are used for flame detection, then dark current is minimized and false alarms are avoided, but these sensors are being phased out due to RoHS restrictions and need replacement
Solution Approach 1:
The patent replaces the discontinued CdS sensor with a silicon photodiode, which is a modern, readily available component. The silicon photodiode achieves comparable or superior performance with lower dark current and broader spectral response, making it a suitable replacement that adheres to RoHS regulations while maintaining reliability in flame detection applications
Solution Approach 2:
The patent changes the operating parameters of the photodetector by using a silicon photodiode operated in photovoltaic mode (zero bias) rather than photoconductive mode. This parameter change reduces dark current significantly and eliminates the need for reverse bias voltage, thereby maintaining low false alarm rates while using an available modern component
2Measurement precision
If the photodiode is reverse biased to operate in photoconductive mode, then detection sensitivity is improved, but dark current increases causing false positives
Solution Approach 1:
Instead of applying reverse bias to improve sensitivity (conventional approach), the patent inverts the approach by applying zero bias (photovoltaic mode). This inversion eliminates dark current generation while maintaining sufficient detection sensitivity through the amplifier circuit, thereby resolving the contradiction between sensitivity and dark current
Solution Approach 2:
The patent employs an operational amplifier with feedback circuitry to amplify the small photocurrent signals from the photodiode. The feedback mechanism ensures stable operation and maintains detection sensitivity without requiring reverse bias on the photodiode, thus keeping dark current minimal while achieving adequate measurement precision
3Power
If an amplifier with high quiescent current is used, then signal amplification is stronger, but false positives increase due to high dark current threshold
Solution Approach 1:
The operational amplifier uses feedback resistors to set the gain while maintaining low quiescent current consumption. The feedback mechanism allows the amplifier to provide sufficient signal amplification for flame detection while keeping the operating current low, thereby preventing false positives that would occur with high quiescent current amplifiers
Solution Approach 2:
The patent changes the amplifier selection criteria to prioritize low quiescent current over maximum gain capability. By selecting an operational amplifier optimized for low power consumption with appropriate feedback network design, the system achieves adequate signal amplification while maintaining a low threshold that prevents false positives from thermal noise and leakage currents
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 effectively minimizes dark current, reduces false positives, and maintains sensitivity to low light levels, ensuring reliable flame detection while being compatible with existing systems and cost-effective, with a focus on minimizing zero-point errors and inhibiting leakage currents.
Implementation Method 1
a sensor (1) with a first and a second sensor terminal, configured to produce a signal offset between its terminals in response to receiving light
Data Source
Figure 1
Figure 2
Figure 3
AI summary
Flame detection for combustion appliances. A sensor configuration comprising a sensor (1) with sensor terminals, the at least a sensor (1) being configured to produce a signal offset when receiving at least 1.1 Lux, and to produce equal signals when receiving less than 1.1 Lux, a differential amplifier (2) produces a current at its output channel (3) in response to the signal offset applied to its input channels, a load member (6) dissipates a first amount of power as a function of the current produced at the output channel (3), the differential amplifier (2) draws a first load current from the supply terminals (7, 8) in response to the signal offset applied by the sensor (1) to its input channels, the differential amplifier (2) draws a second quiescent current from the supply terminals (7, 8) in response to equal signals applied to its input channels (-, +).