Solid-State Anemometer Using Transistor Junction Cooling
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Solution Overview
Problem
Conventional anemometers, such as hot-wires and thermocouples, are expensive, fragile, require frequent cleaning, and need calibration, while existing solid-state solutions face challenges in precision and responsiveness to air velocity changes.
Innovation Solution
A solid-state anemometer using a bipolar transistor with a metal pad thermally coupled to a cylindrical metal rod, where the transistor's base-emitter junction temperature is controlled to measure air flow velocity by analyzing the exponential decay of the junction temperature after heating, with calculations processed on a printed circuit board without the need for calibration.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If hot-wire or thermocouple anemometers are used, then measurement capability is achieved, but the device becomes expensive, fragile, and requires frequent cleaning and calibration
Solution Approach 1:
The patent replaces mechanical anemometer components (hot-wires, thermocouples) with a solid-state bipolar transistor mounted on a printed circuit board. The transistor's base-emitter junction serves as the temperature-sensitive element, eliminating fragile mechanical parts while maintaining measurement functionality through electrical characterization.
Solution Approach 2:
The patent changes the operational parameters by using the bipolar transistor's base-emitter voltage drop as a function of temperature and current. By controlling and measuring these electrical parameters during heating and cooling cycles, the system derives air velocity without requiring mechanical calibration components.
2Ease of manufacture
If conventional solid-state solutions are used, then manufacturing simplicity is improved, but measurement precision and responsiveness to air velocity changes deteriorate
Solution Approach 1:
The patent employs periodic heating and cooling cycles of the bipolar transistor junction. By applying controlled current pulses to heat the junction and then measuring the cooling rate at different air velocities, the system captures dynamic thermal response data that correlates to air flow conditions, enhancing measurement precision through time-resolved characterization.
Solution Approach 2:
The system uses feedback by continuously monitoring the base-emitter voltage drop during cooling and comparing it against reference values. This feedback mechanism allows the microprocessor to calculate air velocity based on the measured cooling rate, ensuring accurate and responsive measurements while maintaining solid-state simplicity.
3Reliability
If the transistor is thermally insulated by its plastic package, then device protection is improved, but response time to air velocity changes becomes slow
Solution Approach 1:
The patent applies preliminary heating to the bipolar transistor junction before measurement, raising the junction temperature significantly above ambient. This creates a large temperature differential that accelerates heat transfer to the surrounding air, thereby speeding up the cooling response time while the plastic package continues to provide mechanical protection.
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 provides a robust, cost-effective, and consistent measurement of air flow velocity without calibration, with the thermal time constant derived from temperature decay curves accurately correlating to air velocity, enabling reliable monitoring and warning signals for adequate air circulation.
Implementation Method 1
Power is applied to the transistor, creating a significant collector-emitter current, which raises the base-emitter junction temperature
Implementation Method 2
the cooling rate is an exponential decay determined by the fixed thermal capacitance of the package/rod and the variable thermal resistance between the junction and the air
Implementation Method 3
A metal rod is affixed to the thermal pad by solder or other thermally conductive adhesive
Data Source
AI summary
An anemometer and method for analyzing fluid flow is described. In one embodiment, a transistor sensor is heated by applying power to cause its base-emitter junction to rise from an ambient first temperature to a second temperature. The power is removed, and the Vbe is measured at intervals as the junction cools. The Vbe equates to a temperature of the junction. The temperature exponentially decreases, and the time constant of the decay corresponds to the fluid flow velocity. A best fit curve analysis is performed on the temperature decay curve, and the time constant of the exponential decay is derived by a data processor. A transfer function correlates the time constant to the fluid flow velocity. The transistor is thermally coupled to a metal rod heat sink extending from the package, and the characteristics of the rod are controlled to adjust the performance of the anemometer.