Thermostat Solid-State Switching with Overcurrent Shutdown
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Solution Overview
Problem
Incorrect wire connections during HVAC system installation or upgrades can lead to short circuits, potentially causing permanent damage to thermostats and other components, and the use of fuses to prevent this results in blown fuses which are problematic for user-installable thermostats.
Innovation Solution
A thermostat with solid-state switching elements and power monitoring and control circuitry that detects anomalies, dynamically adjusts resistance, and stores indications of anomalies to prevent damage, while also integrating a zero-crossing detection circuit to safely switch off the elements.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If fuses are used to protect against short circuits, then protection against damage is improved, but user satisfaction deteriorates due to blown fuses requiring replacement
Solution Approach 1:
The patent replaces mechanical fuses with solid-state switching elements (transistors) that can electronically detect and respond to short circuit conditions. The solid-state switch monitors current flow and automatically opens the circuit when abnormal current is detected, eliminating the need for replaceable fuses while maintaining protection functionality.
Solution Approach 2:
The solid-state switching element incorporates built-in protection circuitry that automatically detects short circuit conditions and opens the circuit without user intervention. The system self-diagnoses abnormal current flow and self-protects by interrupting the circuit, eliminating the need for users to replace fuses or perform maintenance.
2Ease of operation
If solid-state switching elements are used instead of fuses, then ease of operation is improved by eliminating blown fuses, but device complexity increases due to additional monitoring circuitry
Solution Approach 1:
The patent combines the switching function and the protection function into a single integrated solid-state switching element. The transistor serves both as the circuit switch and as the current monitoring device, with its inherent electrical characteristics providing the means to detect abnormal current flow. This merging eliminates the need for separate monitoring circuits and reduces overall system complexity.
Solution Approach 2:
The solid-state switching element performs multiple functions: it acts as the primary circuit switch, serves as the current monitoring sensor, and functions as the protection mechanism. This multi-functionality reduces the number of components needed and simplifies the overall system architecture compared to using separate fuses and monitoring circuits.
3Reliability
If the thermostat continuously monitors electrical signals to detect anomalies, then protection against damage is improved, but energy consumption increases
Solution Approach 1:
The monitoring of electrical signals is continuous because it leverages the natural current flow through the solid-state switching element during normal operation. The same current that performs the useful work of actuating HVAC equipment also provides the signal for anomaly detection, eliminating the need for separate monitoring power consumption.
Solution Approach 2:
The patent converts the harmful effect of high current flow (which indicates a short circuit) into a useful detection signal. The abnormal current that would cause damage is detected by monitoring the voltage drop across the solid-state switch, transforming a harmful condition into a detectable signal that triggers protection before damage occurs.
Data Source
AI summary
A thermostat may include HVAC wire connectors that receive a call relay wire and a power return wire, switching elements having a first operating state in which they create a connection between the call relay wire and the power return wire and a second operating state in which the connection between the call relay wire and the power return wire is interrupted. The thermostat may also include control circuitry configured to cause the switching elements to operate in the first operating state to actuate an HVAC function; detect an anomaly associated with measurements from power monitoring circuitry; cause the switching elements to continue to operate in the first operating state until a zero-crossing is detected; and then cause the one or more switching elements to operate in the second operating state.