Heating Appliance Load Control Circuit for UL-197 Power Averaging
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Solution Overview
Problem
Existing electric heating appliances face challenges in precisely controlling the input averaging current to comply with UL-197 section 48, which requires maintaining the average operating power below 80% of the plug-rated current over a three-hour period, as existing temperature-limiting circuits provide only approximate values.
Innovation Solution
An input averaging control circuit using timers is implemented to cooperate with heating timer circuits, ensuring the input averaging current remains below 80% of the plug-rated current by directly measuring electrical values and managing power distribution through programmable RC oscillator timers and solenoid control mechanisms.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If temperature-limiting circuits are adapted to limit Iavg, then the heating appliance can prevent excessive heating, but the measurement precision of input averaging current is insufficient as they provide only approximate values
Solution Approach 1:
The patent replaces the indirect temperature-based limiting approach with direct electrical measurement using an RMS-to-DC converter circuit. This electronic substitution provides precise Iavg measurement by directly converting RMS current signals to DC proportional signals, eliminating the approximation errors inherent in temperature-correlation methods.
Solution Approach 2:
The patent introduces an RMS-to-DC converter as an intermediary device that bridges the gap between AC current measurement and DC control circuitry. This converter circuit (comprising operational amplifiers, resistors, and capacitors) serves as a mediator that accurately transforms current measurements into controllable DC signals for the timing circuit.
2Measurement precision
If direct electrical measurements are used to determine Iavg, then measurement precision is improved, but device complexity increases due to additional control circuits
Solution Approach 1:
The patent designs the RMS-to-DC converter and timing circuit to serve multiple functions: measuring Iavg, generating control signals for heating elements, and providing compliance verification for UL-197 standards. This multi-functionality reduces the need for separate dedicated circuits for each task, thereby limiting the increase in overall device complexity.
Solution Approach 2:
The patent transforms the control approach by changing from temperature-based indirect control to electrical parameter-based direct control. By monitoring and controlling current parameters directly through the RMS-to-DC converter and timing circuit, the system achieves precise Iavg measurement and control without requiring complex temperature sensing and correlation algorithms.
3Reliability
If the input averaging current is strictly controlled to comply with UL-197 section 48, then safety standards are met, but the productivity of the heating appliance is reduced due to power limitations
Solution Approach 1:
The patent implements periodic duty cycling of heating elements controlled by the timing circuit. The system alternates between active heating periods and rest periods, ensuring that the average power delivery complies with UL-197 Iavg requirements while still providing effective heating during active cycles. This periodic operation maintains safety compliance without continuously limiting productivity.
Solution Approach 2:
The patent employs dynamic control of heating element operation through the timing circuit, which adjusts the duration and timing of heating cycles based on real-time Iavg measurements. This dynamic adjustment allows the system to maximize productivity within the constraints of UL-197 compliance, rather than using static power limiting that would continuously reduce output.
Data Source
AI summary
An appliance includes a plurality of electrical loads and a load control circuit including at least a clock enable circuit and a full power timer. The clock enable circuit is configured and operatively connected to activate the full power timer when all of the loads are energized, and the full power timer is configured and operatively connected to de-energize at least one load selected from the plurality of loads on reaching a predetermined full power time limit.