Wetting Current Diagnostic Circuit for Oxidized Load Switches
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Solution Overview
Problem
Sensed switches in complex electrical systems, such as those in automobiles, face issues due to switch contact oxidation, which can lead to incorrect operation and faults in the wiring harness, preventing the delivery of a wetting current necessary to remove oxidation, thereby affecting the reliability of load control switches.
Innovation Solution
A diagnostic testing circuit and method are implemented to detect and differentiate between various fault conditions, including wetting current faults, by sensing current along the path to the load control switch and correlating it with voltage measurements, ensuring the wetting current is properly applied when the switch is closed, and diagnosing faults like leakage and system offset.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If a sensed switch is used to control loads, then wiring harness complexity and costs are reduced, but switch contact oxidation increases resistance and causes incorrect operation
Solution Approach 1:
The system applies wetting current to the switch contacts in advance and periodically to prevent oxidation buildup before it causes malfunction. This preliminary maintenance action ensures contacts remain conductive without requiring higher operational currents or more complex wiring.
Solution Approach 2:
The control circuit monitors the current path and provides feedback to determine whether wetting current should be applied. This feedback mechanism detects oxidation-related resistance changes and triggers corrective wetting current application to maintain reliable switch operation.
2Reliability
If high current is used in powered switches, then oxidation is burned off and reliability is improved, but wiring harness complexity and costs increase
Solution Approach 1:
Instead of continuously high current, the system uses periodic wetting current pulses to clean oxidation from contacts. This periodic maintenance approach achieves the same oxidation removal effect as continuous high current but with much lower average current and simpler wiring.
Solution Approach 2:
The system changes the current parameter dynamically - using low current for normal operation and temporarily increasing to wetting current levels only when oxidation removal is needed. This parameter modulation maintains reliability without requiring permanently high current capacity in the wiring harness.
3Reliability
If wetting current is applied to remove oxidation, then switch reliability is improved, but faults in the wiring harness may prevent wetting current delivery
Solution Approach 1:
The control circuit incorporates feedback monitoring of the current path to detect faults that prevent wetting current delivery. This feedback enables real-time detection of wiring harness problems and triggers appropriate fault indicators to alert operators to maintenance needs.
Solution Approach 2:
The system performs self-diagnosis by monitoring its own current path and detecting faults automatically. This self-service capability identifies wiring harness problems without requiring external diagnostic equipment, enabling timely maintenance and preventing switch failure.
4Ease of operation
If voltage comparison is used to determine switch state, then operation is simple, but oxidation increases sensed voltage and causes incorrect operation
Solution Approach 1:
The system introduces wetting current as an intermediary that actively maintains low contact resistance. This intermediary action ensures the voltage comparison method remains accurate by preventing oxidation buildup that would otherwise corrupt the voltage signal and cause incorrect switch state detection.
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 enhances the reliability of sensed switches by ensuring the wetting current is effectively delivered to remove oxidation, reducing the risk of incorrect operation and extending the lifespan of load control switches, while also identifying and addressing faults in the electrical system.
Implementation Method 1
The temporary current level is sufficient to remove the oxidation
Implementation Method 2
A current sensor connected in series with the current source along the path to the load control switch generates a current sensor signal indicative of a current level along the path
Data Source
AI summary
A circuit for diagnostic testing includes a current source coupled to a power source and configured to provide wetting current along a path to a load control switch, a current sensor connected in series with the current source along the path, the current sensor being configured to generate a current sensor signal indicative of a current level along the path, a voltage measurement unit having an input terminal coupled to a node along the path through which the wetting current flows to reach the load control switch, the voltage measurement unit being configured to detect a state of the load control switch based on a voltage at the node, and a controller coupled to the current sensor and the voltage measurement unit, the controller being configured to determine a wetting current diagnostic condition in accordance with the current level and the detected state.