Contact resistance is the resistance that appears between conductors in contact with each other. Typically, contact resistance is required to be below 10-20 mΩ, though some switches may require it to be under 0.1-0.5 Ω. Variations in contact resistance can significantly affect sensitive circuits. Many great distributors like Heisener offer a wide range of components of Contact resistance to cater diverse application needs.
Contact resistance consists of several components:
Concentration Resistance: This is the resistance observed due to current constriction or concentration at the actual contact surface.
Film Layer Resistance: This arises from film layers and contaminants on the contact surfaces. It can include both solid film layers and loose impurity contamination.
Conductor Resistance: When measuring contact resistance, it also includes the resistance of the conductors and leads connected to the contact points. This conductor resistance depends on the conductivity of the metal and is characterized by its temperature coefficient.
In practical measurements of contact resistance, a milliohm meter designed based on the Kelvin bridge four-terminal method is commonly used. This device utilizes specialized clamps that attach to both ends of the contact points being tested. The total measured contact resistance, R, consists of three components:
R=Rc+Rf+Rp
where RC is the concentration resistance, Rf is the film layer resistance, and Rp is the conductor resistance. The purpose of contact resistance testing is to determine the resistance generated when current flows through the electrical contact points on the contact surfaces.
Working Principle of Contact Resistance
Contents
The working principle of contact resistance involves the complexity of the actual contact surface. Even with a smooth gold-plated layer, microscopic examination reveals surface protrusions of 5-10 micrometers. The actual contact area is often much smaller than the theoretical contact area because contact occurs at scattered points rather than across the entire surface. These contact points are formed where the interface film has been disrupted by contact pressure or heat, and they constitute about 5-10% of the actual contact area.
Furthermore, the actual contact surface includes portions where contact occurs through a contamination film at the interface. Metal surfaces have a tendency to form oxide films, and even clean metal surfaces quickly develop a few micrometers of oxide film once exposed to air. Even stable metals like gold form organic gas adsorption films on their surfaces. Additionally, dust and pollutants in the air contribute to the formation of a deposition film on the contact surface. Thus, from a microscopic perspective, any contact surface inevitably becomes a contaminated surface, which significantly impacts the actual contact resistance.
Influencing Factors
Factors affecting contact resistance include contact material, normal pressure, surface condition, operating voltage, and current.
Contact Material: Different materials have different standards for contact resistance. For instance, the resistance of copper alloy contacts should be less than 5 milliohms, while iron alloy contacts should be less than 15 milliohms.
Normal Pressure: This refers to the force perpendicular to the contact surface. Increasing normal pressure increases the number and area of contact points, thereby reducing contact resistance. Normal pressure is influenced by the geometry and material properties of the contact components.
Surface Condition: Contaminants and deposits on the contact surface can increase resistance. Common contaminants include dust, oil, and chemical adsorption films, all of which can reduce the contact area and increase resistance.
Operating Voltage: High voltage can break down surface films, lowering contact resistance, but it can also cause nonlinear resistance changes, which may lead to errors in testing and operation.
Current: Excessive current generates Joule heat, which can cause metal to soften or melt, thereby reducing contact resistance.
Measurement Methods
Measuring contact resistance accurately is crucial for ensuring the reliability of electrical connections. Here are common methods used for measuring contact resistance:
Four-Wire (Kelvin) Method
This method uses four separate wires to measure resistance, eliminating the impact of lead and contact resistances. Two wires supply the current, while the other two measure the voltage drop across the contact.
Procedure:
- Connect the current-carrying wires to the contact points.
- Attach the voltage-sensing wires to the same contact points but at different locations.
- Measure the voltage drop and current. The contact resistance R is calculated using Ohm’s law: 𝑅=𝑉/𝐼
Two-Wire Method
This simpler method uses two wires, one for current and one for voltage measurement. It is less accurate due to the inclusion of lead and contact resistances in the measurement.
Procedure:
- Connect one wire to supply current and the other to measure the voltage drop.
- Calculate the resistance with 𝑅=𝑉/𝐼
Micro-Ohmmeter Method
A micro-ohmmeter is a specialized instrument designed to measure very low resistances with high precision.
Procedure:
- Connect the micro-ohmmeter to the contact points.
- The device applies a known current and measures the voltage drop to calculate the resistance.
Bridge Method
This method uses a bridge circuit (e.g., Wheatstone bridge) to measure resistance by balancing two legs of a bridge circuit.
Procedure:
- Set up the bridge circuit with contact resistance in one leg and known resistors in the other legs.
- Adjust the known resistors until the bridge is balanced.
- Calculate the unknown resistance based on the known resistors and the bridge configuration.
Contact Resistance Testers
These are dedicated devices specifically designed to measure contact resistance quickly and accurately.
Procedure:
- Connect the tester to the contact points.
- The tester applies a current and measures the resulting voltage drop to calculate resistance.
Summary
Contact resistance refers to the electrical resistance measured at the point where conductors make contact. It is influenced by various factors including the materials used, contact pressure, surface conditions, voltage, and current. In practice, contact resistance is usually much higher than the theoretical value due to the small area of contact points, surface contamination, and the presence of oxidation films. When measuring contact resistance, it is important to consider these influencing factors to ensure accurate assessment of contact performance and to avoid electrical faults or performance degradation caused by poor contact.
