Analysis of contact resistance of automobile connector terminals-2
Blog | Sep 16,2022
As the automobile connector terminal contact resistance analysis -1 said, the following is more factors on the contact electron influence and description.
Influence of terminal forward force
Terminal forward force refers to the force generated from and perpendicular to the contact surface of the male and female terminals. According to the analysis of the action principle of contact resistance, with the increase of the terminal forward force, the number and area of the actual contact points of the terminal contact interface also gradually increase, so the actual contact area increases with the increase of the forward force.
The additional resistance of the terminal contact interface is calculated by the following formula:
- R is the additional resistance of the contact interface between the male terminal and the female terminal;
- K is the calculation coefficient;
- P is the resistivity of the terminal material;
- H is the hardness of terminal material;
- F is the forward force of the terminal.
According to Equation (L), the additional resistance R of the contact interface between the male and female terminals is inversely proportional to the square of the positive force of the terminals. Therefore, the connector design can ensure the mechanical properties of the premise, by changing the terminal forward force, to reduce the terminal contact interface additional resistance, to ensure the stability of power transmission and signal transmission.
Influence of terminal contact interface surface state
The influence of the surface state on the contact resistance of the terminal contact interface is mainly manifested in the surface roughness of the contact interface and the influence of the surface oxidation film on the contact resistance. The roughness of the contact interface has a certain influence on the contact resistance, which is mainly because the different roughness will cause the different number of microscopic contact points of contact interface, and finally affect the size of contact resistance.
The surface oxide film of the contact interface is produced by the contact between the contact interface and the atmosphere. The chemical corrosion and electric corrosion in use will make the situation more serious. The thickness and state of the surface oxide film determine the resistance of the film layer, so the surface oxide film of the contact interface has a certain influence on the contact resistance.
Influence of using voltage
When the voltage drop on the terminal contact interface reaches a certain value, the oxide film on the terminal contact interface will be broken down, and the film resistance on the terminal contact interface will be reduced, thereby reducing the contact resistance of the terminal. However, due to the thermal effect under the voltage, the chemical reaction near the film layer is accelerated, which has a certain repair effect on the film layer, so the resistance of the film layer presents nonlinear changes.
The small fluctuation of the voltage drop near the breakdown voltage can cause the current to change in the range of dozens of times, so that the contact resistance has a great change. Understanding this nonlinear variation is helpful to correctly analyze the nonlinear variation of contact resistance and avoid errors during testing and use.
When the current of the terminal contact interface reaches a certain value, the joule heat generated at the contact point of the contact interface due to electricity makes the metal soften or melt, thus changing the number and area of the contact point, so that the actual contact area of the terminal contact interface increases substantially, and the contact resistance decreases substantially.
The chemical reaction caused by Joule heat can repair the membrane layer and increase the resistance of the membrane layer nonlinearly, thus increasing the contact resistance. According to practical experience, the joule heat generated by the current will generally reduce the contact resistance.
Test of contact resistance
The establishment of test model
The additional resistance of the connector terminal contact interface can not be measured directly, so it is necessary to establish a suitable test model to achieve the purpose of measurement. The figure above shows a test model of the additional resistance at the contact interface. T1 and T2 in the figure are welding induction points 75mm away from the conductor crimping center. The actual resistance measured is the resistance RC between the two welding induction points T1 and T2 and the resistance RP of the 150mm wire used.
The difference between the two measurements is defined as the contact resistance of the connector terminal as follows:
Where R is the contact resistance between the male and female terminals
RC is the total resistance measured by the test model
RP is the resistance of the 150mm used wire.
As can be seen above, the contact resistance tested by the model includes the additional resistance of the terminal contact interface, the conductor resistance of the terminal, and the conductor crimp resistance between the terminal and the wire.
Voltage drop converts contact resistance
The desired contact resistance L is obtained by testing the voltage drop between the wire crimp and the terminal contact area under nominal current conditions. The required resistence value can be calculated by measuring the voltage drop between the two welding induction points, T1 and T2, and based on the nominal current guaranteed by the adjustable power supply.
According to the analysis on the influence factors of contact resistance, additional resistance terminal contact interface under the effect of voltage and current present nonlinear variation, and splicing device used in automobile, many are in under the action of voltage and current, therefore, measured by this test contact resistance is close to plug in a strong current state of actual use. In particular, the voltage drop test after thermal aging, temperature and humidity cycle, current cycle and other tests can better reflect the actual use state.
Low level contact resistor
When the connector is used for signal transmission in the automobile, the voltage drop and current of the terminal contact interface area are often too low to affect the additional resistance of the contact interface. Therefore, the results measured by the voltage drop conversion contact resistance method cannot effectively reflect the actual contact resistance of the use situation.
The low level contact resistance measurement method uses a miniature OHMmeter to measure the resistance of the wire crimp and terminal contact areas. The required contact resistance can be calculated by measuring the resistance between the two welding induction points T1 and T2 and the resistance of the used wire of length L50mm using a miniature ohmmeter.
According to the working principle of ohmmeter, when using micro ohmmeter to measure resistance, the current in the circuit and the voltage drop of the measured area are far lower than the current and voltage drop needed to affect the additional resistance of the contact interface. Therefore, the contact resistance measured in this test can better reflect the actual use of the connector in the micro current circuit. Especially the low level contact resistance test after thermal aging, temperature and humidity cycle, current cycle and other tests can ensure that the contact interface film is not damaged by voltage drop and current, and can reflect the actual use state.
Contact resistance is the main electrical properties of automobile connectors. The influencing factors of contact resistance include terminal material, contact interface geometry, terminal forward force, terminal contact interface surface state, use voltage and current, etc. The test results of voltage drop conversion contact resistance and low level contact resistance reflect the actual use of the connector in strong current circuit and micro current circuit respectively.
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