The description of contact resistance management Essay

CHAPTER 7
CONTACT RESISTANCE MEASUREMENT MICROOHMMETER
7.1. Definition
The contact resistance is defined as the ratio the voltage across the contact to the current flowed through a closed pair of contacts. It concurred with Ohm's law.
In Fig. 7.1, there is an interface between the metal 1 and metal 2. The current, I, originating from the current source moves through this interface, can be perused from a current meter. And then the voltage drop across the interface can be read from a voltage meter as, U. Then the contact resistance value, RX, can be calculated by (1).
Fig 7.1 The definition of Contact Resistance (RX)
7.2. Methods of measuring Contact Resistance
The Fourterminal is typical normal estimation technique that is constantly applied in measuring lowlever resistance &contact resistance to keep the lead resistance from being included into the measurement.
1) The direct method for nonloop circuit and structure
The nonloop structure means there are no others contact point but the place under measuring. The circuit is open. DMM or microOhm meter can read out the contact resistance value directly, as show in Fig. 7.2.
Fig 7.2 Directly measuring with resistance meter.
2) The indirect method for loop circuit and structure
The simple loop structures are shown in Fig. 7.3(a). We can choose three points on the loop structure the point 1 and 2 areclose to the contact, point 3 is at discretion. The resistance R12across point 1 and 2 is just the contact resistance. The measurement procedure has three steps.
a. Measure the resistance R12 at the point 1 and 2, the value recorded as A
b. Measure the resistance R23 at the point 2 and 3, the value recorded as B
c. Measure the resistance R31 at the point 3 and 1, the value recorded as C.
Fig 7.3 Indirect method for measuring contact resistance.
The equivalent circuit is show in Fig. 7.3(b). According to seriesparallel circuit, the values A, B, C can be expressed as the coupled equations (4)
The solutions of the coupled equations (4) are the final result. The transformation of Yresistance networks will help us to solve the problems. The transformation circuit is show in Fig. 7.3(c). And the transformation formulas are expressed as the coupled equations (5).
According to the Yform resistance network, in Fig. 7.3(c), the values should be measured as A=R1+R2 B=R2+R3 C=R3+R1 .The expressions of R1, R2, and R3
Combine the (6) into (5), we can get the final result as (7)
If there is too much difference of the values between A, B and C, the errors caused by instrument resolution and calculation precision would badly affect the result, so