Low Level Circuit Testing in Modern Appliances
Many years ago the connectors for appliances and the appliances themselves were relatively simple as compared to today’s machines. The electrical circuits were 110 volts with relatively high current. The machines were generally built with electromechanical devices and point-to-point wired with simple quick connect terminals individually hand installed by assembly line operators. The wire installation operation was quite often used to balance labor on the assembly line. If an assembly operator installed a component and had a few spare seconds, they would attach a pre-terminated wire. The connector materials were usually plain brass or tin plated brass for normal temperature connections and steel for high temperature connections. There were no electronics. Most interconnection failures were caused by wires falling off of the components due to improper installation.
Today’s high efficiency and often internet compatible appliances are sophisticated with many solid state low-level circuits along with the high current requirements. The sensors and controls require connectors that have long-term low and stable resistance interfaces for reliable use in the appliance and its use environment.
High normal force tin plated connectors are still used where mating forces are manageable. Generally these are low pin-count connectors. However, high pin-count connectors frequently use gold plated low normal force contacts with low mating forces for attachment to each other and printed circuit boards.
The low level circuits require test plans designed to assess the reliability of the electrical connections in those circuits.
Low Level Contact Resistance (LLCR) measurements are an integral part of any test plan designed to assess the reliability of electric contacts. The measurement conditions are constrained such that the measurement itself is unable to modify the contact interface or any contamination films produced as a result of environmental stresses that the tested component was subjected to will not be disrupted. Any degradation that had been produced will be immediately obvious. The term “dry circuit” is synonymous with those conditions required for low level contact resistance measurements.
Measurement conditions required for low level contact resistance measurements are constrained such that neither the applied voltage nor current will modify (change) the contact interface. The general consensus is that the voltage appearing across the contact must never exceed 20 mv and that the current itself be limited to a value of less than 100ma. The 20 mv value is nominal for most LLCR specifications, however while a maximum current of 100 ma is allowed, the actual current during the measurement itself may be much less—often in the low ma range. As the current becomes less and less, accuracy becomes much more difficult.
With measurement conditions limiting current to low ma levels, the voltage produced across the component under test could be extremely low, perhaps in the low microvolt range. Certainly, LLCR measurements will require very sensitive instrumentation—voltage resolutions of fractions of a microvolt could be necessary.
The most common source of error in low level voltage measurements is that of thermoelectric origin. Thermoelectric voltages (thermoelectric EMFs) are generated when different parts of a circuit are at different temperatures and when conductors of dissimilar metals are joined together. Under certain conditions, these thermal EMFs could seriously affect the accuracy of low level contact resistance measurements. Thermal EMF levels could be greater than the IR drop across the contact itself. We will discuss two measuring techniques that may be utilized to overcome unwanted thermal EMF offsets. These procedures are the current-reversal method and the offset-compensated ohms method.
In this discussion it will be assumed that the contact tested will be properly configured for the measurement. The basic 4-terminal configuration should be implemented such that any contribution of lead/bulk resistance will be minimized. The following derivations will be such that those parameters that need to be controlled and measured include the current and the voltage developed across the sample. It must be emphasized that the polarity of the measured voltages (+ or -) must be noted and incorporated in the calculation. Hopefully, these derivations will illustrate the rational steps necessary to eliminate errors caused by thermal EMFs. While not explicitly mentioned, these steps will also eliminate any errors caused by meter offsets.
Source: Getty Images
The two methods which are about to be described depend either on a complete temperature stabilization or a measurement cycling speed such the temperature remains essentially constant throughout the measurement. This assures that the thermal error is constant and is eliminated as the two measurement voltages are subtracted from one another, in the manner prescribed.
A. Current-Reversal Method
This approach consists of making two voltage measurements, the second with the current reversed. A sensitive voltmeter in conjunction with an appropriate bipolar current source may be utilized.
With positive current applied, the measured voltage is:
V(I+) = V(Thermal) + IR
Upon reversing the current, the measured voltage is:
V(I-) = V(Thermal) - IR
The thermal voltage error will be eliminated if we subtract V(I-) from V(I+), in which case we determine:
V(I+) - V(I-) = 2IR
The sign (+ or -) of the measured voltages must be preserved in the calculation. Finally, and since the resistance must be positive we can state that
R = ABS ((V(I+) - V(I-))/2I)
Where ABS is the absolute value of the terms in parentheses.
B. Offset-Compensated Ohms Method
This method is somewhat similar to the reversed current technique in that two voltage measurements are necessary. However in this case the measurement is alternated between a fixed current and zero current.
When the current source is on, the total voltage measured will be the drop across the contact as well as any thermal EMFs and may be written as follows:
V(I on) = V(thermal EMF) + IR
The second measurement is made with the current off and will be as follows:
V(I off) = V(thermal EMF)
Subtracting we find:
V(I on) - V(I off) = IR,
Again, the sign of the measured voltages must be retained for the calculation,
Thus we obtain:
R = ABS((V(I on) - V(I off))/I)
We hope that this discussion has provided a better understanding of how offset errors may be eliminated in low level resistance measurements. Since change in resistance is taken as a prime factor in determining acceptable performance, accurate measurements are mandatory. This discussion did not include specific instrument control/ software solutions. A number of integrated instruments are available that have the capability of performing the necessary error reduction operations while operating under very demanding low level requirements.