A Helpline customer submitted a large potentiometer that is used in military electronics as both a switch and a variable resistor. The potentiometer had a cold in-circuit failure that showed momentary opens during system testing. The customer requested an identification of the cause of failure.
The EMPF began the analysis with basic electrical probing of the leads and determined that there were two regions of the potentiometer rotation that had momentary opens during resistance readings. These occurred near 470ohm and 4400ohm as the device was rotated in one direction, and at slightly lower values (differing by 20ohm) as it was rotated in the opposite direction.

The EMPF disassembled the potentiometer to inspect the resistance tract and found that it did not display any abnormal wear. The component consisted of two main pieces, which were the wiper and the resistance tract. The resistance tract is a ceramic based material that has a set resistance value per "square" or length.

The wiper (Figure 2-1) is a small black carbon/graphite piece that rides in a guide tract with a spring behind it that forces contact. The spring force that is used must satisfy two fundamental requirements. The spring force must be 1) strong enough to maintain continuity under even moderate vibration, and 2) as small as possible in order to avoid wearing the wiper or the resistance tract.

The wiper is typically composed of graphite and displayed some wear on the dual contact side. There are formulations that can be used to determine the hardness and lubrication of a composition. These compositions have mixtures of graphite and wax binders that automatically adjust the lubrication of the components. As the material wears, slippery graphite layers are deposited on the resistance tract and provide lubrication.

A potentiometer functions in many applications as a voltage divider, distributing a ratio of voltages. As the device rotates, the distance from the base contact point to the wiper contact point increases, and hence, the resistance increases for one pair of terminals and decreases for the other pair of terminals. A potentiometer may also function as a rheostat when used as a variable resistor. In this application, the potentiometer uses the varying distances of the resistance strip to control the voltage and power loads that are applied to devices. A large amount of power is distributed applying heat to the resistance tract. Early model potentiometers have a tendency to exhibit intermittent loss of contact between the wiper and the resistance tract. If a circuit cannot withstand a loss of continuity, an extra wire may be placed in the circuit between the pole and the outer terminal. If a momentary open occurs at the wiper, this wire bypasses an open circuit and enables the potentiometer to provide maximum resistance and distribute power loads across the end terminals.

In harsh environments such as those in military situations, small vibrations will cause mechanical wear of the potentiometer components. If the potentiometer is kept in one position for a long time, the resistance tract and conductive pathway can begin to wear. In addition to poor reliability in vibration environments, potentiometers also suffer from large hysteresis (i.e. adhering to one particular value while rotating).

Inspection of the metal resistance tract (Figure 2-2) showed one area of excessive wear. This area most likely was the depression on the tract that caused the momentary discontinuity. Based on the various readings that were measured while increasing and decreasing resistance during testing, it is probable that the wiper skipped and opened momentarily due to the depression. There was also excessive dark colored material on a portion of the tract which may have caused the wiper to rise off of its path regardless of the spring pressure.

The recommendation to the customer was to contact the manufacturer of the potentiometer and ask them to examine their pin-on-disk tests of the wiper base material when used on copper. This material should wear at a greater rate than copper areas. A chemical analysis of the ratios of graphite to binder material should be examined and compared to that of a product that performs adequately. Examination of the difference between the copper used in functioning and defective products should also be considered. The possibility exists that the copper was exceptionally soft and wore prematurely.