MIL-PRF-39015 Resistors, Variable, Wirewound, \(Lead Screw Actuated\), Nonestablished Reliability, And Established Reliability, General Specification ForThis specification covers the general requirements for nonestablished reliability \(non-ER\) and established reliability \(ER\), lead screw actuated; wirewound variable resistors with a contact bearing uniformly over the surface of the entire resistive element, wound linearly, when positioned by a multiturn lead screw actuator. These resistors are capable of full load operation \(when the maximum resistance is engaged\), at a maximum temperature of 85OC and are capable for continuous operation, when properly derated, at maximum temperature of 150OC \(see 3.6\), with a resistance tolerance of ??5 percent. The ER resistors have life failure rate \(FR\) levels ranging from 1.0 percent to 0.001 percent per 1,000 hours \(see 1.2.1.4\). These FR levels are established at a 60 percent confidence level on the basis of life tests.

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MIL-PRF-39015E

4.8.2 Conditioning (see 3.8). Resistors shall be conditioned in accordance with method 108 of MIL-

STD-202. The following details and exceptions shall apply:

a. Method of mounting: Supported by their terminals (resistor not mounted on life test chassis).

Resistors shall be so arranged that the temperature of any one resistor shall not appreciably

influence the temperature of the other resistor. There shall be no circulation of air over the

resistors other than that caused by heat of the resistors.

b. Temperature and tolerance: 25�C +10�C, -5�C.

c. Initial measurements: Initial total resistance shall be measured after mounting at 25�C +10�C, -

5�C as specified in 4.8.4.1. This initial measurement shall be used as the reference temperature

for all subsequent measurements.

d. Operating condition: DC continuous working voltage or a continuous working voltage from an ac

supply at commercial line frequency and waveform equivalent to 1 watt power dissipation shall be

applied between the end terminals intermittently one and one-half hours "on" and one-half hour

"off" for a minimum of 50 hours +8 hours, -0 hour at a temperature of 25�C +10�C, -5�C. Each

resistor shall dissipate 1 watt (see 3.1).

e. Measurement after conditioning: Total resistance shall be measured at the end of the 50 hours +8

hours, -0 hour as specified in 4.8.4.1 after load has been removed and the resistors stabilized.

f. Examination after conditioning: Resistors shall be examined for evidence of mechanical damage.

g. Test duration: 50 hours +8 hours, -0 hours minimum.

4.8.3 Peak noise (3.9). Peak noise resistance shall be measured with the measuring circuit shown on

figure 3, or its equivalent. The lead screw shall be rotated in both directions through 90 percent of the

actual effective electrical travel for a maximum of 6 cycles. If the product passes on any one of the first

three cycles, then the product is acceptable. Only the last three cycles shall count in determining whether

or not a noise is observed at least twice in the same location. Group A, subgroup 1, product acceptance

may be determined based on one cycle minimum where compliance to the specification is demonstrated.

The rate of rotation of the lead screw shall be such that the wiper completes one cycle in 5 seconds

minimum to 2 minutes maximum. The equivalent resistance shall be calculated using the following

formula:

Epn

Noise =

ohms

0.001

Where:

Epn = the peak-noise signal voltage presented on the oscilloscope screen.