Материал: Стандарт MIL-STD-202G

MIL-STD-202G

METHOD 307

CONTACT RESISTANCE

1. PURPOSE. The purpose of the contact resistance test is to determine the resistance offered to a flow of current during its passage between the electrical contacting surfaces of connecting components, such as plugs, jacks, connectors, and sockets, or between the electrical contacts of current controlling components, such as switches, relays, and circuit breakers. For practical reasons, lead and terminal resistances may be included in the actual measurement, as well as the contact resistance proper. In many applications it is required that the contact resistance be low and stable, so that the voltage drop across the contacts does not affect the accuracy of the general circuit conditions. If large currents are passed through high resistance contacts, excessive energy losses and dangerous overheating of the contacts may occur.

1.1 Precautions. Contact resistance values between two contacting surfaces are influenced by such factors as the resistivities of the surface materials; contact pressure; area; shape; condition (including relative cleanliness, smoothness, and hardness) of surfaces; current; open circuit voltage appearing at the contacts during interruption of current; temperature; and thermal conductivity of leads. These factors should be considered in designing test jigs or clamps, or in performing contact resistance measurements. Contact resistances are usually measured by a 4- terminal procedure, using a Kelvin bridge, or by the voltmeter-ammeter method. The test current used is usually the maximum rated current for the contacting surfaces involved. In measuring contact resistance, it is important to keep the specimen free from vibration, and to prevent changes in normal contact pressure which might result from improper application of test jigs or clamps.

2.PROCEDURE. The resistance of the contacts may be measured directly using a Kelvin bridge, or indirectly using the voltmeter-ammeter method, ammeter-potentiometer method, or other suitable means. The maximum allowable measurement error shall be 5 percent. The point of measurement shall be the point at which the external leads are normally connected to the terminals. Connections between the specimens and the measuring apparatus shall be made as specified, using suitable connecting jigs or clamps, where required. The magnitude of direct current to be passed through the contacts during the measurement and, when necessary, the maximum open circuit test voltage shall be as specified. A series resistor may be used provided the specified open circuit test voltage is not exceeded. The number of activations to cleanse the contacts prior to measurement, the number of test activations, and the number of measurements per activations to be made on each contact shall be as specified.

3.SUMMARY. The following details are to be specified in the individual specification:

a.Method of connection (see 2).

b.Test current (see 2).

c.Maximum open circuit test voltage, if applicable (see 2).

d.Number of activations prior to measurement (see 2).

e.Number of test activations (see 2).

f.Number of measurements per activation (see 2).

METHOD 307 24 October 1956

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MIL-STD-202G

METHOD 308

CURRENT-NOISE TEST FOR FIXED RESISTORS

1. PURPOSE. This resistor noise test method is performed for the purpose of establishing the "noisiness" or "noise quality" of a resistor in order to determine its suitability for use in electronic circuits having critical noise requirements. This method is intended as a standard reference for the determination of current noise present in a resistor, for use in an application with specific current-noise requirements. It is not intended as a general specification requirement. Interference caused by the generation of spurious noise signals in parts tends to mask the desired output signal, thus resulting in loss of information. For low-level audio frequency and other low-frequency circuits, where low-noise parts are used, resistors may become an important source of interfering noise. One source of noise in a resistor is molecular thermal motion which generates a fluctuation voltage termed "thermal noise". It is not necessary to determine the magnitude of thermal noise by measurement since the mean-square value of the fluctuation voltage is predictable from Nyquist's equation, which shows the mean-square value to be proportional to the product of resistance, temperature, and the pass band of the measuring system. Generally, an increase in fluctuation voltage appears when direct current (dc) is passed through resistive circuit elements. The increase in fluctuation voltage is termed "excess noise" or "current noise". The magnitude of current noise is dependent upon many inherent properties of the resistor such as resistive material and other factors such as processing, fabrication, size and shape of resistive element, etc. Since there is no apparent functional relationship between current noise and many of these factors, current noise generally cannot be predicted from physical constants. Therefore, it is necessary to measure current noise to determine its magnitude. The method employed in this test has been designed to evaluate accurately the "noisiness" or "noise quality" of individual resistors in terms of a noise-quality index. The noise-quality index, expressed in decibels (dB), is a measure of the ratio of the root-mean-square (rms) value of current-noise voltage, in microvolts ( v), to the applied dc voltage, in volts. The pass band associated with the noise-quality index is one frequency decade, geometrically centered at 1,000 hertz (Hz). This index is termed the "microvolts-per-volt-in-a-decade" index. In the design of circuits, an added advantage accrues from the definitiveness of the index which allows the estimation of interference attributable to current noise. Conversely, for a given limit of current-noise interference in a particular circuit design, a maximum acceptable value of the index may be established. Ordinarily, it is not necessary to duplicate the operating conditions of the particular circuit design when measuring the current noise. The noise quality of populations of resistors may be reasonably estimated by measurement of the index of representative groups of resistors using suitable sampling procedures. Measurements on sample groups tend to have a normal distribution and once representative parameter values for the distribution have been established (the mean and standard deviation), such parameter values would serve as norms in judging "noisiness" and product uniformity insofar as noise is concerned.

1.1 Precautions. Adherence to the ambient temperature specified in 3.1 is emphasized as an important consideration of this method. It is also necessary, in making noise measurements, using the apparatus of this method, to delay reading the noise meter for a period of time no less than four times the effective time constant of the detector to allow the meter sufficient time to reach at least 98 percent of the representative average value. The effective time constant of the apparatus is normally adjusted to a value close to 1 second and therefore, a minimum time delay of 4 seconds is normally required for the noise meter to indicate a valid average. Immediately after this 4 second delay, the meter should be read even though it continues to fluctuate as the noise signal varies. Normally, the operator in making a visual reading of the fluctuating meter pointer, should estimate an average for a short duration, in the order of 1/2 to 1 second.

METHOD 308

29 November 1961

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