Материал: Стандарт MIL-STD-202G

MIL-STD-202G

METHOD 309

VOLTAGE COEFFICIENT OF RESISTANCE DETERMINATION PROCEDURE

1.PURPOSE. Certain types of resistors exhibit a variation of resistance with changes in voltage across the resistor. This is a measurable characteristic; a test to determine the magnitude of such a characteristic is the Voltage Coefficient of Resistance Determination Procedure.

2.PROCEDURE. The voltage coefficient is applicable only to resistors of 1,000 ohms and over. Unless otherwise specified in the individual specification, all measurements and tests shall be made at a temperature of 25°C ±5°C. Adjust the resistance measuring device to apply 0.1 X rated continuous working voltage to the resistor. Measure the resistance by applying this voltage intermittently for not more than the total of 0.5 second in any 5 second interval. Readjust the resistance measuring device to apply 1.0 X rated continuous working voltage to the resistor and repeat the above intermittent measuring procedure. Compute the Voltage Coefficient (VC) as follows:

VC = (R −r )100

3.PRECAUTIONS. Adherence to 2, applying voltage intermittently for not more than the total of 0.5 seconds in any 5 second interval is emphasized as an important consideration of this method. Failure to comply would result in a voltage coefficient of vast variations. A resistance measuring device capable of withstanding high voltage applications should be used. Certain types of resistors exhibit a variation of resistance with changes in potential difference, this effect being separate and distinct from the change in resistance due to heating effect whether from applied voltage or ambient conditions.

4.SUMMARY. The following detail is to be specified in the individual specification:

The continuous working voltage (see 2).

METHOD 309

27 May 1965

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MIL-STD-202G

METHOD 310

CONTACT-CHATTER MONITORING

1.PURPOSE. This test is conducted for the purpose of detecting contact-chatter in electrical and electronic component parts having movable electrical contacts, such as relays, switches, circuit breakers, etc., where it is required that the contacts do not open or close momentarily, as applicable, for longer than a specified time-duration (see 4.3) under environmental test conditions, such as vibration, shock, or acceleration. This test method provides standard test procedures for monitoring such "opening of closed contacts" or "closing of open contacts".

2.TEST CIRCUITS.

2.1 Selection. In this method there are two test-circuits: A (see 3.1), and B (see 3.2). The selection of the testcircuit depends largely upon the type of electrical contacts to be tested. Test-circuit B is preferred, whenever possible, to avoid contact contamination caused by the formation of carbonaceous deposits on the contacts. The individual specification shall specify the test-circuit and time-duration (see 4.3) required in connection with monitoring of shock and vibration tests. The test-circuits listed herein are "recommended" reference circuits. Any comparable test-circuit which meets the test requirements and the calibration procedures as stated herein, may be used for this test.

2.1.1Selection of test-circuit A. Test-circuit A is for monitoring test-specimens with a single set of contacts, for the opening of normally-closed contacts or false closures of normally-opened contacts (see figure 310-1). Test-circuit A should not be specified for specimens whose capability includes low-level or dry-circuit ratings (10 milliamperes or less and 2 volts or less for openings or closings less than 10 microseconds); since the current through the electrical contacts under test from the test-circuit may cause arcing, thus damaging the contacts.

2.1.2Selection of test-circuit B. Test-circuit B is for monitoring test-specimens with a single set of contacts, for the opening of normally-closed contacts and false closures of normally-open contacts (see figure 310-3). Test-circuit B should not be used for openings or closings of less than 10 microseconds. Test-circuit B does not allow current in excess of 20 milliamperes or an open-circuit voltage in excess of 2-volts during monitoring; which insures that there will be no arcing, which will cause damage, to low-level and dry-circuit test specimens.

3. TEST SYSTEMS.

3.1 Test-circuit A. The test circuit shall be the thyratron circuit shown on figure 310-1 or an approved equivalent circuit. The values for R1, C1, and the suppressor grid-cathode voltage, controlled by R7, principally controls the firing of the thyratron and are so chosen that the thyratron will fire when the duration of the contact-opening exceeds the time-duration specified in the individual specification (see 4.3 and 5). For the longer time-durations, such as above 1 millisecond, it may be necessary to change the values of R2, R5, and R6.

a.To monitor normally-closed contacts, the normally-closed contacts are connected to BP1 and BP2, with switch S1 in the "normally-closed position". The grid of the thyratron is placed at ground potential. The cathode of the thyratron is at a positive potential (depending on the setting of R7), thus providing sufficient negative bias to cut the thyratron "off". Any contact chatter (opening of closed contacts) will cause the grid of the thyratron to rise exponentially to +150 volts at a rate determined by the preselected time constant of R1 and C1. As long as the contacts remain open, the grid potential will continue to rise. If the contacts remain "open" for longer than the specified interval, the grid potential rises to the point at which the thyratron conducts and ionizes, thus lighting DS1. Since, in a thyratron, the grid loses control of conduction as soon as the tube conducts, the contacts being monitored can reclose at any time thereafter without affecting the monitoring circuit. Thus, lamp DS1 will remain "on" until the thyratron is manually reset by operation of switch S2.

METHOD 310 20 January 1967

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