MIL-PRF-27 Transformers And Inductors \(Audio, Power, And High-Power Pulse\), General Specification ForThis specification covers the general requirements for power transformers, power inductors, audio transformers, audio inductors, high power pulse transformers, charging inductors, saturable transformers and saturable inductors for use in electronic and communication equipment. This specification covers transformers and inductors weighing 300 pounds or less or having root-mean-square \(rms\) test-voltage ratings 0f 50,000 volts or less, and also high-power pulse transformers where the peak pulse power is greater than 5 watts. Transformer and inductor assemblies incorporating any other active or passive components do not come within the scope of this specification.

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MIL-PRF-27G

4.7.3 Resistance to solvents (see 3.6). Transformers and inductors shall be tested in accordance with method 215

of MIL-STD-202. The following details shall apply:

The marked portion of the transformer and inductor shall be brushed.

The number of sample units shall be as specified in table V.

Transformers and inductors shall be examined for mechanical damage (see 3.24).

A soft bristle brush may be substituted when testing conformal coated (i.e. varnish or epoxy dipped)

parts using materials with a soft (i.e. Shore Hardness less than 70) finish.

4.7.4 Thermal shock screening and thermal shock test (see 3.7).

4.7.4.1 Thermal shock screening (level T only). This test is applicable only when specified (see 3.1) and shall be

performed in the qualification tests in group II and the group A inspection in subgroup I. Transformers and inductors

shall be tested in accordance with method 107 of MIL-STD-202 except the temperature for step 3 shall be the

maximum operating temperature for the class. The following details and exceptions shall apply:

Number of cycles: The number of cycles shall be as specified (see 3.1) but shall be restricted to 5, 10,

15, 20, 25 or 50 cycles and 25 cycles minimum for product level T parts.

When specified (see 3.1), transformers and inductors shall be continually monitored continuity during the

entire final cycle to verify no intermittent conditions. Continuity monitoring current shall not exceed 100

microamperes. Equipment shall be capable of detecting intermittent opens exceeding 100

microseconds.

Class T parts using magnet wire smaller than AWG 38 shall have dc resistance measured before and

after the thermal shock screen. The change in resistance shall not exceed �3 percent.

4.7.4.2 Thermal shock test. Transformers and inductors shall be tested in accordance with method 107 of MIL-

STD-202. The temperature for step 3 shall be the maximum operating temperature for the class. The following details

and exceptions shall apply:

Test condition - A, except 10 cycles.

After cycling - transformers and inductors shall be examined for evidence of leakage and other visable

damage (see 3.24).

4.7.5 Burn-in. (Applicable to product level T parts only)

4.7.5.1 Power burn-in (applicable for transformers with an output greater than 0.8 watts). Devices shall be

tested as follows:

Test duration: 96 hours minimum.

Test temperature: Maximum rated ambient temperature. (See applicable specification)

Test voltages and currents: Rated input voltage and current at minimum rated frequency and at

maximum rated load.

4.7.5.1.1 No load burn-in (applicable for transformers with an output equal to or less than 0.8 watts). Devices

shall be tested as follows:

Test duration: 96 hours minimum.

Test temperature: Maximum rated ambient temperature. (See applicable specification)

Test voltages and currents: Rated input voltage and current at minimum rated frequency with no load.

4.7.5.1.2 No load burn-in for inductors. Devices shall be tested as follows:

Test duration: 96 hours minimum.

Test temperature: Maximum operating temperature. (See applicable specification)

Test voltages and currents: Not applicable.

4.7.6 Resistance to soldering heat (see 3.8). Transformers and inductors shall be tested in accordance with

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