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

4.7.12.3 DC resistance and resistive unbalance. The dc resistance of the windings shall be measured at, or

corrected to 20�C. The resistive unbalance of center-tapped windings in percent [[(R1 - R2)/R1] x 100] shall be

computed. R1 and R2 are the resistances of each part of the winding, and R1 ≥ R2 . For resistances under 1 ohm,

measurements shall be made with a Kelvin bridge or equivalent.

4.7.12.4 Inductance and inductive unbalance. The inductance of the windings shall be measured at the specified

test voltage and frequency with the specified dc current applied (see 3.1). The inductive unbalance of center-tapped

windings in percent [[(L1 - L2)/L1] x 100] shall be computed. L1 and L2 are inductances of each part of the winding,

and L1 ≥ L2 .

4.7.12.5 Harmonic distortion. The transformer shall be terminated in its proper source and load impedance. A

sine-wave voltage of specified frequency shall be applied such that the specified output conditions are achieved and

the total harmonic distortion shall then be computed or measured (see 3.1).

4.7.12.6 Primary impedance (for qualification only). Transformer primary impedance shall be measured with all

normally loaded secondaries loaded with their specified impedances, and with specified dc currents flowing in the

windings. The resistance and reactance looking into the primary shall be measured at the specified input frequency

and voltage by a bridge or equivalent method approved by the Government (see 3.1).

4.7.12.7 Frequency response. Using the circuit shown on figure 3, with the specified values of impedance,

sufficient EMF from the source of the specified reference frequency should be applied to present the specified voltage

across the load impedance. Keeping the source EMF constant, the output voltage is then measured at the specified

frequencies (see 3.1). The rated dc currents shall flow through the windings. The measuring device shall be a

vacuum-tube voltmeter or equivalent instrument having high-impedance input circuit. The same type of instrument

shall be used to measure the source EMF and the output voltage. Frequency response, in decibels (dB), may be

determined for each instrument as follows:

dB = 20 log E

Where:

Er = reference output voltage.

Ef = output voltage at any other frequency.

A frequency spectrum instrument may be used as an alternate method of test.

4.7.12.8 Insertion loss. Transformers shall be tested in accordance with 4.7.12.8.1 or 4.7.12.8.2, as specified (see

4.7.12.8.1 Insertion loss, for equal input and output impedance. Using the circuit shown on figure 4 with a

constant voltage, Eg, across the generator, the load voltage, with and without the transformer in the circuit, shall be

measured at the specified frequency (see 3.1). The insertion loss in dB, shall be calculated using the following

formula:

I.L. = 20 log10 E

Where:

I.L. = Insertion loss (in dB) with (Eg = E1) constant

E1 = Voltage with the transformer not in the circuit at the specified frequency

E2 = Load voltage with the transformer in the circuit at the specified frequency

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