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

4.7.12.8.2 Insertion loss for unequal input and output impedance. The insertion loss shall be determined as above

using the test circuit on figure 4. The insertion loss shall be calculated using the following formula:

I.L = 20 log10 2E + 10 log10 Z

Where:

I.L. = Insertion loss (in dB)

Eg = Constant voltage across the generator

EL = Load voltage with transformer in circuit

ZS = Source impedance (to match the reflected input design impedance)

ZL = Load impedance.

4.7.12.9 Self-resonant frequency. Unless otherwise specified (see 3.1) the self-resonant frequency shall be

determined as follows, using the test circuit shown on figure 5. Starting at the lowest frequency within the specified

frequency range, the frequency shall be increased and the VTVM or equivalent observed for voltage dip (e.g.

impedance bridges). The frequency at which the minimum dip occurs shall be recorded as the self-resonant

frequency. An alternate test method is to use an impedance analyzer, an oscilloscope (or similar equipment) to

determine the self-resonant frequency.

4.7.12.10 Coupling among units (multi-unit transformers and inductors). With any one of the units energized, as

specified, at the geometric-mean frequency of the specified frequency range, the voltage induced in the highest voltage

or impedance winding of any other unit shall be measured (see 3.1).

4.7.12.11 Electrostatic shielding. With all windings short-circuited and those on the same side of the electrostatic

shield connected together, using the circuit shown on figure 6 the voltage of the signal generator at the specified

frequency (see 3.1) shall be set to give a definite indication on the detector, with switch "S" open. With switch "S"

closed, the generator voltage shall be increased so as to yield the same indication on the detector, and the ratio of the

generator voltages shall be computed. The detector shall have a minimum input impedance of 1 megohm.

4.7.12.12 Magnetic shielding. The transformer or inductor shall be placed in the approximate center of a

Helmholtz structure (see 6.5), consisting of two test coils placed coaxially 1 foot apart and connected in series aiding.

Each coil shall consist of 1,500 turns of 0.00795-inch diameter (AWG size 32) wire, wound on a coil form having a

radius of 1 foot and a length of 1 inch. A 115-volt, 60 Hz alternating voltage shall be applied across the

series-connected coils, the transformer or inductor shall be rotated until the voltage across the highest voltage or

highest impedance winding is a maximum, and this value shall be noted. The detector shall have a minimum

impedance of 1 megohm.

4.7.12.12.1 Alternate test. The specimen under test shall be energized and the external field shall be measured by

a suitable probe.

4.7.12.13 Center-tap balance at low levels (-20 to -80 dB m balance). Using the circuit shown on figure 7, adjust

R1 until V goes through minimum. Calculate the dB balance from:

dB = 20 log R - R

An alternate method may be used for center-tap balance by substituting a ratio transformer for R1 and R2 on figure 7.

L1 and L2 will then replace R1 and R2 in the formula for calculating the dB balance.

4.7.12.14 Polarity. With the transformer windings connected in series as specified (see 3.1), and with a voltage

applied to one of the windings, comparison shall be made between the sum of the voltages across individual windings

and the voltage across the series of windings. Any other suitable method of determining polarity is permissible.

4.7.12.15 Storage factor. Storage factor (Q) shall be measured under the conditions specified (see 3.1).

4.7.12.16 Wave shape. With the source and load conditions as specified (see 3.1), the wave shape of the output

shall be determined.

4.7.12.17 Turns ratio or voltage ratio (as specified). The voltmeter method or any other suitable means shall

determine the ratio.

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