Magnetic Amplifiers and Logic Circuits (Part One)

Magnetic Amplifiers

Magnetic amplifiers do not amplify magnetism but use electromagnetism to amplify a signal. Essentially, the magnetic amplifier is a power amplifier with a very limited frequency response. The frequency range most commonly associated with the magnetic amplifier is 100 Hz and less, which places it in the audio range. As a technical point, the magnetic amplifier is a low-frequency amplifier.

Advantages of the magnetic amplifier are:

1. Very high efficiency, on the order of approximately 90 percent
2. High reliability
3. Very rugged, able to withstand vibrations, moisture, and overloads
4. No warm-up time

Some of the disadvantages of the magnetic amplifier are:

1. Incapacity to handle low-voltage signals
2. Not usable in high-frequency applications
3. Time delay associated with magnetic affects
4. Poor fidelity

The basic operating principles of the magnetic amplifier are fairly simple. Keep in mind that all amplifiers are current control devices. In this particular case, power that is delivered to the load is controlled by a variable inductance.

If an AC voltage is applied to the primary winding of an iron core transformer, the iron core is magnetized and demagnetized at the same frequency as that of the applied voltage. This, in turn, induces a voltage in the transformers secondary winding. The output voltage across the terminals of the secondary depends on the relationship of the number of turns in the primary and the secondary of the transformer.

The iron core of the transformer has a saturation point after which the application of a greater magnetic force produces no change in the intensity of magnetization. Hence, there is no change in transformer output, even if the input is greatly increased. The magnetic amplifier circuit in Figure 12-247 is used to explain how a simple magnetic amplifier functions.

1. Assume that there is 1 ampere of current in coil A, which has 10 turns of wire. If coil B has 10 turns of wire, an output of 1 ampere is obtained if coil B is properly loaded.
2. By applying direct current to coil C, the core of the magnetic amplifier coil can be further magnetized. Assume that coil C has the proper number of turns and, upon the application of 30 milliamperes, that the core is magnetized to the point where 1 ampere on coil A results in only 0.24 ampere output from coil B.
3. By making the DC input to coil C a continuous variable from 0 to 30 milliamperes and by maintaining an input of 1 ampere on coil A, it is possible to control the output of coil B to any point between 0.24 ampere and 1 ampere in this example.

The term “amplifier” is used for this arrangement because, by use of a few milliamperes, control of an output of 1 or more amperes is obtained.

Saturable-Core Reactor

The same procedure can be used with the circuit shown in Figure 12-248. A saturable-core reactor is a magnetic-core coil whose reactance is controlled by changing the permeability of the core. Varying the unidirectional flux controls the permeability of the core.

By controlling the extent of magnetization of the iron ring, it is possible to control the amount of current flowing to the load, since the amount of magnetization controls the impedance of the AC input winding. This type of magnetic amplifier is called a simple saturable reactor circuit.

Adding a rectifier to such a circuit would remove half the cycle of the AC input and permit DC to flow to the load. The amount of DC flowing in the load circuit is controlled by a DC control winding (sometimes referred to as bias). This type of magnetic amplifier is referred to as being self-saturating.

To use the full AC input power, a circuit such as that shown in Figure 12-249 may be used. This circuit uses a full-wave bridge rectifier. The load receives a controlled DC by using the full AC input. This type of circuit is known as a self-saturating, full-wave magnetic amplifier.

In Figure 12-250, it is assumed that the DC control winding is supplied by a variable source, such as a sensing circuit. To control such a source and use its variations to control the AC output, it is necessary to include another DC winding that has a constant value. This winding, referred to as the reference winding, magnetizes the magnetic core in one direction.

The DC control winding, acting in opposition to the reference winding, either increases (degenerative) or decreases (regenerative) the magnetization of the core to change the amount of current flowing through the load. This is essentially a basic preamplifier.

Logic Circuits

Logic is considered the science of reasoning—the development of a reasonable conclusion based on known information. Human reasoning tells us that certain propositions are true if certain conditions or premises are true. An annunciator being lit in the master warning panel is an example of a proposition, which is either true or false. For example, predetermined and designed conditions must be met in order for an annunciator in a master warning panel to be lit. A “LOW HYDRAULIC PRESS” annunciator may have a simple set of conditions that cause it to be illuminated. If the conditions are met, such as a hydraulic reservoir that is low on fluid causing the line press to be low, then the logic is true and the annunciator lights. Several propositions, when combined, form a logical function. In the example above, the “LOW HYDRAULIC PRESS” annunciator is on if the LED is not burned out and the hydraulic press is low or if the LED is not burned out and the annunciator test is being asserted.

This section on logic circuits only serves as an introduction to the basic concepts. The technician encounters many situations or problems in everyday life that can be expressed in some form of a logical function. Many problems and situations can be condensed down to simple yes⁄no or true⁄false statements that, if logically ordered, can filter a problem down to a reasonable answer. The digital logic circuits are well suited for this task and have been employed in today’s integrated circuits found in virtually all of the devices that we take for granted in modern aircraft. These logical circuits are used to carry out the logical functions for such things as navigation and communications. There are several fundamental elements that form the building blocks of the complex digital systems found in line replaceable units (LRUs) and avionics card cages. The following is a very basic outline of what those elements are and what logic conditions they process. It is far beyond the scope of this text to cover digital logic systems due to the vast body of knowledge that it represents. However, this serves as an introduction and, in some limited cases, is useful in reading system block diagrams that use logic symbols to aid the technician in understanding how a given circuit operates.

Logic Polarity

Electrical pulses can represent two logic conditions and any two differing voltages can be used for this purpose. For example, a positive voltage pulse could represent a true or 1 condition and a negative voltage pulse could then represent a false or 0 logic condition. The condition in which the voltage changes to represent a true or 1 logic is known as the logic polarity. Logic circuits are usually divided into two broad classes: positive polarity and negative polarity. The voltage levels used and a statement indicating the use of positive or negative logic is usually specified in the logic diagrams provided by the original equipment manufacturers (OEMs).

Positive

When a signal that activates a circuit to a 1, true or high condition, has an electrical level that is relatively more positive than the other 0 or false condition, then the logic polarity is said to be positive. An example would be:

Active State: 1 or True = +5 volts direct current (VDC)

0 or False = −5 VDC

Negative

When the signal that actives a circuit to a 1, true or high condition, has an electrical level that is relatively more negative than the other 0 or false condition, then the logic polarity is said to be negative. An example would be:

Active State: 1 or True = 0 VDC

0 or False = +5 VDC

Pulse Structure

Figure 12-251 illustrates the positive and negative pulse in an idealized form. In both forms, the pulse is composed of two edges—one being the leading edge and the other the trailing edge. In the case of the positive pulse logic, the positive transition from a lower state to a higher state is the leading edge and the trailing edge is the opposite. In the case of the negative logic pulse, the negative transition from a higher state to a lower state is the leading edge while the rise from the lower state back to the higher state is the trailing edge.

Figure 12-251 is considered an ideal pulse because the rise and fall times are instantaneous. In reality, these changes take time, although in actual practice, the rise and fall can be assumed as instantaneous. Figure 12-252 shows the non-ideal pulse and its characteristics.

The time required for a pulse to go from a low state to a high state is called the rise time, and the time required for the pulse to return to zero is called the fall time. It is common practice to measure the rise and fall time between 10 percent amplitude and 90 percent amplitude. The reason for taking the measurements in these points is due to the non-linear shape of the pulse in the first 10 percent and final 90 percent of the rise and fall amplitudes. The pulse width is defined as the duration of the pulse. To be more specific, it is the time between the 50 percent amplitude point on both the pulse rise and fall.

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