You will need to know about alternating and direct current for the Electronics subtest of the ASVAB. A current doesn’t always flow in one direction. A direct current (DC) does — it only and always flows in one direction. An alternating current (AC), however, constantly changes direction in a regular pattern.
Higher voltages are easier to obtain with alternating current, and transferring high voltage down a power line is ultimately cheaper than transferring low voltage, so most electricity comes in the form of AC.
Figuring out frequency
The number of times an alternating current changes direction per second is known as its frequency. Hertz (Hz) is the unit of measurement for frequency. One hertz (Hz) equals one complete cycle per second. In other words, the current makes two complete alternations of direction.
The AC (alternating current, not the air conditioner) in your house probably completes 60 alternating cycles per second. Therefore, the AC in your house has a frequency of 60 Hz. Most electronic devices operate at higher frequencies; therefore, frequencies may be measured in kilohertz (kHz, 1,000 hertz), megahertz (MHz, 1 million hertz), or even gigahertz (GHz, 1 billion hertz).
AM radio stations often broadcast in the 530–1,700 kHz range. Television stations may broadcast at as low as 7 to as high as 1,002 MHz. Radar operates in the 1–40 GHz range.
Impedance: Join the resistance!
Resistance interferes with the flow of current in a circuit. But the flow of current is also impeded by two properties of alternating currents:
Capacitive reactance (capacitance): Capacitance is the storage of energy that occurs in a nonconductor. This property resists any change in voltage in a circuit.
Inductive reactance (inductance): Inductance is the property that causes an electromotive force (another way of saying voltage) to be induced in a circuit.
These two types of reactance combine to impede the flow of current. Impedance can be expressed as the ratio of electromotive force to the current:
Electronic devices often require a specific capacitive or inductive reactance to work. Capacitors and inductors are devices used in circuits to provide the type of reactance needed. Capacitors are rated in microfarads , and inductors are rated in millihenries (mH).
You can relate impedance to Ohm’s law in reference to AC circuits. Simply substitute resistance in Ohm’s law with impedance and voltage with electromotive force.
Sorting out capacitors and inductors
Capacitors store or hold a charge of electrons. In an AC circuit, because AC voltage goes positive and negative in each cycle, the capacitor is constantly charging and discharging. The rate of the charging and discharging acts as opposition to the changing AC voltage — as a resistive effect called capacitive reactance.
Inductors are coils of wire that make use of the properties of a magnetic field. The property specifically desired is the flow of current through the wire. With full current, the magnetic field is at its maximum. However, if you take away the current, the field doesn’t disappear immediately.
It decays gradually, and the decay continues to push electrons in the path they were going. But in an AC circuit, the current constantly reverses. The rate of changing current flow and the resulting collapse and regeneration of the magnetic field in the coil act as opposition to changing AC current — a resistive effect called inductive reactance.
Rectifying the situation: Going direct
Certain electronic circuits are engineered to change alternating current to direct current. The process of making the change is called rectification, and the circuits that perform the rectification are called rectifiers.
Rectifiers contain semiconductor diodes, a component made of a material with conductivity somewhere between that of a conductor and an insulator. Diodes conduct electricity in only one direction. Rectification also often requires inductors and capacitors.
Rectification helps appliances run at cooler temperatures and allows them to run at variable speeds. Devices typically need direct current to run properly. The process of rectification changes the incoming AC to DC.
Turning up the old transistor radio
A transistor is a semiconductor (an object that conducts electricity poorly at low temperatures) that controls the flow of electricity in a circuit. It’s usually made of germanium or silicon. This electrical device can amplify a signal, which is why it’s used in transistor radios. Transistors have many properties:
Unlike rectifier diodes, a transistor doesn’t require a vacuum to operate.
Transistors are small, require little power, and last a long time.
A transistor contains at least three terminals:
The emitter is the voltage output.
The base acts like a gate, and the voltage at the base controls the flow of current through the transistor (and therefore the voltage).
The collector is the voltage input.