Basic Circuit Components Created by Jesse Kuzy for
Resistors • A resistor is a circuit component which has electrical resistance; it slows the movement of electrons through it. • Resistors dissipate electrical energy, converting it to heat.
Resistors in Circuits • Resistors lower voltage across an active circuit; the voltage on the positive end will be higher than the voltage on the negative end. • The voltage across is a resistor is proportional to the current flowing through it. • The symbol for resistors is a zigzagging line. It resembles a type of resistor called the wire-wrapped resistor, which is wire wrapped around a ceramic core.
Capacitors • Capacitors are circuit components that store electrical charge. • Capacitors have two conductors separated by an insulator called the dielectric. • When there is an electric potential across the capacitor (a difference in the voltage), electrons cannot flow across the gap; instead, one end becomes positively charged and the other becomes negatively charged, and an electric field forms between the conductors.
Capacitors in Circuits • When a circuit first comes on, the charge in the capacitor begins to build. Electrons gathering on one end and vacating the other create a temporary current as they move. As they do so, the voltage across the capacitor increases and the current decreases. • After the circuit has been on for a long time (steady-state), there is a voltage across the capacitor and no current through it. At steady-state conditions, a capacitor acts like a break in the circuit. • The symbol for a capacitor is like two plates near one another; this resembles the construction of basic capacitors. [Picture of plate capacitor]
Inductors • An inductor is a circuit element that develops a magnetic field as current flows through it. This field resists and slows the movement of electrons in the inductor. • Most inductors consist of coiled wire.
Inductors in Circuits • The amount that an inductor resists electrical current depends on the voltage difference across it, and vice-versa. • When a circuit first comes on, the voltage across an inductor is high and no current flows through it. • Over time, the voltage drops and the current through the inductor increases as the magnetic field develops. • At steady-state conditions, there is no voltage across an inductor and current flows through at a constant rate. An inductor behaves like wire at steady-state. • The symbol for an inductor is like coiled wire.
Comparing Inductors and Capacitors • The properties of inductors and capacitors are complements in many ways. Consider:
Comparing Resistors to Inductors and Capacitors • Inductors and capacitors act differently when a circuit is going from off to on or from on to off than when the circuit has been on for a long time. This is called transient behavior. Generally speaking, their behavior is time-dependent. • By contrast, resistors act the same at steady-state as they do in changing systems. • Inductors and capacitors behave differently in AC circuits than in DC circuits. Their behaviors are much more complicated in AC circuits, where the current and voltage are constantly fluctuating. Naturally in DC circuits, where the current and voltage stay the same, they are much less complicated. Resistors, because they are unaffected by current and voltage changes, behave the same way in both AC and DC circuits
Ideal vs Real Components • Resistance, capacitance, and inductance are properties that all circuit elements have. Well-designed elements tend to focus on just one of these. It is possible to have a component designed to focus on more than one property. • When represented in circuit diagrams, elements only have the property that they are designed for; resistors don’t have capacitance, inductors don’t have resistance, and so on. • If an actual component does have two or more of these properties to a significant degree, it is often represented in diagrams by multiple elements which together account for all of the component’s properties. This keeps circuit analysis clean and simple. • For example, if an inductor has a non-negligible resistance, it may be represented in a diagram as an inductor in series with a resistor.
Semiconductors • Semiconductors are materials that fall between conductors and insulators. • They may act as insulators in some conditions and as conductors in others. • Semiconductors can be doped; this is when another substance is added to the semiconductor to change its properties. • Donor dopants produce an excess of electrons in the semiconductor. Semiconductors doped with donors are called n-type. • Acceptor dopants produce an excess of positive “holes” where there are no electrons. Semiconductors doped with acceptors are called p-type.
Diodes • A diode is a circuit element which essentially is a resistor with polarity; it has a different resistance in one direction than in the other. • Most diodes have no resistance in one direction and very high resistance in the other, so that they only allow current to flow in one direction. These diodes are called rectifiers. • Recall that semiconductors may change from insulators to conductors under certain conditions. For semiconductor diodes, the diode behaves as an insulator until a certain voltage is achieved across the diode. It then behaves as a conductor, allowing current to pass. When this happens, the diode is forward-biased. • The symbol for a diode looks like an arrow that points in the direction of current flow. The diode shown below would allow current to flow from left to right.
Transistors • Transistors are circuit components made of semiconductors that amplify and switch currents. • A good example of how transistors work is the Bipolar Junction Transistor (BJT). In the NPN BJT, a layer of p-type semiconductor separates two sections of n-type semiconductor. When there is a voltage across the two n-type layers, no current can pass through. When positive voltage is applied to the p-type layer, however, the transistor becomes conductive, and current can pass through. • In PNP transistors, two p-type semiconductors are separated by n-type semiconductor material. When positive voltage is applied to the n-type layer, it is closed; when negative voltage is applied, it is open.
Parts of a Transistor • The terminal that receives current is called the collector. • The terminal that releases current is called the emitter. • The terminal that controls whether the transistor is on is called the base. Collector Base Emitter