There are three basic types of circuits: series, parallel, and series-parallel.

Series circuits

A series circuit is a one-path circuit; that is, it never has more than one conductor connected to one terminal and there is only one path from the source to the load, then back to the source.

In a series circuit, the loads are connected end to end. In a 12-V automobile battery, for example, six cells are connected so that the individual cell voltages add together. In a 6-V battery, three cells are connected the same way.

Think of the most common series connection, found in a three-cell flashlight. The cells are connected so that the positive terminal of the first cell connects to the negative terminal of the second; the positive terminal of the second connects to the terminal of the third; and so on.

This is a series connection, because the same current flows through all three cells. Since the individual electromotive force (EMF) is 1.5 V, the overall EMF is 4.5 V.

Figure 1 shows a different type of cell. Again, the three cells are wired together in series. The voltages add together because between cells, the opposite-polarity terminals are connected. Thus, the three 1.5-V cells provide a total EMF of 4.5 V.

With the series connection, the total voltages across the battery are equal to the sum of the individual values of each cell. However, the current capacity of the battery does not increase. Since the total circuit current flows through each cell, the current capacity is the same as for one cell.

In another example, lightbulbs with voltage starting at 120 V are represented in Figure 2. As the voltage travels through each resistance, there is a drop in voltage proportionate to the amount of resistance.

Upon return to neutral, the voltage is 0. With 120-V resistance, there is a drop of approximately 20 V across each resistance.

If only three bulbs are used, each bulb glows more brightly, with each having about twice the voltage drop (40 V) to produce more light. In a continuous circuit, it does not matter which side of the resistance is wired to the hot wire (H), and which continues to neutral.

Current passes through each resistance in sequence, from the first to the second, to the third, to the fourth, and so on. As there is no alternate path, when one bulb is unscrewed, it acts like a switch, and the entire circuit de-energizes.

This is why several safety devices controlling the same component are wired in series with the component. If any one of them senses trouble, it takes the component off-line. Switches are always wired in series, and ahead of the device they control.

When there are several unequal resistances in a series circuit, the voltage drop is the greatest across the highest resistance. A 100-W bulb has larger wire than a 40-W bulb, so the resistance of the 100-W bulb is much less than the 40-W bulb.

If both are wired in series, most of the voltage is used to light the smaller bulb, and the larger one does not have enough voltage travelling through to light it.

The bulbs glow brightest when the circuit is first energized. The bulbs then dim somewhat after a short period of time because as the bulbs warm up, they offer more resistance, causing less current flow.

Parallel circuits

As mentioned earlier, the series connection of cells increases the output voltage but not the current capabilities of the cells.

However, there is a way to connect cells so that their current capabilities add together. This is called a parallel connection.

In Figure 3 you see that the loads are thought of as being side by side (parallel) rather than end to end (series).

A parallel circuit has more than one path between the two terminals of the power source.

The total resistance of the parallel circuit is always less than the smallest individual resistance of the components. Current (amps) divides equally among the branches, should the resistances in a parallel circuit be equal.

For example, if two bulbs, each with a resistance (R) of 1,200 ohms, are wired in parallel, the total R in the circuit is 600 ohms.

Voltage divided by resistance equals the total current. In this case, 120 V divided by 600 ohms equals 0.2 A, and each if the parallel circuits draws 0.1 A. Should the resistances differ, the amps always divide proportionately.

Major components in the hvacr system, such as blower motors, use parallel circuits, because the voltage drop remains constant and is the same as the supply voltage.

Series-Parallel circuits

In an actual hvacr system installation, there are combinations of series and parallel circuits. Some series circuits are in parallel with each other, and a series circuit may include components wired in parallel with each other.

These circuits are called series-parallel or parallel-series.

The cells are connected in series-parallel when both a higher voltage and an increased current capacity are required. For example, when four 1.5-V cells need to be connected so that the EMF is 3 V, the current capacity is twice that of any one cell. This is achieved by connecting the four cells as shown in Figure 4. To achieve 3 V, cells 1 and 2 are connected in series.

However, this does not increase the current capacity. To double the current capacity, a second series string (cells 3 and 4) must be connected in parallel with the first. The result is the series-parallel arrangement shown.

Whether a circuit is in series or parallel depends on its electrical relationship with another circuit or component. In Figure 5, circuit AC is a series circuit wired in parallel to series circuit BC. Both are in parallel to circuits DF and EG.

To be part of the series circuit, a component may be wired in parallel with another component.

This article was excerpted from Starting in Heating & Air Conditioning Service, published by Business News Publishing Co. For more information, call 800-837-1037.

Sidebar: Wiring diagrams

Wiring diagrams denote one wire for hot (H); this wire is black. The other wire is marked (N) for neutral, and this wire is white.

Single-phase, 120-V current requires only these two wires.

The neutral wire is sometimes called a ground wire; this should not be confused with an earth ground wire. The neutral wire is actually the neutral or zero-voltage side of the circuit.

A third wire, called an earth ground, is added to certain circuits. The earth ground wire is green.

Most installations include only the ground (neutral) wire in the outlet, which will accommodate a three-prong plug. On wiring diagrams, the earth ground wire (third wire) is not usually shown.