The electrical circuitry in modern heating, cooling, and air conditioning systems is important to service technicians who install and service these systems, so a thorough understanding of the basic concepts is essential for troubleshooting and repairing them.
The most important kinds of circuits are parallel and series circuits. The parallel circuit has more than one path through which electricity can flow and it is designed to supply more than one load in the system. A series circuit has only one path through which electricity can flow and it is usually used for devices that are connected in the circuit for safety and control.
The series-parallel circuit is a circuit that has a combination series and parallel circuits. Most electrical systems in equipment and controls are made up of combination circuits. If we look at a series circuit we see that the switches and controls are wired in series with each other in control of one or more loads.
The simplest and easiest electric circuit to understand is the series circuit. The series circuit allows only one path for current flow through the circuit. In other words, the path of current flow must pass through each device in the circuit. Series circuits are incorporated into most control circuits used in our profession. A control circuit is an electric circuit that controls some major load in a system. Therefore, the opening of any switch in a series circuit will open the circuit and stop the flow of current to the load as shown in figure 1.
Parallel circuits have more than one path for the electron flow. That is, in the parallel circuit the electrons can follow two or more paths at the same time. The electric loads are arranged in the circuit so that each load is connected to the supply voltage conductors. Parallel circuits are common in this industry because most loads used operate from line voltage.
Line voltage is the voltage supplied to the equipment from the main power source of a building and typically has a value of 115 volts or 240volts. The parallel circuit allows the same voltage to be applied to all the loads connected in parallel, as shown figure 2.
Parallel circuits are used in the industry to supply the correct line voltage to several different circuits in a control system and are called power circuits. Figure 3 is a typical power circuit.
In figure 3 you see a control system with several circuits in parallel being fed line voltage. As you can see there are many different paths for electron flow in this parallel schematic. Parallel circuits are used in all power wiring that supplies the loads of heating, refrigeration, and air conditioning systems. The electric loads of a system must be connected to the power supply separately or in a parallel circuit to supply the load with full line voltage. Parallel circuits react differently from a series circuit. When a break occurs in a conductor, only the current in that path is interrupted, whereas in the series circuit, (when a break in the conductor occurs) all current is interrupted.
Series-Parallel (Combination) Circuits
Combination circuits are used on most types of heating, cooling, and refrigeration equipment. This type of circuit allows the voltage to be applied to all the electrical loads in the circuit. In the parallel circuit, the switches used to control the loads are connected in series. See figure 4 below for a typical series-parallel arrangement where switches are in series and loads are in parallel.
Notice that the compressor, condenser fan motor, and indoor fan motor are connected in parallel to the supply voltage. This is the power side of this circuit. On the low voltage side (24 volts) we have the indoor fan relay and the contactor coil in parallel with the 24 volt transformer supplying voltage to the loads. In addition we have all the various switches such as the system switch, the auto and on switch on the thermostat, and the various contacts on the power side that control the loads.
The three types of electric circuits used in our profession are series, parallel, and series-parallel. The series circuit has only one path for current flow and the most common type of circuit is parallel circuit which has more then one path for current flow. This type of circuit allows applied voltage to reach all loads. The series circuit is used for most of the control circuits, because when any switch in the circuit opens or closes, the load will stop or start, depending on the position of the switch. The series-parallel circuits are circuits that are used most often to operate equipment.
copyright(c)2009 Roger J. Desrosiers
About the Author: Roger is a contributing faculty member of HVACReducation.net He has over 40 years experience in Air Conditioning and Refrigeration. He is also a member of R.S.E.S., CM, The Association of Energy Engineers, Certified Energy Manager, ASHRAE, Certified Pipe Fitter United Association and is 608 Universal Certified.