The article provides an overview of different types of electrical circuits, including complete (closed), series, parallel, and series-parallel circuits, explaining their characteristics and functions. It also discusses faulty circuit conditions such as open, short, and ground circuits, highlighting their causes, effects, and troubleshooting techniques.
Complete Circuit
A complete circuit is a path that electricity can flow through. It is also known as a closed circuit. The simplest complete circuit includes a power supply, a load or resistance, and interconnecting wires. Figure 1 illustrates a complete circuit. It includes a power source, a load (the electric heater in the upper Figure), a motor load (the lower Figure) and interconnecting wires. Switches, thermostats, and pressure control devices are placed in the circuit as optional devices for convenience and safety during operation of the system. This is not a common circuit; it is used merely to illustrate a complete circuit.
Figure 1 A complete circuit is a path that electricity can flow through. It is also known as a closed circuit.
An adequate power source that will push (voltage) electrons (current flow) through the circuit is required. A load is required to consume and dissipate some of the electron flow in the circuit. The load performs useful functions such as operating a compressor motor (to pump refrigerant) or a fan motor (to move air). The path for electron flow or wires interconnects the load and power source to create a complete circuit.
A complete or closed circuit can be a simple series circuit, parallel circuit, or series-parallel circuit. Series-parallel circuits are also known as combination circuits. There are also open circuits, short circuits, and grounded circuits. Next we discuss the various types of circuits.
Series Circuit
A series circuit is shown in Figure 2. A series circuit has the same current flow at every point in the circuit. If there are two loads in a series circuit, the current flow will be the same everywhere in the circuit, but the voltage will be divided across the loads. If there are two equal loads in a series circuit, as shown in Figure 2, the voltage drop (measured voltage) across each heater will be the same. This voltage drop can be calculated by dividing the supply voltage by 2. This is just an example to explain series circuits. By the way, most heat strips and most HVACR components have only one load in series with the power supply, not two as shown in this example.
Figure 2 This series circuit has electric heat strip loads H1 and H2, which have the same resistance. The current flow is the same everywhere in the circuit. The voltage will be split across each heat strip since they have the same resistance.
Tech Tip
Understanding how the current and voltage flow in series, parallel, and combination circuits will help you troubleshoot a circuit. It is also important to know how total resistance changes as these circuit types change.
Parallel Circuits
Parallel circuits are arranged so that the power source is the same across all the loads. In Figure 3, the power source is in parallel with three electric heating elements. The voltage supplied across each element is the same. The current flow through each element is the same because the resistance of each heater is the same. The total current flow equals the sum of the amperage draw of each branch or heater.
If the resistance of the heating elements is different, the current flow through each branch will be different. Using Ohm’s law, you can calculate that the heating element with the lower resistance will develop higher current flow because there is less opposition in the heating element. Heating elements with higher resistance develop lower current flow because they oppose the flow of electrons. With this in mind, a shorted heating element will draw extremely high amperage and cause a fuse or breaker to trip.
Figure 3 In a parallel circuit, the same voltage is applied across each of the heat strips. The total current is equal to the sum of the current in each of the parallel circuits.
Series-Parallel Circuits
Combination circuits have series and parallel components in the same circuit, as shown in Figure 4. This is the most common type of HVACR diagram. Let’s trace through part of the series-parallel circuit shown in this Figure.
Figure 4 This simple ladder diagram of a series-parallel circuit shows parallel loads in series with switches that control their operation.
Point 1: Figure 4 shows the contactor circuit, highlighted in yellow, in series with the series load of the compressor and run capacitor
Point 2: Below and in parallel with the compressor line, highlighted in red, is the switching relay, which is in series with an indoor fan and condenser fan. These fans are in parallel with each other.
Point 3: The primary of the control transformer is in parallel with the components above it.
Point 4: The primary side of the transformer, highlighted in blue, is also in series with the thermostat, limit switch, and gas valve.
Point 5: This is the secondary or 24-V side of the transformer. It will be used to energize the contactor coil and the relay coil.
Some components are in series and some are in parallel, which is why the word combination is applied to this circuit: It combines series and parallel circuits together in the same circuit. In combination circuits, the voltage is the same across each parallel branch. The current flow depends on the amount of resistance or impedance in each branch. The total current flow is the sum of current flow in each of the branches. Impedance is the resistance to current flow in inductive components such as motors, transformers, or relay coils. An inductive device is any component that uses magnetism to do work. For example, motors require magnetism to create rotating motion.
Simply measuring the resistance of an induction component and using the Ohm’s law formula will not give a technician the current flow requirement in a circuit. A special impedance formula is needed to calculate this information. Knowing the exact impedance is not important to the HVACR tech.
A complete circuit can be found in many different configurations. It is important to identify the type of complete circuit in order to understand the voltage and current flow and requirements. Knowing what to expect in the circuit makes the troubleshooting process easier to master.
We have discussed complete or closed circuits. We now turn to a discussion of open, short, and grounded circuits.
Figure 4 This simple ladder diagram of a series-parallel circuit shows parallel loads in series with switches that control their operation.
Open Circuit
An open circuit means that there is a break or “opening” in the wire or load that stops the flow of electrons. Figure 5 has one open switch that creates an open circuit. Figure 6 shows an open contactor in a series circuit. A common way to open a circuit is with the use of a thermostat to open cooling, heating, or refrigeration circuits when they are not required.
Figure 5 This is an open circuit with no current flow. The switch is open, blocking the current flow.
Figure 6 The contactor is a magnetic switch that opens and closes a contact or set of contacts to operate a compressor or fan load. A thermostat is used to control the 24 volts going to the contactor coil. This is an open circuit, since the contacts on the left side are open.
Short Circuit
A short circuit has the voltage going to ground or around the load. The word short means that voltage is taking a shortcut around the load. A short circuit does not need to be a direct short to ground or a routing of the voltage around the load. For example, a condition in which the load has a very low resistance is a short-circuit condition. A load that is supposed to be 10 V but is actually 2 V is a short- circuit condition.
Figure 7 is a diagram of a short-circuit condition. The short circuit is not a circuit by design, but a problem. A short circuit is usually a wiring mistake or a defect that causes the load to be bypassed. A short-circuit condition causes excessive amperage draw, which may open the fuse or circuit breaker or even burn a wire open.
As stated earlier, a short circuit is generally thought to have no resistance. A circuit can be considered as shorted even if it has a resistance reading. Another example: Consider what will happen if the resistance in a parallel branch is 5 V and a relay coil in that branch shorts out and reduces the resistance from 5 to 1 V. This low resistance will create a high current flow in that part of the branch. If this part of the circuit is served by a transformer, it will cause the transformer to overheat or the fuse to blow. A short circuit can be difficult to troubleshoot because there is resistance in the circuit. Each component needs to be inspected for abnormal appearance, such as a burned coil or overheating. If this does not reap results, each of the parallel branches needs to be checked individually for resistance. Also, remember that a wire or terminal can be touching the case ground and cause a shorted condition.
Figure 7 This is a short circuit or a power supply that is bypassing the load. A red wire is causing the power supplied to be shorted out. The power supply or wire will be damaged in this shorted condition.
Ground Circuit
A ground circuit is similar to a short circuit. A ground circuit is one where the component has less resistance than normal to ground, as shown in Figure 8. Normal resistance to ground is called “infinity” or at least has a resistance of millions of ohms to ground. A good motor will have a reading of millions of ohms to ground. If the motor winding to ground is 100 ohms, this would be considered a grounded condition.
Consider another example: A good motor winding may have a resistance of 10 V. If some of the windings are shorted to the case, the resistance will be less than 10 V, maybe even 0 V. The lower resistance means that the component is grounded or some of the windings have a voltage circuit to motor case. This lower resistance to ground stops the motor operation and excess current draw is observed. The excess current will open a fuse or trip a circuit breaker. Sometimes the terms short circuit and ground circuit are incorrectly used interchangeably, but the results are still the same: A defective component is created by a ground or short.
Figure 8 This heating element is shorted to ground. This will result in high amp draw, which will possibly trip the circuit breaker. The shorted heat strip will reduce heat output.
Tech Tip
Do not test transformer voltage using the spark method. The spark method involves quickly touching a 24-V transformer’s leads to each other. Some techs do this to see if a spark is created. A spark indicates that there is some voltage at the output of the transformer. But some transformers have internal or external fuse protection that will blow if this procedure is used. So it is recommended that a voltmeter be used to check the secondary voltage instead of the spark method.
Types of Circuits Key Takeaways
In conclusion, understanding the various types of electrical circuits—complete, series, parallel, and series-parallel—as well as identifying faulty conditions such as open, short, and ground circuits, is crucial for practical applications across multiple industries. These concepts are fundamental in designing and troubleshooting electrical and electronic systems, including HVACR, automotive, and industrial control circuits. By recognizing how voltage, current, and resistance behave in different circuit configurations, professionals can ensure efficient operation, prevent malfunctions, and enhance system reliability.
