Sources and Common Returns

Part 1: Fundamental Concepts: Sources and Common Returns

Most of the circuits we have looked at so far have had a voltage source included as part of the schematic.  These have been generic representations of an ideal source of power used to drive the circuit.  This has been useful in visualizing how things work.

Real world devices are either line powered (plugged in the wall) or battery powered.  In these devices, it is very common to construct circuits that take the incoming power and perform the function of an ideal voltage source.  These circuits are called power supplies and voltage references.  Power supplies are used to power the circuitry (this will make more sense when we get into active circuits) and drive the circuit’s load, while voltage references provide an accurate voltage level used to set an operating point in a circuit.  Depending on the design of the power supply or voltage reference, they can within the limits of their design, behave as ideal sources.

The following figure depicts both the DC voltage source you are used to seeing on the left, and an AC voltage source on the right.  If you don’t understand what AC is, don’t worry.  It will be covered in a later chapter.  In a vast majority of circuits, power supplies and voltage references are DC.  It is worth noting that the symbols shown in Figure 1 would not appear in a schematic for an actual device.  They are useful for simulation purposes, for instance in LTspice, and for problem solving.  In an actual device schematic, these symbols are replaced with the components that make up the power supply or voltage reference.

The symbols presented in this section are not universal.  There have been long standing differences between the symbols used in the US and a number of other countries.  To complicate matters, there have been changes in the standards that govern schematic symbols over time.  This has unfortunately lead to a some schematics being drawn with a hodgepodge of symbols.  See the appendix on drawing schematics for more detail.

Figure 1. A DC and AC voltage source.

The primary characteristic of ideal voltage sources is that they will supply exactly the voltage indicated, no matter what the load is.  This requires that they can supply an unlimited amount of current.  This, of course, is not the case when we replace the voltage sources in real devices with power supplies or voltage references.  What is true, is that they can be designed to supply the current needed by the circuit and its loads, with the amount of accuracy, or voltage stability, required.  In other words, we can approximate an ideal voltage source in practical devices.

If we have a source of power, an ideal voltage source, that does not change with load, this tells us something about its resistance.  Consider the voltages sources shown in Figure 1 as black boxes that, inside, are made up of other parts.  If we change an external resistance connected to it, without its voltage changing, it is either able to violate Ohm's law (a change in current must result in a change in voltage) or it has no, or a 0Ω,  internal resistance.  It is of course impossible to build a circuit without resistance.  Power supplies and voltage references do act in this ideal way, within their design limits, by monitoring their output and adjusting the output based on the amount of load they see.

When drawing schematics, particularly complicated ones, having a large number of lines on the schematic can lead to confusion and difficulty interpreting the circuit’s behavior.  For that reason, particularly when it comes to power distribution, the common practice is to replace interconnecting lines with a symbol.  Whenever you see the symbol, it means that point has a connection to all other identical symbols on the schematic.  The following figure shows how this looks with a voltage source having a return or circuit common (all too often referred to as ground) symbol on the negative terminal and a power supply symbol on the positive.

Figure 2. A DC Voltage source with net symbols.

In a schematic that contained Figure 2, all other components, or nets (another name for a circuit node), with an up pointing triangle labeled +10V, would be connected to the up pointing triangle labeled +10V in Figure 2.  It is common to have more than one supply voltage in a circuit.  Given that ideal voltage sources are independent elements, it is possible to interconnect them in any way needed.  This is shown in the following figure.

It is common for schematic to be drawn with a common return, or ground, symbol.  When a schematic is drawn this way, unless otherwise indicated, always assume that all voltages are measured in reference to the common return.

Figure 3. A triple supply with net symbols.

Figure 3 depicts three sources that are interconnected.  This is a surprisingly useful, and often times necessary, thing to do.  In this case there are three voltages (+5V, -5V, and +10V) available to the circuit in reference to a common return.  Triple power supplies are quite common in circuits that deal with mixed analog and digital signals.  The symbol names (+5V, +10V, -5V) are chosen by the circuit designer and can follow any convention they prefer.  The most common notation used is to label the triangle with the actual voltage.

There is another type of source, similar to a voltage source, called a current source.  Although used much less frequently, current sources fill an indispensable role in some designs.  The symbols most commonly used to represent an ideal current source are shown in the following figure.

Figure 4. A current source.

In the same way that a voltage source maintains a constant voltage on its terminals, a current source maintains a constant current flowing from, and returning to, its terminals.  If you connect a resistor across a current source, there will be a voltage across the resistors and the current source’s terminals.  There has to be, because Ohm's law must be satisfied, and the current source will produce an output voltage so that the current flowing from its terminals is the specified value.  If you change the value of the resistor, the voltage will change because the current source will always do what is needed to maintain a constant flow of current.

Practical current sources, the ones that are implemented in real world circuits, operate within limits imposed by their design.  It is possible, however, to design practical current sources, that act like ideal current sources sufficiently well for practical applications.  It is also interesting to know that some types of electrical components (they will be discussed later), act as if they are current sources.  The amount of current that flows from their terminals is constant (or nearly so) regardless of the voltage present on those terminal.

Voltage sources act as if they have no internal resistance.  They produce a constant voltage regardless of load.  Current sources produce a constant output current regardless of the load.  For this to occur, current sources behave as if they have an infinitely high internal resistance.  They appear, from the circuits standpoint, like an open circuit from which a current is flowing.

Remember that a schematic for a functioning piece of electrical hardware will not contain the symbols shown here for a voltage or current source.  The use of net symbols, the up and down pointing triangles (or other symbol at the designers discretion) are commonly used when drawing schematics of practical devices.  A schematic which uses these interconnection symbols is shown below in Figure 5.

In Figure 5, all of the resistors are equal value. The right facing arrows are used to indicate the voltage at that point and are not meant to be interpreted as a net symbol.  Those voltages are there so you can look over the schematic and verify that you understand what is happening.  As a side note, the arrow symbol is most often used to indicate a connector pin or test point. 

Everything should make sense with what you know with the possible exception of the connections made to R7 and R8.  Both V1 and V2 (the 5V and -5V sources) are connected to circuit common.  As a consequence, they are connected to each other and current can flow between them.  The voltage divider formed by R7 and R8 is connected between 5V and -5V.  It has a difference of 10V across it.  The output voltage of the voltage divider will therefore be 5V above its lowest point.  In this case, the lowest point is -5V which results in voltage divider output of 0V.  Intuitively you can see this by knowing the 0V is halfway between +5V and -5V.