One of the most common errors people make when they build their first electronic circuits is to not bother connecting up the 0 Volts or Earth lines. This is partly the fault of engineers for leaving these wires off most diagrams, but it also seems to come from thinking something like, "0 Volts is nothing - therefore - this wire/connection does nothing - therefore - I don't have to bother with it". Alas, leaving off the Earth connections usually has the same effect as not connecting the power supply voltage - nothing works!
To see why the 0 Volts connections are important, remember that all the voltages we're talking about are really Potential Differences. i.e. they refer to a difference in electrical potential between two different places. Currents will only flow in circuits when we apply differing voltages to different places in them. It is very common to draw diagrams like figure 1 which shows the signal as a 'voltage on a wire'. In fact, the signals are voltages or potential differences between pairs of wires. These wires form loops around which the signal current flows.
Engineers often say something like, "The voltage on this resistor is 10 Volts." This is misleading. When they say this they really mean that there is a 10 Volt potential difference applied between the two wires connected to the resistor. Try not to be mislead by this kind of statement and resign yourself to the sad fact that enjuneerz ain't much good at Inglish!
Another potentially confusing (deliberate awful pun!) thing is that sometimes systems don't use a wire to connect places at 0 Volts together. This is because many signal generators, amplifiers, etc, have a connection to the earth pin of their mains plugs. All these earths on the wall/bench sockets are connected together, so we can sometimes use these earth wires as the 0 Volts connection to 'return' currents and provide a place all our potential differences are referred to. Note, however, that this is bad practice. Mains earth wires are provided for safety reasons. They ensure that the metal cases of equipment are always at 0 Volts even when the equipment is faulty, hence preventing us from getting a shock. We shouldn't assume that they will help us carry signals around.
Summary.
You should now know that Circuit Diagrams are a picture language for describing how electronic circuits work. That they are a sort of shorthand which shows the functional relationships of the items in the circuit, but don't show how the components are placed in a real circuit, nor do they always include every detail (e.g. power connections are sometimes ignored). You should also know that electronic systems process information in the form of patterns of electrical voltage & current. These signals must have a non-zero power, which means that both the voltage and the current must be non-zero. Since all voltages are potential differences, signals require pairs of wires or connections which form loops around which signal currents can flow. It should now also be clear that systems which amplify signals require a power supply, and that this usually means connecting both power lines or wires and 0 Volt/Earth wires.
Content and pages maintained by: Jim Lesurf (jcgl@st-and.ac.uk)
using HTMLEdit2 on a StrongARM powered RISCOS machine.
University of St. Andrews, St Andrews, Fife KY16 9SS, Scotland.