Reading Schematic diagrams
This page was created in response to all the email I receive, on how to
read a schematic. Learning to read a schematic diagram, is similar to
map reading. You need to know which wires connect to which component
and where each wire starts and finishes. With a map book this would be
equivalent to knowing your origin and destination points and which
roads connect to the motorway network, etc. However schematics are a
little more complicated as components need to be identified
and some are polarity conscious (must be wired up in the circuit the
correct way round) in order to work. You do not need to understand
what the circuit does, or how it works, in order to read it, but you do
need to correctly interpret the schematic. Here are some basic rules
that will help with reading a diagram. Look at the circuit diagram
shown below:
The blue lines
represent wires and for simplicity i have labeled them as A,B,C. There
are just three components here and it is easy to see
where each wire starts and ends, and which components a wire is
connected to. As long as the wire labelled A connects to the switch and
negative terminal of the battery, wire B connects to the switch and
lamp, and C connects to the lamp and the battery positive terminal then
this circuit should work.
Before moving on, it is important to realise that any
schematic may be drawn in a number of different ways.
In Fig 1 and Fig 2 i have drawn two electrically equivalent lamp dimmer
circuits, they may look very different, but in fact, if you mentally
label the wires
and trace them, you will see that in both diagrams each wire starts and
finishes at the same components on both diagrams. The components have
been
labelled and so have the three terminals of the transistor.
In Fig1 there are two wire junctions as indicated by a
"dot". A wire cpnnects from battery positive to the C (collector)
terminal of the transistor, and also
a wire runs from the collector terminal to one end of the
potentiometer, VR1. The wires could be joined at the transistor
collector, battery positive or even
one end of the potentiometer, it does not matter, as long as both wires
exist. Similarly, a wire runs from battery negative to the lamp, and
also from lamp
to the other end of VR1. The wires could be joined at the negative
terminal of the battery, the lamp, or the opposite tag of VR1. In
drawing Fig 1, I could have drawn the wires from the lamp and bottom
terminal of VR1 back to the battery negative terminal and placed the
dot there, it would still be the same.
If you now look at Fig 2, you will see that one wire junction appears
at the negative battery terminal, the other junction in a similar
place.
Circuit Layout
Sometimes the way a circuit is wired up may
compromise its performance. This is particularly important for high
frequency and radio circuits, and
some high gain audio circuits. See the diagram below:
Although this audio circuit has a voltage gain of less
than one, wires to and from the transistor, should be kept as short as
possible. This will prevent a
long wire picking up radio interfereance or mains hum from a
transformer. Also, in this circuit input and output terminals have been
labelled and a
common reference point or earth is indicated. The earth terminal would
be connected to the chassis or metal framework of the enclosure in
which this
circuit is built. Many schematics contain a chassis or earth point.
Generally its just to indicate the common reference terminal of the
circuit, but in radio work, the earth symbol usually requires a
physical connection to a cold water pipe or an earth spike buried in
the soil.