Building simple
resistor circuits
In the course of learning about electricity,
you will want to construct your own circuits using resistors
and batteries. Some options are available in this matter of
circuit assembly, some easier than others. In this section,
I will explore a couple of fabrication techniques that will
not only help you build the circuits shown in this chapter,
but also more advanced circuits.
If all we wish to construct is a simple
single-battery, single-resistor circuit, we may easily use
alligator clip jumper wires like this:
Jumper wires with "alligator" style spring
clips at each end provide a safe and convenient method of
electrically joining components together.
If we wanted to build a simple series
circuit with one battery and three resistors, the same
"point-to-point" construction technique using jumper wires
could be applied:
This technique, however, proves impractical
for circuits much more complex than this, due to the
awkwardness of the jumper wires and the physical fragility
of their connections. A more common method of temporary
construction for the hobbyist is the solderless
breadboard, a device made of plastic with hundreds of
spring-loaded connection sockets joining the inserted ends
of components and/or 22-gauge solid wire pieces. A
photograph of a real breadboard is shown here, followed by
an illustration showing a simple series circuit constructed
on one:
Underneath each hole in the breadboard face
is a metal spring clip, designed to grasp any inserted wire
or component lead. These metal spring clips are joined
underneath the breadboard face, making connections between
inserted leads. The connection pattern joins every five
holes along a vertical column (as shown with the long axis
of the breadboard situated horizontally):
Thus, when a wire or component lead is
inserted into a hole on the breadboard, there are four more
holes in that column providing potential connection points
to other wires and/or component leads. The result is an
extremely flexible platform for constructing temporary
circuits. For example, the three-resistor circuit just shown
could also be built on a breadboard like this:
A parallel circuit is also easy to construct
on a solderless breadboard:
Breadboards have their limitations, though.
First and foremost, they are intended for temporary
construction only. If you pick up a breadboard, turn it
upside-down, and shake it, any components plugged into it
are sure to loosen, and may fall out of their respective
holes. Also, breadboards are limited to fairly low-current
(less than 1 amp) circuits. Those spring clips have a small
contact area, and thus cannot support high currents without
excessive heating.
For greater permanence, one might wish to
choose soldering or wire-wrapping. These techniques involve
fastening the components and wires to some structure
providing a secure mechanical location (such as a phenolic
or fiberglass board with holes drilled in it, much like a
breadboard without the intrinsic spring-clip connections),
and then attaching wires to the secured component leads.
Soldering is a form of low-temperature welding, using a
tin/lead or tin/silver alloy that melts to and electrically
bonds copper objects. Wire ends soldered to component leads
or to small, copper ring "pads" bonded on the surface of the
circuit board serve to connect the components together. In
wire wrapping, a small-gauge wire is tightly wrapped around
component leads rather than soldered to leads or copper
pads, the tension of the wrapped wire providing a sound
mechanical and electrical junction to connect components
together.
An example of a printed circuit board,
or PCB, intended for hobbyist use is shown in this
photograph:
This board appears copper-side-up: the side
where all the soldering is done. Each hole is ringed with a
small layer of copper metal for bonding to the solder. All
holes are independent of each other on this particular
board, unlike the holes on a solderless breadboard which are
connected together in groups of five. Printed circuit boards
with the same 5-hole connection pattern as breadboards can
be purchased and used for hobby circuit construction,
though.
Production printed circuit boards have
traces of copper laid down on the phenolic or fiberglass
substrate material to form pre-engineered connection
pathways which function as wires in a circuit. An example of
such a board is shown here, this unit actually a "power
supply" circuit designed to take 120 volt alternating
current (AC) power from a household wall socket and
transform it into low-voltage direct current (DC). A
resistor appears on this board, the fifth component counting
up from the bottom, located in the middle-right area of the
board.
A view of this board's underside reveals the
copper "traces" connecting components together, as well as
the silver-colored deposits of solder bonding the component
leads to those traces:
A soldered or wire-wrapped circuit is
considered permanent: that is, it is unlikely to fall apart
accidently. However, these construction techniques are
sometimes considered too permanent. If anyone wishes
to replace a component or change the circuit in any
substantial way, they must invest a fair amount of time
undoing the connections. Also, both soldering and
wire-wrapping require specialized tools which may not be
immediately available.
An alternative construction technique used
throughout the industrial world is that of the terminal
strip. Terminal strips, alternatively called barrier
strips or terminal blocks, are comprised of a
length of nonconducting material with several small bars of
metal embedded within. Each metal bar has at least one
machine screw or other fastener under which a wire or
component lead may be secured. Multiple wires fastened by
one screw are made electrically common to each other, as are
wires fastened to multiple screws on the same bar. The
following photograph shows one style of terminal strip, with
a few wires attached.
Another, smaller terminal strip is shown in
this next photograph. This type, sometimes referred to as a
"European" style, has recessed screws to help prevent
accidental shorting between terminals by a screwdriver or
other metal object:
In the following illustration, a
single-battery, three-resistor circuit is shown constructed
on a terminal strip:
If the terminal strip uses machine screws to
hold the component and wire ends, nothing but a screwdriver
is needed to secure new connections or break old
connections. Some terminal strips use spring-loaded clips --
similar to a breadboard's except for increased ruggedness --
engaged and disengaged using a screwdriver as a push tool
(no twisting involved). The electrical connections
established by a terminal strip are quite robust, and are
considered suitable for both permanent and temporary
construction.
One of the essential skills for anyone
interested in electricity and electronics is to be able to
"translate" a schematic diagram to a real circuit layout
where the components may not be oriented the same way.
Schematic diagrams are usually drawn for maximum readability
(excepting those few noteworthy examples sketched to create
maximum confusion!), but practical circuit construction
often demands a different component orientation. Building
simple circuits on terminal strips is one way to develop the
spatial-reasoning skill of "stretching" wires to make the
same connection paths. Consider the case of a
single-battery, three-resistor parallel circuit constructed
on a terminal strip:
Progressing from a nice, neat, schematic
diagram to the real circuit -- especially when the resistors
to be connected are physically arranged in a linear
fashion on the terminal strip -- is not obvious to many, so
I'll outline the process step-by-step. First, start with the
clean schematic diagram and all components secured to the
terminal strip, with no connecting wires:
Next, trace the wire connection from one
side of the battery to the first component in the schematic,
securing a connecting wire between the same two points on
the real circuit. I find it helpful to over-draw the
schematic's wire with another line to indicate what
connections I've made in real life:
Continue this process, wire by wire, until
all connections in the schematic diagram have been accounted
for. It might be helpful to regard common wires in a
SPICE-like fashion: make all connections to a common wire in
the circuit as one step, making sure each and every
component with a connection to that wire actually has a
connection to that wire before proceeding to the next. For
the next step, I'll show how the top sides of the remaining
two resistors are connected together, being common with the
wire secured in the previous step:
With the top sides of all resistors (as
shown in the schematic) connected together, and to the
battery's positive (+) terminal, all we have to do now is
connect the bottom sides together and to the other side of
the battery:
Typically in industry, all wires are labeled
with number tags, and electrically common wires bear the
same tag number, just as they do in a SPICE simulation. In
this case, we could label the wires 1 and 2:
Another industrial convention is to modify
the schematic diagram slightly so as to indicate actual wire
connection points on the terminal strip. This demands a
labeling system for the strip itself: a "TB" number
(terminal block number) for the strip, followed by another
number representing each metal bar on the strip.
This way, the schematic may be used as a
"map" to locate points in a real circuit, regardless of how
tangled and complex the connecting wiring may appear to the
eyes. This may seem excessive for the simple, three-resistor
circuit shown here, but such detail is absolutely necessary
for construction and maintenance of large circuits,
especially when those circuits may span a great physical
distance, using more than one terminal strip located in more
than one panel or box.
-
REVIEW:
-
A solderless breadboard is a device
used to quickly assemble temporary circuits by plugging
wires and components into electrically common spring-clips
arranged underneath rows of holes in a plastic board.
-
Soldering is a low-temperature
welding process utilizing a lead/tin or tin/silver alloy
to bond wires and component leads together, usually with
the components secured to a fiberglass board.
-
Wire-wrapping is an alternative to
soldering, involving small-gauge wire tightly wrapped
around component leads rather than a welded joint to
connect components together.
-
A terminal strip, also known as a
barrier strip or terminal block is another
device used to mount components and wires to build
circuits. Screw terminals or heavy spring clips attached
to metal bars provide connection points for the wire ends
and component leads, these metal bars mounted separately
to a piece of nonconducting material such as plastic,
bakelite, or cerami
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