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 ceramic.
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