|by Carol Fey
Or, why we don’t need all that math from electricity class.Over
the past year in this column, we’ve talked about the basic components
of a circuit. Now let’s talk about how they’re wired together
into a circuit.
If you have one of each of the basic components of a circuit
(power supply, switch and load), there’s only one way to wire
them — connect all three together in a circle. It doesn’t matter
what order they go in.
When you add loads or switch components (one power supply, two
switches and two loads, which might be a transformer, thermostats
and zone valves), there are several different ways they could
be wired together. How you wire them makes a huge difference
in how the circuit performs.
There are several different names for circuit wiring configurations.
The two that we commonly use in control circuits are “series”
and “parallel.” Let’s look at the series circuit.
Series means a group of things strung together in a row. For
example, the World Series is a string of baseball games. Series
wiring puts controls together in a row.
A circuit might be a transformer, a thermostat and two zone
valves (a power supply, a switch and two loads). In this case
we say the loads are “in series.” Because of that, the whole
circuit is a series circuit. The components can be in any order
so long as one control is connected to the next, is connected
to the next, and the last is connected back to the first.
Let’s take a moment to note that we think of basic controls,
such as the ones just mentioned, as having two “sides.” Each
has either two wires or two screw terminals coming out of it.
(If you’re thinking of controls with more than two wires, forget
about them and think of controls with only two.)
On the control, for the most part, it doesn’t matter which we
call the “first terminal” or the “second terminal.” It’s just
important to distinguish between the two. So, the second terminal
of the first control is wired to the first terminal of the second
control. The second terminal of the second control is wired
to the first terminal of the third control. And so on. For the
last control, the second terminal is wired to the first terminal
of the first control.
Recall from an earlier column about switches (“Switches Is Just
Switches,” March 2003) that the switch turns the electricity
on and off in the circuit. No matter where the switch is placed,
it turns off the electricity to the whole circuit. That means
that both loads are controlled simultaneously by that switch.
Picture a series circuit of power supply-switch-load-switch-load,
and back to power supply. It might be tempting to think that
each switch controls one load, and not the other. That’s not
the case because any switch that’s turned off turns off the
electricity to the whole circuit.
That brings up the fact that there are several disadvantages
of series circuits for wiring controls. In control work, it’s
usually a bad idea to have loads in series.Wasted Math
If you’ve taken an electricity class, chances are you either
dropped out or did a fair amount of math with fractions. Some
of this math was probably about Kirchoff’s Law. Kirchoff’s laws
let you do math to predict how loads in series will perform.
Whether or not you made it through the math, you probably noticed
that you don’t often use it on the job. That’s because in control
work, we almost never put loads in series. That math is for
folks who do electronics — putting resistors, transistors and
diodes in series to make all kinds of magic happen.
This doesn’t mean that series circuits are all bad for controls.
We use switches in series to great advantage. In fact, the whole
concept of safety controls is mostly about switches in series.
We’ll look at that in a future column.
Let’s go back to loads in series. The classic example is, you
guessed it, holiday tree lights. I’m talking about the inexpensive
sort that we had up until a few years ago. When one burnt out,
the whole string went out. That meant you had to figure which
was burned out and replace it before any would work again. (I
was one of those folks willing to sit down under the tree and
try a new bulb in each socket. And of course it took all night
because there was more than one burned out.)
The reason the whole string went out was that the filament in
the bulb was like a switch. If it broke, it opened the circuit.
Like an open (turned off) switch, it kept the electricity from
the whole circuit.
Here’s a thought, just in case you’re wondering. Why wouldn’t
the bulbs “before” the burned out bulb light anyway? The reason
is that electricity has to complete the circuit and get back
to where it came from before any load works. A break in the
circuit is like unplugging the whole string of lights.
Today’s tree lights are not wired in series. They’re wired in
parallel. But that’s a future column.Path Of Least Resistance
Besides the fact that one burned-out (open) load disables the
whole circuit, there are other disadvantages of loads in series.
Let’s look further.
Imagine wiring a circuit of a power supply, a switch and two
60W light bulbs. Turn on the power, turn on (close) the switch,
and the bulbs come on. But something’s wrong. Those don’t look
like 60W bulbs. They’re dim. They look like, maybe, 30W bulbs.
And that’s exactly what they represent. When bulbs are in series,
they have to share the available electricity. Since there are
two bulbs of the same wattage, they each get half of what they
The same thing would happen if the loads were something other
than light bulbs. None of the loads would get as much electricity
as they need to behave right.
Let’s go a step farther. We’ll replace one of the 60W bulbs
with a lesser wattage bulb. Let’s make it a 25W bulb. Power
the circuit, close the switch and what do you suppose happens?
Take a moment and guess.
Some of the possibilities are: both bulbs are the same brightness
as each other because each gets half of the electricity, or
the 25W is brighter, or — my personal favorite — the 60W is
brighter because “the big guy always wins.”
And what we see is (tah-dah) none of the above. We see that
the 25W bulb is on and the 60W bulb is off. Wait a minute! That
can’t be. Remember the holiday lights? When one’s out, the whole
string should be out.
If we look closer at the 60W bulb, though, we can see a tiny
bit of light across the filament. When we cup a hand around
the bulb, we feel heat, which means the bulb’s on. And, if the
60W bulb were truly out, when we unscrewed it, nothing would
change. But when we do unscrew it, the 25W bulb goes out. That,
too, proves that the 60W bulb has electricity going through
What’s going on here is explained by the simple statement, “Electricity,
like water and children, takes the path of least resistance.”
Most of the electricity will go through the 25W bulb because
it’s easier. That makes the 25W bulb its normal brightness.
But in order for the circuit to work, a little electricity has
to go through the 60W bulb. It’s not enough to make it bright,
but it is enough to create some heat and a slight glow across
This is a third illustration why we don’t put loads in series
in control circuits. Depending on the relationship between the
size of the two loads, one of the loads can look like it doesn’t
work at all. If you’re into troubleshooting by replacing parts,
you could replace that larger load as many times as the supplier
would put up with your returns, and it still wouldn’t work.
Look for next month’s column where we’ll discuss switches in