Series Circuits

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 need.

 

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 it.

 

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 the filament.

 

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 series.

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