Figure 1

Figure 2

## Power Supplies

There are two different types of power supply: line voltage (usually 120V) and low voltage (24V). Boilers, furnaces and air conditioners run on line voltage. Their controls usually run on low voltage. There's a different symbol for each type. Figure 2 is the symbol for a line voltage power supply. It's easiest to just accept this as the symbol for where line voltage comes from, and let it go at that. But many of us want to know a little more. So here's what this symbol is about. There are two arrows pointing in the same direction. What are they pointing to? Intuitively, we want them to point a direction for the electricity to go. But if you try to make sense out of that idea, it just doesn't work. I've asked many engineers and no one knows for sure why that symbol is the way it is. My electrical engineer buddy, Mike, said he never thought about it - that's just the way it is. But when pressed about what it might be, he said he thinks the arrows represent the male side of a connection. They're like the plug on the end of the toaster cord; they show where the circuit plugs in. But plug into what? Before I got Mike's conjecture, I made up my own explanation. It works, too. Those two arrows are pointing to where the electricity comes from - the power plant. In real life, the connection is simply going to be to where electricity enters the building. Sometimes, but not always, the two lines from the arrows have “L1” written on one, and “L2” on the other. L1 is the “hot side” or “hot leg.” L2 is “neutral.”

Figure 3
Even though they're not always labeled, in real life it can be important to know the difference. L1 is the wire that can hurt you, and L2 isn't. The switch in the circuit (Figure 3) should be placed in the L1 leg because we need to be able to turn off the hot electricity before it gets into the circuit. If the circuit is low voltage (24V) rather than line voltage, the power supply will be a transformer. Notice that the two arrows are still there from the line voltage symbol. That side of the transformer is connected to line voltage. The left side of the transformer is low voltage (magically “transformed” inside the transformer). This second side is called the “secondary,” and it is the source of electricity for a low-voltage circuit.

Figure 4

## Switch

The second symbol found in a circuit is a switch. Figure 4 shows symbols for a switch. A switch may be shown either “open” or “closed.” Open is just like a drawbridge. That means it's off, because there's no path for the electricity to go on.

Figure 5