We have been talking about thermostats and the fact that the heating business is really the comfort business. A thermostat is one of the three components of a basic control circuit (power supply, switch and load). Now let’s look at another of the three components — the power supply.
I want to talk about transformers. In any low voltage circuit, the transformer is the power supply. But first ...
My dad was an electrician. His idea of a great family vacation was to put us in a car with no a/c in August and visit power plants while driving cross-country from Ohio to Arizona. What I remember most about these trips is that they were hot.
This isn’t complaining. I’m just saying, boy, do I ever know where electricity comes from. I don’t think a lot of people know that. Actually, I don’t think many people care. But, to understand controls, it helps to know where electricity comes from.
Electricity comes from power plants, which are big places, usually far away. Or electricity comes from batteries, which are little things nearby. Power plant electricity is called AC, which stands for “alternating current.” It’s called that because in the electricity lab there’s a device called an oscilloscope, which to me always looked like my grandma’s old radio with the big knobs.
The oscilloscope has a screen with a graph paper-like grid on it that shows what electricity looks like. AC makes a sine wave, which is a squiggle that alternates equally above and below a base line. Thus, we have alternating current.
Battery electricity is called DC, for “direct current.” On the oscilloscope, it makes a straight line. Direct, get it?
AC/DC
So now you know the difference between AC and DC, if you didn’t already. If you see the notation VAC on a wiring diagram, you know it means volts alternating current, not vacation or vacant or vacuum.In controls circuits, we almost always use AC. That’s the stuff that comes out of the wall socket or the service box.
There’s an interesting story about how AC came to be the standard. Allegedly back before there was a national standard for electricity, Misters Edison and Westinghouse were locked in a competitive battle. Edison was fighting for AC, and Westinghouse was pushing for DC.
Edison, ever the inventor, came up with the perfect device to prove his point — the electric chair. He invented the electric chair to use DC. Then he argued that AC was preferable because it didn’t kill people like DC did. (And we think competition is ruthless now!)
As it turns out, DC wouldn’t have worked in our cross-country distribution system anyway.
Power plants make “huge” amounts of electricity to distribute over huge distances. As it travels toward its destination, it is “stepped down” several times along the way by transformers. The gray barrel-like things we see hanging high on electric poles are one type of transformer. We use a very small one for control circuits. More on that soon.
Electricity from the power plant is called “line voltage.” When we get it, it has been stepped down to voltages such as 240, 220, 120 or 110. By the time it gets to us it’s called “house current.”
Line voltage has polarity. You may be familiar with the “hot leg” vs. the “neutral leg” of line voltage. The hot leg hurts to touch, but the neutral leg doesn’t. The terminology is “L1” for hot and “L2” for neutral.
Here’s what’s going on: Picture an electricity train coming from the power plant. The train is full of big, powerful electricity. That’s the hot leg.
When the electricity train gets to its destination, the electricity is unloaded and used in lights and motors and things. The electricity energy is transformed into another form of energy (light, motion). Now the electricity train is empty of energy, and it has to go back to the power plant to be reloaded. The electricity train going back is the neutral leg.
Line voltage, though, doesn’t work really well for control circuits. The big reason is that comfort issue I keep talking about. Low-voltage controls make it possible to control within that two-degree temperature range, which is the definition of heating comfort. Of course, there are line voltage controls, but the temperature swing is bigger, which means less comfort.
Stepping Down
So in the quest for comfort, we transform line voltage to low voltage using a small transformer to “step down” from 120V to 24V. This is called a step-down transformer. Transformers also come in “step-up” versions, but we don’t have much use for them in our business.The device for changing between AC and DC, by the way, is called a converter. We don’t use that much either.
A transformer consists of two “sides” and a “core.” On a brand-new transformer, the side with the wires hanging from it is the “primary.” Like kids go to primary school first, electricity first goes to the primary side of the transformer. The primary is made up of a coil of wire. It’s wound many times around a spool, or bobbin, like a spool of thread. The electricity simply enters one end of the coil, goes around and around, and leaves.
What the primary accomplishes is to create magnetism. Hold that thought.
The middle, heavy part of a transformer is called the core. A transformer is one of the things left in the world that really is better the heavier it is. Heavier means more iron, and more iron means more ability for the core to transfer magnetism from the primary side to the secondary side of the transformer.
The other side of the transformer is called the secondary. When new, this side usually has just two screw terminals, no wires. There’s a coil of wire on the secondary side, too, but there are many fewer windings. There are actually about one-fifth as many. This difference is how voltage gets stepped down.
Let’s say there are 500 windings on the primary side. If the line voltage coming into the primary is 120V, those windings transform that into 120 volts worth of magnetism.
The iron core transfers that magnetism for the secondary to pick up. On the secondary there are one-fifth as many windings, say 100. Even though 120 volts worth of magnetism are available in the core, the secondary can only pick up one-fifth of it. To see how much voltage that is, divide 120V by 5 and you get 24V. This is called a ratio of 5:1.
Did you ever wonder why line voltage seems to end in nice round zeros (110, 120, 220, 240), and then it goes to 24V for low voltage? It’s because 120V divided by 5 is 24V.
Transformers come in a lot of different shapes and sizes. For low-voltage control circuits, we need a transformer that will take whatever line voltage we have available and turn it into 24 volts. So if there’s 120V available, we want a 120V primary. If you want to be ready for any situation, a “multi-tap” transformer lets you use a variety of different line voltages by selecting the correct wires on the primary.
The second sizing factor is “VA,” which amounts to how much 24-volt electricity you can get out. A transformer has a rating of say 20VA or 40VA. The total VA of the loads you connect to the transformer must not exceed the VA rating of the transformer.
Most of us have a bit of the rebel in us and sometimes we just have to push these limitations. What happens if you exceed the VA rating? Why does it seem like sometimes we can get away with it and sometimes we can’t? Tune in next month for the answer to that mystery.
In the meantime here’s a transformer joke: Why do transformers hum?
Answer: Because they don’t know the words.
