So far in this column you've read that you -- not an electrician -- should be doing your controls wiring. I have written about the fact that a control circuit is a simple thing. It is a "circle" of at least one of each of these three things: power supply, switch and load.
With a control circuit we're trying to make things go on and off at the right times. That's done with switches.
Everyone's used to the idea that you turn something on and off with a switch. Every control circuit must have at least one switch. Many familiar controls -- thermostats, aquastats, humidistats -- are simply switches. In fact, anything that contains the letters "-stat" is probably a switch.
If you come from plumbing, it may seem odd that a switch is closed when it's on, and open when it's off. That can seem backwards because with valves, open means that stuff flows, right? Closed means that stuff stops. Switching action is the opposite of valve action.
Inside A Simple SwitchA switch is like a drawbridge. Picture the drawbridge closed. There's a path for the electricity to flow on. Then picture the drawbridge open. There's a break in the path, and the electricity cannot flow.
Here's my electricity drawbridge fantasy. It's a glorious sunny day. The local track team made of fast, hot little electricity guys is out for its daily run. In fact, they're wearing tiny electric blue jogging shorts that glisten in the sun. Every day they start out and make a circuit through the town, crossing the bridge, back to where they started. They trot along effortlessly, hardly even varying their pace.
Then one day the bridge is up. They have to stop and can go no further. Until the bridge goes back down, they can keep jogging in place. Or they can sit down and watch the birds fly by. But they can't go around, and they can't go over the bridge until it closes. But as soon as the bridge closes, they start up again and complete their circuit back to where they started.
Well, if switches are like drawbridges, then they're pretty simple. Then along comes our electrician buddy, and he says, "OK plumber dude, you think you know so much, is that switch of yours a SPST or SPDT?" Isn't that how it goes? As soon as you think you know something, some guy's gotta point out that you don't!
What is this four-letter switch stuff?
Gone FishingSwitches are sometimes identified with a four-letter code. S stands for single, D for double, P for pole, and T for throw.
You already understand single and double.
But before we go any farther, let's go back to the drawbridge. The drawbridge makes me think of a river. A river makes me think of fishing. And fishing makes me think of a fishing pole. The pole in a switch is a lot like a fishing pole.
A pole connects two points. If you hold one end of a fishing pole and your buddy holds the other, we're talking about a single pole. If you have two more buddies standing beside the two of you, and a separate pole connects them, you have double poles. There's no connection between the two poles. The pole situation is the same in switches. But in switches, the two poles have to connect or disconnect at the same time.
Keep thinking about fishing. Now let's talk about throws. You're holding the pole. When you cast, or throw, so that the pole connects you and your buddy, that's single throw. If you then cast behind and connect with another buddy, that's double throw. The rule is that you can connect with one buddy or the other, not both at once. The same rule applies to poles and throws in switches.
Four-Letter WordsPut all of this together, and you have four possible combinations of poles and throws--SPST, SPDT, DPST and DPDT. Don't sweat these. In control work almost all our switches are SPST. That's the simplest type, just like a light switch. We turn one thing on or we turn that same thing off -- single pole single throw (SPST). Turning it on is making the connection, which we call the throw. Turning it off is removing the connection.
Let's talk about that in another way. The 1950s and'60s were an SPST world. When I was a kid, my grandma lived in an old wooden farmhouse with an old wooden porch and an old wooden screen door. On Sunday afternoons, the adults would sit inside and talk and talk and talk and I would get restless and b-o-r-e-d. Finally Grandma would say, "Why don't you go out and play on the porch?" I'd head for the door.
It was one of those floppy ones with the spring attached. I'd push the door open, go through it, let go, and it would slam shut with a "ka-whap." Hey, that's fun, my kid brain noticed. Let's do that again. From the outside, I'd pull the door open again, and let it go. Ka-whap. Next time I'd pull it open even farther. Ka-whap it would go, even louder. I'd keep that up until a grownup voice from inside yelled, "Stop slamming that door!" The switching action on that door was single pole, single throw. I applied power to open and hold it open. When I let go, I removed power and the spring slammed the door shut. That was a simple SPST world.
My own kids grew up in an SPDT world of doors. With air conditioners and airtight houses, the old wooden screen door with the spring became a thing of the past. Regardless of the season, the solid front door is to be shut. You open it, and (supposedly) you close it behind you. Power open, power closed. One throw to power it open. The other throw to power it closed. Instead of, "Stop slamming that door!" my kids heard, "Shut that darn door, will ya?"
Committee MeetingNearly all of the switches we use in control circuits are SPST. The switches are very simple. Yet switching can appear to be very complex. In fact, switching is probably what makes controls seem complicated. How can a simple thing be complicated? Switching gets complicated because we often use many switches in a single circuit. That's because we want many conditions to be just right before the circuit can operate.
I think of switches in a control circuit as a committee. Each switch is in charge of making a particular decision. The thermostat is in charge of deciding if heat is needed. The high-limit switch (probably an aquastat) is in charge of whether or not the temperature inside the equipment is too hot. The low-water cutoff is in charge of making sure there's enough water in the boiler.
Like in a company or a family, if there is just one decision-maker, it's easy to understand where the decision is coming from. But if there is a committee where all have to agree, it gets harder to understand how a decision got made.
Now picture those controls -- the thermostat, high limit and the low-water cutoff -- all standing in a row. In technical talk, this would be called in series. The question up for vote is, shall we bring on the burner? All three have to vote yes for it to happen: yes, there's a need for heat; yes, it's safe to bring on the burner because it's not already too hot in the equipment; and yes, there's enough water in the boiler.
Ah, yes, one more piece of complexity. Ever work with a group of decision-makers when you don't know who they are or how many there are? That happens with controls too. And the bottom line is, once you figure out who they are, folks is just folks, and switches is just switches.