Thermostatic Control of Heat tape

[Hoomi]

Basic Wiring Sketch using standard thermostat and Solid State Relay

Some very basic electrical principles:

Electricity is commonly measured by three criteria: Voltage, Current, and Power. Easiest way to think of these is by comparing to a water pipe. Voltage equates to water pressure. It measures how much "force" is behind the electricity. Current, logically, equates to the flow. We might measure water in Gallons per minute, or some similar unit, and we measure electricity in amps. Power is the combination of the two. Mulitply the voltage times the amps, and you get the watts. Hence, if you have a light bulb in your car which is drawing 10 amps at 12 volts, you have a 120 watt bulb. Electricity MUST have a complete circuit for the system to work. It isn't enough to supply the electrical power to a device. You also have to provide a "return" path for the electrical current. That's why your heat tape or UTH has two wires connected to it, with a two prong plug.

That should be enough basic theory for what we're looking at here.

In the drawing above, we start out with two wires leaving the AC Adapter. This is a basic AC Adapter like you would use with a personal electronic device, and for our purposes, we'll use one with a 6 VDC output. One wire from the adapter goes directly to the Solid State Relay control connection. The control connection will most likely be "polarized", in that you need to observe the "plus" and "minus" of the voltage. One other note on this: Most Solid State Relays use such a low current draw that a battery pack could be substituted for the AC Adapter. Using 4 AA batteries wired for 6 VDC output (essentially wiring them end to end) will likely last more than a year controlling a device like this.



A Pair of Solid State Relays

As you can see in the picture, each Solid State Relay has 4 screw contacts on top. One pair is labeled either "Input" or "Control", and the other is labeled either "Output" or "Load". We're going to connect the plus voltage from either our battery or adapter to the screw labeled '+' on the input or control side. The other wire from either the adapter or battery pack will go to one connection on the thermostat. We'll connect the other connection on the thermostat to the other "input" or "control" screw on the Solid State Relay. When the thermostat drops below the set temperature, it will connect the two wires together, applying the "minus" side of the electricity to the relay. This tells the Solid State Relay to "turn on".

Looking back at the drawing, you'll see that we have an household style electrical plug at the lower left. This is our supply power for the heat tape. The "Hot" side of this connection will go to one side of the "Load" or "Output" screws of the Solid State Relay. The "neutral" line from the electric plug will go to the neutral connection on the electric socket we're going to plug the heat tape into. The "hot" side of the socket will be connected to the last remaining connection on the Solid State Relay. If you're not sure of how to determine "hot" and "neutral" on your household sockets, the normal convention is that the taller slot is neutral. If you're not certain if your house is wired this way, you can pick up an inexpensive checker at Sears which will indicate how it's wired.

How it works, wrapped up:

When the thermostat registers a temperature below the desired setting, it connects the two wires for the furnace control together internally, allowing the electricity in the one to continue on its way through the other. This electricity, when applied to the Solid State Relay, tells the SSR to "connect" the two "output" or "Load" connections together, allowing the household electrical current to flow to your heat tape. When the temperature rises above the setting point, the thermostat will disconnect the two wires from each other, which in turn will tell the SSR to disconnect the electrical current from the heat tape.

Solid State Relays can be purchased from places like Graingers or other electrical supply houses.

How it compares to a Rheostat

A thermostat and a rheostat are two completely different methods of controlling heat. The thermostat uses the sensed temperature to determine whether to turn on the heat or to turn it off. A rheostat, on the other hand, does not monitor the temperature (unless it's a thermostatically controlled rheostat). What a rheostat does is to control the amount of electrical current applied to the heat tape or UTH. Think of your rheostat as the electrical equivalent of a variable water valve. You set it for a certain amount of electrical flow, and that's where it stays until YOU change it again. Heat tape and UTH's are devices which convert electrical current into heat. If you lower the amount of current passing through them, you lower the amount of heat they produce. Your heat tape or UTH will carry a wattage rating. Remember our theory above. Wattage is a product of voltage times current. Lower either one, and you lower the product unless you raise the other to compensate.

Hopefully that makes things clearer. If not, feel free to fire away with questions, and I'll do my best to answer them in non-technician language.

[BallKingdom]

Top of the line work Hoomi, I'll make it a sticky later.

[RPlank]

Aaaaahhhhhhhh..........

Electricity for Dummies! Thanks, Rick!
There is now a light bulb turned on over my head! :idea: (Not that I know how it's wired, but it's there!)

I think even I can manage to put that together! Thanks for taking the time to write this post, and to make it understandable, even to me! (The pic helped a lot!) Great post!!!!!

[Wizill]

That's really cool Hoomi. Randy is right, great post.