As I mentioned in my last post, I’m lucky enough to have a large heated wood shop to work in during the brutal midwestern winters.
In this post, I keep my promise to let you know how I control the loose cannon in my shop, which is my “vintage” tube-style heater. Just imagine this heater, but much older and very dirty.
Let’s start this discussion off right, with a reference to Pop Culture.
Perhaps you’ve heard a certain heriess mutter the words…
It is a wonder to me how words so simple were used to fan the flames of fame.
Today we’re going to look at a few neat ways that thermostats detect when things get “Hot…”.
First we need to convert “That’s Hot…” back to its original meaning. No fashion opinions here. We just want to detect the fact that the temperature has risen. For that we need devices that change in response to a change in temperature, or, in engineering lingo, Delta T.
Delta T (ΔT)
As a woodworker, we learn about wood movement and how the dimensions of a piece of lumber are affected by humidity. Here is a link to a great webpage explaining wood movement.
Electronics also change in response to humidity, but not as dramatically as they do to changes in temperature.
Just as we choose species and cuts of wood for specific project pieces based on our knowledge of lumber, temperature sensitive component engineers choose materials based on their knowledge of that specific materials response to Delta T. These devices often pair one material that does not change much in response to temperature with another that changes dramatically. Since they are constructed with two different metal alloys, these materials are referred to as “Bi Metal Clad” materials.
The iconic “The Round” thermostat by Honeywell is a good place to start a discussion of thermostats.
Jammed within this classy and smooth case is not one, but TWO bi metal coils. One to display ambient temperature. The other Bi Metal coil is where the magic happens within this beauty (I seem to have lost it, calling a thermostat “beauty”). The other coil is fitted with a mercury bulb switch on the end. As the coil shrinks and expands, the mercury travels around within the bulb like the fluid in a levels vial.
Contacts within the bulb are closed by the conductive mercury when the bulb is at the correct angle. This triggers additional electronics to control the heat source. The temperature is set on this thermostat by rotating the Bi Metal coil.
Here is an image showing the movement vector of a Bi Metal coil.
The mercury vial would be attached to point A in this illustration.
This webpage shows many other Bi Metal designs that create motion in different vectors.
Pros / Cons
This thermostat technology is limited to stationary applications. Perfect for home installation, but today we are looking for a thermostat that can be installed onto a mobile heater.
Mercury is a hazardous material, so why keep it around?
Digital thermostats like this one use a Thermistor to sense temperature. A Thermistor (wiki link) is a small electronic device that changes resistance based on ambient temperature.
To understand how a Thermistor works, we need to dig deep down into electronics theory, all the way down to the structure of an atom. Too deep for ya, ya me too. So here is a campfire story to sum it up.
Atoms are constructed of three parts.
The first two parts protons (positively charged) and neutrons (neutral charge), sit in the atoms nucleus. Think of this like a campfire on the 4th of July. If your family is like mine, the parents sit around the fire most of the time and chat. These parents are the nucleus.
Running around this nucleus of parents are their kids. These are the negatively charged electrons. The kids are bonded (or attracted to, because opposite poles attract) to their parents at different levels, depending upon how close it is to supper time. Mostly, these electrons are just spinning around, waiting for something with some energy to excite them.
All of the sudden, BANG! Energy is introduced into the atom structure in the form of a fireworks display.
The kids are much more drawn to the sight of explosions and off they go. These electrons have just jumped from the atom’s valance band to the conduction band.
Back to our Thermistor example, when the atoms of a Thermistor get excited by a source of heat, conduction between the atoms goes up, causing the resistance of the device to go down. A device with these properties is said to be a NTC or Negative Temperature Coefficient device, which is just a fancy way of saying resistance goes down when temperature goes up.
Digital thermostats are outfitted with a microcontroller that is used to monitor the Thermistors.
Pros / Cons
Digital thermostats are great. This technology can very accurately measure temperature, but this precision comes at a cost and that makes them a bit too expensive for my shop budget. After all, Im not after a specific temperature. Im just looking for not cold.
Another drawback of the last two thermostats is that they dont switch line voltage like we need to. So Ill kill any suspense now and tell you that I use this next thermostat style. It requires NO additional circuitry. In fact it doesnt use any electricity at all. Pretty neat, huh. Lets take a look.
Bi Metal Thermostat
This thermostat uses a clever mechanical adaptation of the Bi Metal expansion coefficient that almost directly controls our heat source.
Like the coil in “The Round” thermostat, this unit uses a movement vector to open and close a circuit.
The front view of the Bi Metal Clad plate can be seen in the picture below.
The plate has a set screw in the middle. The tip of the set screw is positioned directly over a high voltage / high current switch.
When the ambient temperature rises, the plate flexes downward. When it is flexed enough, the heater is switched OFF.
Temperature switch point is adjusted by rotating the dial, which has a cam profile on the underside. As you move the dial towards a lower set point, you push that end of the cantilevered clad plate toward the switch.
One thing to note is that although this is a simple device, just like the more complex options above, it needs to be wired according to the manufacturer’s instructions. Red to line, or source voltage, and Black to load, or the heater.
Here is a link to the manufacturers website.
Knowledge is Power
So much of both woodworking and engineering is rooted in knowing the characteristics and properties of the materials and using that knowledge to make things happen.
In the next post, I’ll show you how to wire this device in line with a heater.
Id like to know what heaters and thermostats you are using in your shops. Leave a comment below.