DIY Adjustable Temp Switches

This article was first published in 2003.

This is a story for those who like doing things differently, for those who like to make use of things that other people throw away. If you don't get a thrill - even a little technical one - from producing something yourself, then you probably won't be much interested. But if you want to save money and make some very useful devices, then this is for you!

So what are we talking about? Variable trip point temperature switches - often called electrical thermostats - that can be used to control radiator or intercooler fans, an intercooler water spray, illuminate over-temp and under-temp dash warning lights, drop boost when the intake air temp is high.... there is a bloody stack of uses!

For the cost of a box, a relay, a couple of LEDs, two resistors and a knob, you can have a professional looking, reliable and accurate adjustable temp switch that looks like it should have cost at least AUD$100. But divide that by about ten to get the real cost... Those parts still cost too much? Well, if you don't care what it looks like and you only want to switch something off when it gets too hot, forget those bits and you can have it at zero cost!

But where do you get these thermostats? Out of discarded electric frypans and woks, clothes irons, water bed heater controls - even electric hot water systems! Basically, if it is a consumer level device that has to hold a regulated temperature and which has been available for decades, it almost certainly uses a thermostat.

Bi-Metallic Strip Thermostats

What makes these thermostats so easy to use is that they are mechanical in nature. (As opposed, say, to a thermistor that works in conjunction with an electronic circuit). They way they work is this.

A special piece of metal - called a bi-metallic strip - is the foundation of the design. As its name suggests, this piece of metal is actually two pieces of different metal joined together. The two types of metal have very different expansion rates, so as they are heated, one of the pieces of metal gets longer faster than the other. This causes the strip to bend. When it has bent far enough (ie it's got hot enough!) it breaks the connection between two electrical contacts, turning off the circuit. In order that the temperature at which it switches can be adjusted, a threaded rod is used to vary the distance that the electrical contacts are from the bimetallic strip.

As you can imagine, in this type of design the electrical contacts open and close very slowly. In certain applications this could cause arcing, so some thermostat designs use a 'snap action' approach, where a small leaf spring causes the contacts to fly apart once the trip point has been reached.

The Different Sources

The thermostats from difference appliances have different characteristics and appearances.

• Frypan

Electric frypans (and electric woks) use a thermostat design that's integrated into the module that plugs into the handle. (That is, the box at the other end of the power cord from the wall plug.) The module has a knob on it (for setting the temp) and a stainless steel probe, about the length and a bit smaller in diameter than a little finger. When the module is plugged into the frypan, the probe slides into the body of the pan, allowing it to sense the temperature accurately. Also in this box are a neon bulb and the contacts that pass power to the frypan.

Typically, these designs are suitable for sensing temperature from about 40 - 200 degrees C. Their sensitivity is high (when set appropriately they can be triggered by body heat) and the probe design makes them suitable for sensing applications where the probe can be pushed into something (eg into intercooler fins) or through a grommet into a pipe (eg into the intake air stream). The hysteresis (difference between switch-on and switch-off temps) is fairly small and they react quite quickly.

Both 'click' and 'slow moving' types of thermostats are used in frypans, with later models more likely to use the 'click' type.

• Clothes Irons

Electric clothes irons also use bimetallic thermostats. They are buried right inside the iron - despite being controlled by a knob normally placed on the upper surface, one end of the strip is actually bolted to the inside of the aluminium baseplate of the iron. In all the irons I salvaged, the thermostats were of the 'click' type.

Thermostats from irons react more slowly than frypan thermostats. (This is good if spikes in temperature need to be ignored.) As a result their hysteresis is also larger. Typically, these designs are suitable for sensing temperature from about 60 - 200 degrees C. Because in the iron the whole thermostat is heated (although the bimetallic strip gets hotter more quickly than the rest, because it's actually bolted to the baseplate), this type of thermostat is useful where the temperature of the environment needs to be sensed - rather than the temperature of something within it. So the sensing of general underbonnet temperature, for example, or the sensing of the temperature in a closed boot in which an amp is working hard.

• Water Bed Heaters

Old water bed thermostats use a slightly different design - there's no bimetallic strip inside the controller. Instead a copper bulb is connected to the control box by a thin copper capillary tube. Inside the bulb and tube is a liquid that expands as it is warmed. This pressure acts on a diaphragm that in turns pushes on a click-action microswitch. (Note that modern water bed thermostats use a purely electronic design - the older type can be recognised by the copper tube and bulb.)

Because it's designed to keep a water bed at around 30 degrees C, if you want to sense temperatures around this area quickly and accurately, a water bed thermostat is ideal. It reacts very quickly and has low hysteresis.

Testing Testing the thermostat operation is important, especially if you want to see how quickly a thermostat reacts. One simple test that we did was to control a small PC fan with the thermostat, turning it on when the temperature rose to a certain level. The thermostat sensing probe was placed in the airflow generated by the fan, and then the whole assembly was positioned on top of a computer monitor, being subjected to the heat rising out of the monitor's vents. We were then able to assess how quickly the fan activated and then how quickly the thermostat cooled down before it shut off the fan. As a guide, the frypan thermostats would cycle the fan on about every 30 - 60 seconds, while the clothes iron thermostats did the cycle about every 3 minutes. The water bed thermostat was similar to the frypan designs. A heat gun or electric hairdryer can be used to produce quick changes of temp to see how quickly the thermostats react in rapidly changing conditions.

• Other Appliances

Electric storage water heaters use a mechanical thermostat, usually with clearly marked temperatures on them. They work over a range of 50 - 80 degrees C. They are designed to be bolted to the side of the water storage tank.

Electric toasters use a bimetallic strip system, but it doesn't usually lend itself to use in other applications.

Where Do You Get Them?

Obviously if you go out and buy a new iron or a water bed temp control or a frypan just to get the thermostat, you're nuts! Instead, be on the lookout for these items at zero or nominal cost. Municipal dumps, junk shops, garage sales - anywhere that people are getting rid of stuff that they regard to be of no further use.

It doesn't matter if the frypan handle is broken, it doesn't matter if the iron no longer produces steam, it doesn't matter if the water bed has a hole in it. In all cases where there's no obvious physical damage to the assembly housing the thermostat (or in the case of the water bed thermostat, the capillary tube as well), then it's very likely that the thermostat will be fine. In fact, of all the thermostats I have salvaged, only one was getting a bit past it - the thread on the adjusting rod was worn.

If you want to sort out the better 'snap action' types, put your ear up against the housing and rotate the adjusting knob very slowly. At one point in the rotation there should be a distinct click. (Ignore those people who sarcastically ask you what the football scores are as you indulge in this bizarre behaviour!)

Using Them

So that's all fine and good, but how do you actually use these thermostats in a car?

• Low Temp Warning Light

The simplest automotive application is to turn off a warning light when the temperature reaches a certain level. For example, to illuminate a red warning on the dash when the engine oil temperature is still low - all engines should be warmed-up before they are driven hard, and the coolant temp gauge doesn't always reflect oil temp.

In this application, either an iron or frypan thermostat is ideal. Because it won't matter if the warning light flickers a little before turning off, it doesn't need to be a snap-action thermostat - in fact, it's probably useful to use a non snap-action design because you can then see that the oil is nearly up to temperature.

The first step is to disassemble the frypan controller. Once you have opened-up the plastic housing (which may require breaking it as you pry it apart) you should be left with a thermostat assembly. There are three wiring connections - an earth to the body of the thermostat and two connections to the switch contacts. The earth is no longer needed, and you can either cut off the switch contact wires (and solder new wires in their place) or solder new wires to the wires that are already there. While it's apart, use some very fine sandpaper to clean up the contacts - if you cut a thin strip you can slide it between the contacts and then pull it back and forth a few times.

These thermostats are easy to mount in small plastic electronics 'jiffy' boxes, using two long screws with multiple nuts on them to hold the thermostat by the flange that is spot-welded about half way along it. Put a knob on the adjusting collar (often you'll need to break the original knob to get it off), run the wires out through a hole, seal up the openings with silicone and you've finished!

The wiring is also easy: supply an ignition-switched 12V to one side of the thermostat, connect the other side of the thermostat to the warning light, and earth the other side of the light. The sensing probe can be placed in contact with the sump.

• High or Low Temp Switching

But what if you want to switch something on when the temp reaches a certain level? For example, radiator cooling fans, an engine-off turbo cooling fan, an intercooler water spray - even just a warning light? Well, again it's pretty easy but it will need a new component - a relay.

Relays Many automotive enthusiasts are as happy as Larry in wiring up relays to control heavy loads - lights, fans and so on. After all, there are just two wires that supply power to the coil to switch on the relay, and two other terminals that get connected together when the relay is on. But it's when relays have five and eight connections that some start to sweat. But if a few overall ideas are kept in mind there's no need for hassle. Firstly, just as with those simple car relays, all relays have two terminals for the coil. A relay is just an electromagnet-based switch, and the coil makes the electromagnet happen. A normal car relay is a single pole, single throw (SPST) design. This designation refers to the switching part of the relay where when it's activated, one wire (a "single pole") can be connected only one way (a "single throw"). Just like a simple on/off switch, when you power up the relay's coil, the connection is made; when you unpower the coil, the connection is broken. In this diagram the relay's coil is yellow. Near to the coil you can see a switch, which is open. This is called a Normally Open contact - it's open when there's no power being applied to the relay. When power is applied to the relay, the single contact closes. This is a Single Pole Single Throw relay - SPST. But wouldn't it be good if we had a contact that was broken at the same time as the above switch was made? That's what happens in this design. Another contact has been added that is Normally Closed. When the relay is energised, this contact is broken and the other one (the Normally Open contact) is closed. We still have only a single pole to be switched, but now it can be connected two ways - a double throw design. This type of relay is therefore called a Single Pole, Double Throw relay. As you can see, it has both Normally Open (NO) and Normally Closed (NC) contacts. Some people call this a changeover relay. In the above design a single circuit could be switched in two different directions. But why not switch more circuits at the same time? That's what this design does - it's a Double Pole, Double Throw relay. The 'Double Pole' bit just means that it has two separate inputs that can be switched - and we now know what the 'double throw' stuff means. With this type of relay when the relay is activated you can: - turn on two completely independent circuits - turn one off and one on - turn off two completely independent circuits Even more complex designs of relay exist, but if you analyse their specs on the basis of how many poles and throws they have, it becomes a helluva lot easier to understand! Note that a typical 12V, SPDT or DPDT relay costs only about AUD$5 - and even less from an electronics disposals outlet like www.rockby.com.au

From the above you might have worked out that to switch something on when the thermostat switches off, we'll need at least a SPST relay. And you'd be right!

Here a fan has been wired into place using the SPDT relay's Normally Closed contacts. When the thermostat reaches its trip point it opens, the relay's coil loses its power, and the Normally Closed contacts shut, turning on the fan. When the fan cools down whatever it is blowing air on, the thermostat closes, energising the relay and so closing the Normally Open contact. This contact doesn't lead anywhere, so the fan switches off.

The box that was used with the Low Temp Warning Light has enough free space inside to fit a small SPDT or DPDT relay, so it's not hard to turn the previous design into one that can switch things off as well as on. However, it's best to use a snap-action thermostat with the relay, otherwise a situation can occur where the relay chatters.

The use of the relay makes the addition of two more ingredients easy and useful. Two LEDs can be built into the box to show the status of the thermostat. For example, a green LED can be illuminated when there is power going to the system but the fan is yet to be switched on. Then that LED can go out and a red LED come on when the fan springs into operation. The extra parts are the two LEDs and two 560 ohm current-limiting resistors.

Conclusion

So where does that leave us? Firstly, collect all the junked irons and electric frypans and water bed temp controllers that you can find!

Secondly, when you want to switch on the basis of temp, consider what characteristics you need:

	Electric Frypan Thermostat	Clothes Iron	Water bed
Temp range	Wide, eg 40-200 deg C	Fairly wide, 60-200 deg C	Narrow, eg 25 - 50 deg C
Hysteresis	Small	Large	Small
Sensitivity	High	Low	High
Action	Many not click type	Most click type	All click type
Sensing	Short stainless steel probe	Whole thermostat	Remote copper bulb

Thirdly, consider how fancy you want the system to be. The red and green LEDs, for example, can be easily placed on the dashboard so you can see at a glance what's going on.

Finally, have fun! After all, it's not everyone who's driving a car around with a frypan part in it doing great service!

Another Use It's not strictly automotive but we thought you'd probably be interested, anyway. The clothes iron thermostats make a great PC fan controller. Here one's been mounted in a jiffy box that comes with an aluminium lid. The aluminium conducts the heat to the thermostat that's been bolted to it, with the thermostat detecting the temp inside the PC case. When the temp rises high enough, extra cooling fans come on. Note that the thermostat was made a little bit more sensitive to low temps by bending the bimetallic strip a tiny amount. The slow response time and higher hysteresis works perfectly in this application, preventing the fans from cycling on/off too frequently. If you want to measure the temp build-up at a specific location inside the case, use a water bed thermostat and mount the bulb at the critical location.

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