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Zero Cost Temp Switches

Zero cost high quality temperature switches.

by Julian Edgar

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We recently showed you how to get together some super sensitive pressure switches at zero cost - this time we move up a notch to temperature switches. Again they'll be at zero - or near zero - cost, but they're still quality snap-action precision in small packages!

But forget most washing machines though as a source of these switches - instead it's time to head for the dishwashers. And here's another benefit - dishwashers also contain the more sophisticated type of multi-level pressure switch. So, with a bit of luck, just a single junked dishwasher can result in the snaffling of a quality pressure switch, and two - yep, two - temperature switches.

Why two temp switches? Well, in most dishwashers there's a switch that keeps the water at the right temp - it controls the heating element and so acts as a thermostat. These are usually rated around 55 degrees C or so. The thermostat switch has only a small hysteresis - that is, its switch-on and switch-off temps are fairly close together, being typically about 10 degrees C apart.

Then there's another switch, this time almost always rated at 85 degrees C. The latter's a safety cut-out switch - if the first switch goes mad and keeps the heating element on, you don't want a situation where the water gets so hot that it starts to boil.... The safety cut-out has a wider hysteresis - typically it switches back at about 55 degrees C. (Which makes sense cos then its putting the temp back at the control level of the thermostat.)

Thermal Mass
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The term 'thermal mass' refers to how quickly a substance changes in temperature with the input of heat. The rapid detection of temperature changes is carried out by sensors with a small thermal mass - they can therefore change in temperature very quickly. A bead type thermocouple is an example of this sort of rapid response sensor. The temperature switches covered here have a much greater thermal mass than a bead thermocouple, and so are slower to respond. As a result, using a dishwasher temp switch will not work very well if it is being used to sense the real-time temperature of (say) the intake air after a turbo. The switch will take a minute or so to respond, so if the temperature peak comes and goes in 10-15 seconds, the switch won't operate - even though the temp may briefly have been much higher than its trip point.

The relationship between the switch's thermal mass and hysteresis, the changing temperature characteristics of whatever is being sensed, the way in which heat is conducted to the switch, and the required response time; all mean that the temperature rating, mounting location and mounting technique are best sorted out experimentally.

Some of these switches are 'normally closed' (ie they go open circuit when the temp is reached) while others are 'normally open' (they close at the designated temp).

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On one side of the switch you'll find a flat metal disc, most often about 20mm in diameter. This is the sensing part of the switch - in dishwashers you'll find this part pushed up hard against the outside of stainless steel bowl that forms the dishwasher inner. The heat from the water is conducted through the bowl to the switch, with that heat transfer often aided by a heatsink compound smeared on the two surfaces. The switches are usually held in place with spring metal clips, so removing them is a case of just pulling back the clip and slipping them out. Others are simply pushed through a rubber grommet - these pull out even more easily.

This type of switch is a heavy-duty snap-action type, typically rated at 12 amps at 250V AC.

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More modern dishwashers have much smaller switches of the sort shown here. These are generally have a lower power handling and so should be used in conjunction with a relay, rather than switching heavy-duty loads directly.

Instead of having just two quick-connect spade terminals, some temp switches have three or four terminals. These switches appear to integrate into the one body the two different functions - thermostat and safety cut-out.

As with the washing machine and dishwasher pressure switches covered previously, invariably old dishwashers are sent to the tip with these switches still in place. So you can obtain them directly from repairers of dishwashers - at no cost if you're permitted to go through the machines being discarded.

Deciding What You've Got

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Many of the switches will have their temp trip point written on the body. For example, '85NC' means that the switch is normally closed but opens at 85 degrees C. Other times, the temp with be marked in degrees F - one of the switches shown here had 'T130' marked on it; testing showed the switch to have an activation temp of 57 degrees C (near 130 degrees F). However, other switches just have incomprehensible makers' codes on them, so usually some sorting out is required.

The easiest way of finding out what you've got is to apply a multimeter's continuity function across the switch terminals, so that when the switch is closed, the beeper sounds and when the switch opens, it stops. (Of course you could use a battery and a light - or any similar circuit - to do the same thing.) You then heat the switch and monitor when it closes.

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With the switches shown here, I checked their operation by mounting each switch so that the sensing part was clamped hard against the outside of a metal container full of water. I then placed the container on an electric stove and heated the water, monitoring the water temp and so being able to see the temp at which the switch closed. The water was then allowed to cool (helped along by some ice cubes) and the temp when the switch opened was also noted.

Using Them!

So the most common temperatures at which the switches click over is ~55 degrees C and ~85 degrees C. But what if you want to operate something at a temperature that is a little different to these tripping points? The switches are not adjustable, but you can still alter the temp setting of the system. How? By the way in which you mount the switch.

Rather than looking at the ideas in isolation, let's look at some potential applications.

  • Cool Engine Warning Light
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Some European cars illuminate a Cool Engine Warning Light - an indication that you shouldn't drive the car hard until the light turns off. The Normally Closed switches are perfect for this application, with the light able to be triggered in a number of ways. You could use an 85-degree switch on the hot side of the radiator, or you could use a 55-degree switch on the sump to detect engine oil temperature. Alternatively, if you wanted the engine temp to be higher before the light goes out, you could mount the 85-degree switch on the sump.

  • Cool Engine Boost Limit

In a turbo car running an anti-wastegate creep boost control and a fairly small turbo it's easy to inadvertently get more boost than is really wise with a cold engine. But if a temp switch is used to activate a Cold Engine Boost Limit solenoid, it's easy to prevent this happening.

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The solenoid valve has been added to the 'Audi-type' boost control previously covered in AutoSpeed at "The Audi's DIY Boost Control - Part 2", with the new components shown in brown. The solenoid is opened by the application of power from the normally closed temperature switch. This allows air from the manifold to bypass the boost control system, feeding air pressure straight to the wastegate. In all cars running factory electronic boost control, the wastegate springs are quite light, so this approach will lower boost substantially over the level normally seen. Once the temp being monitored rises above the switch point, the switch opens, turning the bypass solenoid off and so returning the boost control to its normal operation.

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Here is the low-boost addition to the simpler 'EXA-type' boost control previously covered in AutoSpeed at "Project EXA - Part 3 - DIY Boost Control.". Again the solenoid bypasses the part of the system that increases boost, and again if it's applied to a car which normally has ECU boost control, the developed peak boost in 'engine cold' conditions will be quite low. In cars that didn't originally use ECU boost control, the boost level will return to stock (eg 7 psi) when the engine is cold.

Temperatures which could be monitored by the switch in this system include the coolant or engine oil. Adding a dashboard switch wired in parallel with the temp switch will also give you high/low boost, if you wish.

  • Hot Conditions Boost Limit

In boosted engines where the system is close to detonation at times (eg the intercooler is too small or doesn't exist), you may well want to drop boost when the day is hot or the intake air temp starts to rise. This can be done by using the systems described above, but swapping the normally closed switch for a normally open design. In that case, the solenoid will stay off until the trip temperature is reached, after which it will be fed power and so open to allow the boost control system to be bypassed.

Temperatures which could be monitored by this system include the temperature of the intake air (but see the breakout box on Thermal Mass) or just the general underbonnet temperature. The latter reflects really well when the highest intake air temps are experienced in most turbo cars - after idling on a hot day in city traffic with the air-con switched on. Depending on the car and the mounting location, either the 55- or 85-degree switches could be used.

  • Brake Temps

While none of the temp switches that you will find in dishwashers will be able to directly measure brake temps (cos brake temps are often way too high!), another approach can be used which is nearly as effective. If for example the triggering of a brake water spray is your desired application, then a normally open switch can be mounted in the airflow behind the brakes. Or, it can be mounted far enough away from the caliper or disc that radiant heat increases the temp of the switch when the brakes are hot. The switch can then be used to turn on a water pump working with a misting nozzle located in the duct ahead of the brakes.

  • Turbo Cooling Blower
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In the same way that brake temps cannot be monitored directly but the dishwasher temp switches can still be used, so turbo temps can also be measured. By placing a normally-open temperature switch near to the turbo, a marine-type 12V bilge blower can be activated when the lack of airflow through the engine bay causes increased exhaust housing temps (eg because you've just come to stop and turned off the engine). By placing the switch on a bracket near to the turbo exhaust housing, it's possible to have it trigger the blower to push air over the turbo for a minute or so after the car has come to a halt. If the bracket is designed so it can be pivoted to vary the distance from the switch to the turbo, the sensitivity of the system can be easily and effectively be altered. The 85 degree C temp switch could also be used to directly sense exit oil temp from the turbo bearing.

Other Uses

The above uses comprise just a quick overview of just some of the potential switch applications - there's also the triggering of additional radiator cooling fans (direct sensing from radiator or indirect from the airflow that has passed through the radiator), triggering of intercooler fans (from intercooler core or intake air temps), even the possible switching on of the interior fan when the car is parked in the sun in a hot climate and the inside temp exceeds 55 degrees C.

And as with last week, if you come up with a really tricky use for these switches, we'd love to hear of your project!

Junkyard Dawg - Part 1
Junkyard Dawg - Part 3


Typical Standard Specs

Klixon S-20

  • Single Pole Normally Open contacts
  • Closes at 57 degrees C and opens at 48 degrees C
  • Marked 'T130'
  • Full size body

Elth 340

  • Single Pole Normally Closed contacts
  • Closes at 78 degrees C and opens at 55 degrees C
  • Marked '85NC'
  • Full size body

TOD 36TX02

  • Single Pole Normally Open
  • Closes at 70 degrees C and opens at 52 degrees C
  • Miniature switch

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