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Universal Temperature Switch

Turn on pumps, fans or warning lights

by Julian Edgar

Click on pics to view larger images

At a glance...

  • Pre-built electronic module
  • Remote mount temp sensor
  • Adjustable set-point
  • Adjustable difference between switch-on and switch-off temps
  • 0-110 degrees C range
  • Switch with rising or falling temps
  • Three different switchable output configurations
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It’s easy to get carried away writing about electronic car modifications, to think that everyone wants complex solutions to simple problems. So here’s a simple solution to a simple problem – a temperature switch.

Amongst many other applications, you can use it to show when there’s a problem (think of a coolant temp warning light) or you can use it to switch on fans or pumps (think radiator fans and intercooler spray pumps). The point at which the switch trips is infinitely adjustable, and the difference between the switch-on and switch-off temps is also adjustable.

You can also easily configure the output to suit your application.

The electronic module is prebuilt and tested, and you’ll need just two additional low-cost components.

Temperature Switches

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In the past, temperature switches were most often mechanical designs. They used an internal bi-metallic strip – a piece of metal that bent as it got hot. When the piece of metal bent sufficiently, electrical contact was made and the switch was therefore turned on. These switches have been used for decades – there’s nothing much wrong with them.

Or is there?

Here are some problems with these sorts of switches:

  1. The set-point is normally not adjustable. That means you must select the switch to suit your application, and there may not be such a switch available.

  1. The hysteresis is not adjustable. Hysteresis is the difference between the switch-on and switch-off points. If the hysteresis is too low, the output will chatter. If the hysteresis is too high, the control system may not be effective.

  1. Temp switches available in either ‘normally open’ or ‘normally closed’ versions. You must have the right version to suit your application – eg normally open (closes when it reaches the right temp) to trigger a radiator fan.

  1. The switch is ‘dumb’ – it turns on or off. If you want to flash a warning light or do anything else tricky, you’ll need additional electronics.

As you might have guessed from that preamble, the electronic temperature switch that we’re about to cover overcomes all those problems.

Using the eLabtronics Voltage Switch

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The eLabtronics Voltage Switch is a prebuilt electronic module. It costs AUD$59 from the AutoSpeed shop.

To turn it into a temperature switch, you need add only two low-cost components – a 100 kilo-ohm nominal thermistor, and a 200 kilo-ohm resistor. Separately, these components cost only a few dollars – bought as the accessory pack from the AutoSpeed shop (the pack includes a light sensor and some other components) the cost is just under AUD$13 – see Performance Modules Accessory Pack. A suitable box is also available Performance Modules Box - Bulkhead - Black .

The thermistor alters in resistance with temperature – it’s good for the range of about 0 – 110 degrees C. By using it in conjunction with the resistor and the 5V regulated supply available from the electronic module, the Voltage Switch becomes a temperature switch.

The first step is to mount the thermistor.

Mounting the Sensor

The thermistor comes as bare electronic components. To wire it into place, you’ll need to do two things: solder it to extension wiring and mount it.

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Here’s a bare thermistor

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Shorten the leads and then solder two insulted wires to the leads.

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Use insulation tape (when working with relatively low temps) or good quality heatshrink (high temp sensing) to insulate the connections.

If the sensor is detecting just ambient conditions (eg in-cabin temperature), the sensor can simply be positioned appropriately and held in place with a cable tie. However, if the sensor is working in a much tougher environment, use high-temp epoxy to mount the sensor in a threaded brass fitting so that it can be securely mounted.

Wiring the Voltage Switch

The next step is to wire up the voltage switch. We suggest that you test the operation of the switch on the bench before connecting a load or mounting it in a car.

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This diagram shows how the thermistor and resistor are connected to the electronic module.

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The mode selection is made by altering the position of the Options DIP switch positioned on the board. Orientate the board so that the terminal strip is on the right when checking switch positions. Set the DIP switch to the positions shown in the above diagram and not in this photo!

Connect power and ground wires and turn on the power supply power supply. Adjust the hysteresis pot fully anticlockwise and then turn it clockwise about half a turn. Adjust the set-point pot until the on-board LED comes on, and then a little further back the other way until it just goes off.

(Note: Both the set-point and hysteresis pots are multi-turn so don’t expect to make only one rotation when setting them. Multi-turn pots also don’t have clear end-stops [although they can sometimes be heard clicking when they’ve reached the end of their adjustment]).

Heat the thermistor (eg on a cool day by holding it in your fingers) and the LED should light up, then go off as the thermistor is allowed to cool. You can also use a soldering iron, heat gun (etc) to warm the thermistor.

If the LED flickers at the turn-on or turn-off points, the hysteresis is set too low – turn it clockwise to increase it. If the difference in temps between turn-on and turn-off is too high, decrease the hysteresis (turn the pot anti-clockwise). (More on how to best set hysteresis in a moment.)

Connecting a Load

The next step is to install the temperature switch module in a car and then connect a load. The ‘load’ is whatever is turned on by the switch. For example, you might want to illuminate a bright warning light whenever a temperature exceeds a certain level. In that case, the load is the light.

Or, you might want to turn on a radiator fan. In that case the load is the fan.

The module can directly drive quite big electrical loads. However, for large loads a heatsink will be needed on one of the module components. For even bigger loads, a separate electrical module or relay will be needed – see the breakout box at the end of this story.

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As this diagram shows, the load is connected between the Output of the module and ground. If the load has a polarity, positive goes to the module.

Configuring the Output

Once tripped, the output of the switch can be any one of three modes:

  • Output turned fully on (eg, control of fans or pumps)

  • Output pulses (eg, flashing of warning light or pulsing of audible alarm)

  • Output pulses twice only (eg, indication that something is wrong)

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The third mode is useful where you don’t want to be distracted by an annoying alarm – the output can be used to pulse a buzzer twice and then the illuminated LED (arrowed) reminds you the problem is still present.

The output mode selection is made by altering the position of the DIP switch positioned on the board.

Switch Options

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The eLabtronics Voltage Switch has a four-position DIP option switch. Position the board so that the terminal strip is on the right and then the following switch positions give the listed behaviour. Note that the position of the last switch doesn’t alter.

  1. Output Switches on as Temperature Rises Above Set-point

X

X

X

X

Switches on as temperature rises above set-point, then on-board LED illuminates and output stays fully on. LED and output switch off when temperature falls below set-point. This mode will be one of the most often used, and is the mode used when the module was tested earlier in this story.

X

X

X

X

Switches on as temperature rises above set-point, then on-board LED illuminates and output constantly pulses. LED and output switch off when temperature falls below set-point.

X

X

X

X

Switches on as temperature rises above set-point, then on-board LED illuminates and output gives two pulses. LED and output switch off when temperature falls below set-point.

X

X

X

X

In addition, there is another that holds the output constantly on, irrespective of the input signal. This mode can be used in testing eg to check that the load has been wired correctly. Output is constantly on (useful during set up only).

If you want the Temperature Switch to turn on its output when the temperature falls below the set-point, simply move the position of the third switch to its uppermost position.

Hysteresis

In the text above we’ve said that the output switches off when the temperature moves back past the set-point. In fact, the amount of hysteresis that’s been set will determine exactly when the output turns off.

The smaller the hysteresis (ie more anticlockwise the hysteresis pot), the smaller will be the difference between switch-on and switch-off temperatures.

Let’s take a look at some examples.

You might want a warning light to illuminate when intake air temp exceeds 60 degrees C. If the hysteresis is set high, the light will come on at 60 degrees C – but may not go off until the temp drops to 40 degrees C. That’s no good – the light will still be on when the warning isn’t needed. In this case, you’d set the hysteresis very low – giving a difference between switch-on and switch-off values of (say) just a few degrees.

But if you’re running a radiator cooling fan, you’d want far more hysteresis than you dialled-in for the warning light. If you run just a few degrees C hysteresis, the fan will tend to cycle on and off a lot. In this case, you might instead set the pot to a value that results in (say) 10 degrees C hysteresis.

Big Loads

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The output MOSFET (transistor) of the module is rated to handle a continuous 10 amps – but that’s when it is fitted with a big heatsink. How hot the MOSFET (and the circuit board) get depends not only on the output current but also whether or not the output is being pulsed or held continuously on.

For short pulses, the heatsinked MOSFET will handle up to 15 amps.

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As a rule of thumb, no heat sink at all will be needed if you’re operating warning lights or LEDs – even high powered ones.

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If you are switching on a string of low power filament lamps, a small heatsink will be needed.

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If you’re turning on a pump or small fan, a medium sized heatsink will usually be needed.

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Finally, if you’re switching loads like car radiator fans, a large heatsink will be needed.

The heatsink needs to be isolated from ground and positive supplies, so either mount it so it fits inside a box (and can’t touch anything metallic!) or mount the heatsink to the MOSFET using an insulating spacer and nylon nut and bolt. In either case a smear of heatsink compound will be needed between the MOSFET and the heatsink.

Always make an initial check that the MOSFET is not getting too hot when the switch is driving the load. It might be very warm to touch but you should be able to keep your fingers on it.

For ultra high currents like multiple radiator fans, you’ll need to buy a relay. You can use a conventional car relay or a solid state design. Solid state relays cost more but they’re fully electronic, so have no moving parts.

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In addition to being very durable, an electronic relay can switch very large currents. When equipped with a suitable heatsink, the relay shown here can handle 100 amps continuously and cope with a very short term switch-on current gulp of 240 amps.

When using a relay (either traditional or fully electronic), the module doesn’t need to use a heatsink, so packaging becomes easier – the electronic switch can easily fit into a box and the relay can mount remotely.

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This diagram shows how an electronic relay is wired to the module. The electronic relay is available from the AutoSpeed shop for AUD$40 – see Solid State Relay

Conclusion

If you need to switch anything on and off within a temperature range of about 0 – 110 degrees C, the simple addition of two components to the eLabtronics Voltage Switch will allow you to do it.

Not only can you switch the device on and off, you can also pulse a warning buzzer (or flash a light) or have a more subtle two-pulse indication that a temperature level has been passed. In addition to those features, you’ve also got very fine adjustment of the set-point, a hysteresis range that can be set over a wide range, and a one-switch adjustment that allows the device to trip with either a rising or falling temp.

And that list sure beats a mechanical temperature switch!

The Voltage Switch is available fully built and tested from the AutoSpeed shop.

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