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
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:
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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.
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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.
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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.
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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
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.
Here’s a bare thermistor
Shorten the leads and then solder two insulted
wires to the leads.
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.
This diagram shows how the thermistor and resistor
are connected to the electronic module.
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.
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:
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Output turned fully on (eg, control of fans or
pumps)
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Output pulses (eg, flashing of warning light or
pulsing of audible alarm)
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Output pulses twice only (eg, indication that
something is wrong)
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
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.
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Output Switches on as
Temperature Rises Above Set-point
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.
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.
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.
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).
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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.
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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
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.
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.
If
you are switching on a string of low power filament lamps, a small heatsink will
be needed.
If
you’re turning on a pump or small fan, a medium sized heatsink will usually be
needed.
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.
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.
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
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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!
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The
Voltage Switch is available fully built and tested from the AutoSpeed shop.
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