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Motor Speed Control Module - Part 1

Build your own motor speed control module for performance car applications.

by Julian Edgar

Click on pics to view larger images

This DIY module lets you vary the speed of an intercooler fan or air/water intercooler circulation pump, or control the amount of water flowing through an intercooler spray nozzle or water injection system. The control system can be either manual or automatic, and it's cheap and very effective.

  • Variable speed electric motor control module
  • Cheap DIY kit construction
  • Control the speed of an electric fan or pump by turning a knob
  • Coming articles will cover versions where pump or fan speed varies automatically with engine load

Uses include:

  • Water injection
  • Air/air intercooler fan speed control
  • Water/air intercooler pump speed control
  • Intercooler water spray control

So, what's this - another electronic project? Yep, and we make no apologies for it.

Why? Well, when we've seen products that attempt to do the same thing as this cheap module being sold for ten times the dollars, we figure that if you can do better while also being cheaper, that's a good thing!

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But what does this module do? Basically, it's a device that steplessly varies the voltage going to a 12V load. For example, built exactly as the kit's instructions show, by turning a trimpot you can vary the speed of an electric fan, fuel pump or electric water pump. That has some good uses - eg you can easily make your cabin ventilation fan infinitely adjustable rather than just having the two or three standard speeds. But it gets even better: by making some changes, it's also possible to have the electric motor speed variation occur automatically with variations in engine load.

Yes, as the engine load increases, the pump or fan (or whatever) will increase in speed at the same time, without any human input being needed.

And that suddenly opens up a whole new world.

Let's say that you're running a water/air intercooling system in your turbo or supercharged car. When you set the system up you had a few choices as to its operation. You could:

  1. Run the intercooler water circulation pump at full speed all of the time
  2. Turn on the intercooler water circulation pump only when on boost
  3. Run the intercooler water circulation pump at slow speed (via a hefty dropping resistor) off-boost, and then at full speed when on-boost

The last system is the best, but if you've ever built that sort of design you'll find it quite clumsy - a big heatsink'd resistor, a pressure switch - and lots of fiddling to get it right. Well, instead of doing all of that, just use this module - it lets you easily set the minimum running speed of the pump, then automatically ramps it up to full speed as engine load increases.

And we're not finished yet! You can also use the module to cheaply control an intercooler water spray or water injection system.

In as-supplied form, the module will handle loads of up to 10 amps, while adding another mosfet (power transistor) and a few heatsinks (there's provision on the board for the extra mosfet) doubles this to 20 amps, which is enough for pretty well any automotive application.

And you want some more good news? Depending on the box that you put the module in (and stuff like that), it's quite likely that this module will be cheaper than using a boost pressure switch and a relay! So not only can it do far more than taking one of the conventional approaches, at the same time it's also cheaper.

But because the uses are so varied - and some PCB component changes are required for the different uses - we'll cover them one at a time in three articles.

But Firstly...

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So what are the negatives? There are two.

Firstly, the kit isn't as simple to build as the others that we have covered. It's still a small one - and if you've built lots of electronic kits before, you'll have no problems - but we would suggest that it's not for a complete beginner. Why? Well, the kit instructions aren't very good (although we'll supplement them with lots more tips here); there are quite a few components that if you get around the wrong way, kaput; and Jaycar has recently changed the design of the printed circuit board, which adds a bit to the complexity.

But if you use the instructions, check resistor values with a multimeter before soldering them in place, and look really carefully (ie click on to enlarge) the photos of the completed board included in this article, you shouldn't have any problems.

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Secondly, the module's automatic load sensing is done via the output signal from the car's standard airflow meter or MAP sensor. So this means that the module will only work to automatically increase the speed of the pump or fan (or...) when it sees a signal from the airflow meter or MAP sensor that:

  • Rises with load
  • Has a low-load output of 1-2 volts and reaches 4-5 volts at full load

These two characteristics are typical of most MAP sensors and hot-wire airflow meters, but if you're at all unsure of whether the load sensor in your car is suitable, you might want to measure the sensor's output before starting to build the module.

Voltage Loss

Like most electronic speed controls, the maximum speed of the motor will be a little slower when using the module compared with the battery alone - even when the module is at full output. Specifically, you'll lose about 1.5 volts at full noise. So if you've got a (say) a pump that absolutely requires 13.8 volts (or whatever your car's running voltage is) to deliver the output that you need, then the best it will see after the module has been installed is around 12.5 volts. That's no problem with normal fans and water pumps, but it might be a problem if you're trying to get the utmost out of a fuel pump.

Building It

The Jaycar Electronics kit is cat no KC5225 - the "10A Motor Speed Controller". It is to a design developed by electronics magazine, Silicon Chip. Like most electronic kits, it comprises a printed circuit board (PCB) and a handful of components - resistors, capacitors, transistors and an IC. The kit costs AUD$23.95.

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After you have identified what all the components are, orientate the PCB as it is here and start soldering them to the PCB. Measure the value of each resistor then place its leads through the right holes, soldering it to the PCB pad on the back of the board. Then cut off the long surplus leads and do the next one. When you have installed all of the resistors, it should look like this.

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The capacitors go onto the board next. Unlike the resistors, some of these (the cylindrical ones) are polarised - they go onto the board only one way around. The polarity of these capacitors is indicated by a line of negative signs down one side - the other lead is obviously the positive. Make sure that the polarised capacitors are orientated as shown here. The small square caps can have some confusing labels (but at least they aren't polarised!), but it's easier when you realise that the two 0.1 caps go close to the IC.

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Once all the resistors and caps are installed, solder the rest of the components into place. Be careful that you don't swap the transistors, get the IC in upside down (a notch at one end shows its orientation), and that the voltage regulator and mosfet aren't swapped or installed in the wrong orientation. And don't forget the wire link!

In our application, the large square diode is installed directly on the output terminals. (In the instructions it states that this - and a capacitor - should be mounted on the motor. We didn't find any need for the capacitor.) Note in this picture how it is mounted- it replaces the 'stakes' that the instructions show to be in all four terminal positions.

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Compare the module that you have built very closely with the photos shown here - it should look just the same. (However, note that some components can look a little different, depending on the manufacturer they were sourced from.) As provided, the kit has a 10-amp capability; if you need to control even larger loads a second mosfet can be added (there is room for it on the PCB) which doubles the current handling capability.

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It's vital that the module works exactly as it should, before you start to modify it (or install it in a car as it is). Connect a low power 12-volt bulb across the 'motor' terminals and apply 12V and ground to the module. By turning the trim-pot with a screwdriver, the light brightness should be able to be varied. If there are problems, check that no solder bridges have been created across the PCB tracks, that the polarised components are all orientated correctly, and that all the components are in their correct places.

Using It

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In this form the motor speed control module can be used to manually vary the speed of a water/air intercooler pump, electric radiator fan, and so on. If you want to have easy on-dash control of the speed, simply unsolder the trim-pot from the PCB and replace it with an external 5 kilo-ohm pot, as shown here. The wires between the pot and the board can be a couple of metres long without any problem.

Next week: automatically varying intercooler pump or fan speed with engine load

Available in the AutoSpeed Shop : Kit KC5225 '10A 12VDC Motor Speed Controller Kit'

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