DIY Thermal Differential Controller – Part 2: Component Selection

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by Tim Fulton on September 18, 2009

This entry is part 2 of 4 in the series Thermal Differential Controller

In the last article we talked about what a thermal differential controller is and what the goal of this project is. This time we will learn about the necessary components needed to make the controller.

The first thing we need is a programmable controller. The programmable controller will monitor the temperature sensors for us and send signals to turn the pump on and off. It will let us adjust the on and off temperature differential, as well as allow us to do other things like set a maximum tank temperature, or even use it for data logging if we wanted. For this project, I decided to go with the Ardunio. Its a popular micro controller that is very versatile. It is easy to use and is quite inexpensive. You can get a fully assembled and tested unit like the one above for around $30. There are also stripped down versions or kits that you must solder together that can reduce this cost.

AC solid state relay

The next thing on the list is a relay of some sort.  The relay will receive the signal from the controller to turn the pump on and off and do the actual switching for us.  Which relay you use will differ depending on your specific setup.  If you are planning on using an AC pump, you’ll want an AC relay.  If you are planning on using a DC pump, you’ll want a DC relay.  After you know that, we have another choice to make.  We can go with a mechanical style relay to save some money, or go with a more expensive solid state relay (shown above).  The downside to the cheaper option is that it is more prone to failure and will not last as long.  However, their failure rate and longevity are usually nothing to be concerned with.  A mechanical relay will only cost you about $2 where as a solid state relay will run about $13.

The other option is to try to find a relay that will accommodate both AC and DC pumps.  This will be a bit trickier to find, but keeping the design the same between the two will be worth a bit more hunting.  This is what I am currently looking into.


Photo: ッ Zach Hoeken ッ

The last thing on the list is sensors. Most commercial controllers use 10k thermistors. I see no reason why we can’t use them for this project as well. This gives some flexibility in the design and assembly. You can order 10k thermistor sensors that are already premade for around $13. Alternatively, you can buy a bare thermistor for around $0.60 and make your own sensor casing for it. This is the route I will be pursuing for this project to keep the cost low.

For more info on the controller, visit the thermal differential controller forum thread.

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