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Tempmaster Electronic Thermostat Mk.2

Want to convert an old chest-type freezer into an energy-efficient fridge? Or convert a spare standard fridge into an excellent wine cooler? These are just two of the jobs this low-cost and easy-to-build electronic thermostat has been designed to do. It can also be used to control 12V fridges or freezers, as well as heaters in hatcheries and fish tanks. It controls the fridge/freezer or heater directly via their power cables, so there's no need to modify their internal wiring.

by Jim Rowe

Back in the June 2005 issue of SILICON CHIP, we described an electronic thermostat intended mainly for converting an old fridge into a wine cooler or a chest-type freezer into an energy-efficient fridge. Dubbed the “Coolmaster”, it turned out to be a very popular project, especially with people wanting to reduce their power bill and reduce their “carbon footprint”.

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The switched IEC connector is snap-fitted to an aluminium plate and this assembly is then secured to one end of the case using Nylon screws & nuts.

Converting a chest freezer into a “chest fridge” results in much lower energy consumption than a normal “vertical” fridge of the same internal capacity, because cold air doesn’t fall out every time you open the door and chest freezers tend to be better insulated anyway.

The project became even more popular when the people in Jaycar’s kit department came out with a slightly modified version which could be used to control heating elements as well as fridges and freezers. This modified version was called the “Tempmaster”, to describe its expanded capabilities.

Unfortunately, some constructors did experience problems with the project. In most cases, this seems to have been due to spurious triggering of the control Triac due to inductive spikes fed back from the motor in the compressor of the fridge/freezer, causing noisy and/or

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The other end of the case carries access holes for the sensor jack plug, trimpot adjustment and DC power supply.
hesitant switch-on or switch-off. This problem was solved in most cases by fitting a mains filter circuit between the Tempmaster and the motor but it did point to one shortcoming in the project’s use of a Triac for power control of motors.

Of course, a Triac can only be used for controlling AC in any case, and this meant that the first Tempmaster could not be used to control fridges, freezers or heaters which run from 12V DC – shortcoming number two.

We also received criticism from energy conservationist Dr Tom Chalko, who complained that the Coolmaster/Tempmaster was mediocre in terms of energy efficiency. This was because of its own quiescent energy consumption and it would pose problems for those using electronic inverters to produce 240VAC from a solar or wind generating system, by preventing the inverters from ever being able to switch into “sleep” mode.

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