Remote Bulb Digital Temperature Controller

I love these things.

They're less than $20 from eBay or Amazon and can be used for many things. They turn on/off power depending on the temperature at the remote probe. The original units were Centigrade only but now they can be found in Fahrenheit or selectable. Most have a 10 amp switching rating but if that's not enough a relay can be used. There are models that operate on 12, 24, 120, 220 volts and the switched leads can be independent of the power supply. You could use 120VAC for power to switch a 12VDC circuit or vice versa. Most are selectable for cooling or heat and some can do both.

My first project was a DIY fish tank heater that I wanted to run on 12vdc. I didn't mount any of the components, just had wires going everywhere.

The second project was converting a chest freezer into a refrigerator. Off the shelf the freezer could only be set between -4° and zero F. I wanted to put my beer in the freezer and keep it at 28°. This time I built a tidy little stand alone box with a plug for 120 power and a receptacle for the freezer plug.

 

Big toggle allowed switching off the controller so the freezer could be uses as a freezer.

Click here for more on the freezer to fridge project. 

My next project was for a fellow who wanted to maintain a stable temperature in a room in his barn for eggs. Sometimes the eggs would need to be heated, sometimes they'd need to be cooled. He didn't want to have to monitor them constantly and I knew one of these gizmos would do the trick. I simplified the construction by just mounting the components on a board. The pic was before I wired the additional recepticle for the heating circuit.

The white connector block is available at most hardware stores and online. Look for: 

Wire Connection 12 Position Barrier Screw Terminal Strip Block 

My current project is my boat's refrigerator. When the 40 year old factory 12/120 volt unit gave out I decided to replace it with a considerably cheaper 120 VAC only unit. I figured to power the new unit via my boat's house battery through an inverter. I bought a dorm fridge that didn't include the tiny ice maker - the half dozen ice cubes it might provide take up too much room inside inside the fridge. This system worked great except the fridge would occasionally solidity/freeze my wife's lettuce.

I still don't know why I couldn't find the correct thermostat setting. Maybe it's because the unit is used on the road and on the water. Maybe it's because the unit isn't on a permanent leveled surface. Maybe it's because the inverter messes with the internal refrigerator components. Rather than research all the possible causes I took the easy way out and installed a controller.

Wire up the controller:

I cut a cheap extension cord in half for the plug and receptacle wires.

 Set the controller cooling temp for 35°...that's when the controller will switch off the current to the fridge. Set the controller 'slew' to 5°...any time the probe sees a temperature 5° above the cooling temp the controller will begin feeding power to the fridge.

Plug the controller into the power source. Set the refrigerator to maximum cooling. Insert the remote bulb into the refrigerator. Plug the refrigerator into the controller. Now the guesswork is over. The power comes on at 40° and goes off at 35° and I can see the inside fridge temp on the controller.

What a comfort to know the temp inside the fridge!

The original fridge was larger than this dorm unit. The grates above and to the right are fillers to the original opening. I left those areas open for ventilation.

I didn't want to put any holes in the dorm fridge when I installed it. I secured the new unit in the old space with a reverse clamp setup.

I soldered some long nuts to a piece of brass angle. Then using bolts of the appropriate length, I ran them up out of the long nuts until the fridge was wedged in place. The wing-nuts keep the bolts from working back down. The brass angle was a handy place to mount the controller. This setup has been bulletproof.

And if you're curious, here's an early heating unit with a 30 amp power relay.

This was before I discovered the convenience of the Terminal Strip

What's the square red button for?

This rig was used to cut the power to a hotplate when a given temperature was reached. The controller will cycle on and off depending on the temperature at the probe, but the relay only allows power flow after the button is pushed. So to initiate power to the hotplate, the temperature has to be below the set point AND the momentary red button has to be pushed. When the set point is reached the controller shuts off the power. As things cool the controller will cycle on again but the hot plate cannot get power unless the button is pressed again; the cooking is a one time event. So now, the next time you make caramel and are supposed to turn off the heat when candy thermometer shows 250°F you don't need to be there - this thing will turn it off at the desired temp and it won't go back on.