Building a Temperature Controller Box

The goal for this project is to build a small, medium cost, functional temperature controller for laboratory/custom project use (fish tank temp. control). We take a few shortcuts; I used an enclosed power supply and internal power plug since they were lying about on my work bench. Ideally, you would use a small ac-dc power adapter that could be directly wired in without the additional plug adapter – although I have seen such designs in other very expensive laboratory equipment. The electronic box can be purchased online from just about any vendor, this one is from Omega. Design considerations:

  1. Control Power: We separate the control power from the heating element power. This means that we don’t use the on-board relays in the temperature controller to handle the full heating element load. So, the temperature controller activates the relay, the relay connects the heating element to the power. This protects your temperature controller ($200-$1000)
  2. Power Wires: We have the relay cut the “hot” power wire. You would be surprised how many boxes cut the neutral line… stupid… if you damage the heating element and the power finds a neutral, you have problems (you might be electrocuted). So, we remove the power from the element when it is not “on”.
  3. Relay Type: I use mechanical relays. Why? This is good question and there are lots of people that love the newer solid state relays (SSRs). These non-mechanical SSRs are good for fast switching, smooth heating and quiet operation, but can be expensive ~$80 w/ heat sink. Generally, SSRs require a heat sink that is about 10% of the load power. This means for a 600W heating element we need to dissipate 60W of heat from the control box… that is about 1 rice cooker worth of wasted heat and a very large heat sink and fan assembly to keep things cool. Mechanical relays run cooler, they make noise, no fan is needed, they are generally well reliable if sized correctly and they are cheap (~$4). I have good luck with PID switching/pulsed output to a relay lasting for 5-10+ years with daily operation.
  4. Fuses: Fuse the power line. Things happen. Maybe the heating element gets broken; you want a fuse to protect you. If you decide to use a SSR, you need to buy a fast acting fuse that is closely matched to your heating element. SSRs tend to fail in the “on” position, that is they will keep the heating element powered and never shut off. For mechanical relays, you have some room for different fuses. Set the fuse about 10% higher than the heating element. Remember that normal wall plugs reliably supply about 10A of power. I know they supply more… but things get hot with more current, even if it is rated for higher loads… usually that is 15A or 20A for an entire line, not per-plug. Keep this in mind when sizing a fuse and heating element.
  5. Ground Wires. You need to ground a metal box and any external plugs. Even if you only use a 2-pin heater cable, you need to provide the ground for later use, in the event someone decides to plug in a 3-pin heater.
  6. Power Switch: Make sure the power switch cuts the “hot” wire for all the heating elements and control box.
  7. Heating Elements: Almost all heating elements require a dissipation heat sink, else they burn open. So, don’t use your heating element if it isn’t in contact with some type of heat sink.

This project will need the following components:

  1. Temperature controller
  2. Box to hold the controller
  3. Power switch
  4. Power plug assembly with fuse
  5. Relay (I like traditional mechanical relays, they don’t need heat sinks)
  6. Wire strippers and cutters
  7. Wire – strip off the insulation on the ends (about ¾ inch)
  8. Outlet plug assembly (optional if you heater has a plug)
  9. Internal power supply to operate the low voltage control, or buy an AC relay
  10. Terminals & wire nuts
  11. Diode
  12. Resistor
  13. Thermocouple
  14. Saw/drill/hot glue

Wiring Diagram


We start with the box. Cut out the mounting holes for the power plug assemblies, power switch and temperature controller. These need machined out, or you can use a rotary cutting tool and file to smooth down the edges. Snap in the panel mount plugs and switches.



Next, we connect the wires to the receptacles (back left two blocks). Notice the black (feed), green (ground) and white (neutral) wires connected to the output receptacles. This type uses a terminal… I soldered the wires in to the connectors. So, each output connecter gets three wires connected to it.

Now, cut in the power lines for the relays (right middle), connect with crimp-on terminals. Black wires are for the line power. The power feed comes into the common “C” pin on the relay – attach a black wire here (for now this won’t connect anywhere – we will connect it at the end). The plug (output to the heater) black wire goes to the output of the relay, or the normally open “NO” pin. That is that the heater is off until turned on. The “NC” or normally closed pin is “on” until turned off. So, you should have a wire going from the relay to the plug, and from nowhere to the relay.


Now, insert the temperature controller (bottom left) and start to connect its wires. The feed power for that has been roughed in, white and black wires connected by terminals (top right on the controller). The braided silver line is the thermocouple. What you don’t see is that there will also be control wires from the controller output 1 and output 2 (both on the bottom side of the controller). These are low voltage wires that will carry 12VDC to activate the relays.


Next, solder wires onto the power inlet receptacle (top right). The solder lugs are labeled “N” for neutral (white wire), “G” for ground (green wire, in the center), and “B” or “L” for line (black wire). The black wire lug will be under the neutral, since this assembly has a built in fuse holder.

From the power inlet (top right), connect the black wire directly to the power switch (bottom right). Then solder a new black wire from the other switch terminal for a common feed line. This way the switch cuts all the power for the entire box. This common feed line will then connect to all the other black wires, providing them with power.

Now, we add the internal powersupply and internal receptacle to power it (this will have three feed wires – white, black, and neutral). As shown in the wiring diagram the negative terminal of the power supply will goto the common pin of the temperature controller output. If using two outputs (as shown here), a jumper will connect and provide power to both the commons of the two outputs (on the controller). Then a wire goes from the normally open terminal of the temp. ctrlr. through a resistor (you need to provide this, see calculation in wire diagram above), and then to control pin of the relay. We add a resistor to keep the voltage and current low to the relay input. Here we have two outputs and two relays. Each relay will connect to one outlet receptacle – you want to label these so you know which is output 1 and 2.

Here the relay control power feeds are connected by crimp-on terminals. We add a small diode to allow the relay coil to “drain” when it is switched off. Sometimes a phantom voltage or current can build up on the line and cause the relay to stay latched-on. This diode provides a reverse path for the power stored in the relay coil to drain back and discharge the relay. Think of it releasing the pressure after shutting off the garden hose.

We complete the DC control voltage loop by connecting the other relay control pin to the positive line on the powersupply (see wire diagram above). Keep in mind that the diode is a directional flow controller. It only allows current to flow in one direction. We want the diode to only work after the temperature controller turns off. So, we install the diode as to not short cut the power flow through the relay. If we install the diode the wrong way, the power will flow directly through the low resistance diode and never go though the relay.

Before we finish, we need to add a green – ground wire to ground the case. This case is coated in paint. I took a bit of sand paper and removed the paint in one area and soldered the ground wire to the case. It is hard to see in the picture, it is the green wire on the far upper left that goes under the receptacle.

Now, the last step is to connect all the power wires together. Make sure the wires are stripped back about ¾ of an inch. Lightly bundle and twist all the black wires together, screw on the wire nut. Do the same for the white and green wires.

Review: All the white wires are neutral lines, they are at the same potential. So, we have white wires come in through the power inlet (top right) and connect to the controller box, powersupply, and outlet receptacles. All the green wires are ground. The ground wires come in from the power inlet connector and go to the case and outlet receptacles. The black wires are feed lines. We have a black feed wire that comes in through the power inlet, goes directly to the switch, then comes out of the switch and goes to the common terminals of the relays, the DC power supply, and the controller.


Finally, use the hot glue to pin down the power supply, relays, and reinforce the panel mount switches and receptacles. Attach the cover.




One last note: If you plan to build something like this, you need to make certain that your wire can handle the load. Check out wire size and load capacity charts. This project is for a very low power heater and fan for cooling… it is using computer cable for power feed (about 18 AWG wire) rated for about 10A (and the fuse is rated for 5A).