# Adjustable Temperature Controller for Heating Elements

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## Introduction: Adjustable Temperature Controller for Heating Elements

There are a lot of different products that use heating elements. Unfortunately, many of them are not adjustable. For instance, most soldering irons are either on or off. It would be much more useful if you were able to change the temperature of the iron. This would allow you to use the soldering iron for lower temperature applications.

So I designed a simple control circuit that will let you adjust the output of a heating element.

## Step 1: Watch the Video

Here is a video walkthough of the project.

## Step 2: Materials

Here are the materials and tools that you will need for this project.

Materials:

Tool with a Heating Element

5 Volt Power Supply

Perf Board

555 Timer IC

1 kohm Resistor

22 kohm Resistor (may change depending on your application)

100 kohm Potentiometer (may change depending on your application)

Jumper Wires

5 Volt Relay

Diode

Twist on Insulated Connector Caps

Insulated Project Enclosure

Knob that fits the shaft of the potentiometer

Tools:

Soldering Iron and Solder

Knife

Wire Cutters

Wire Strippers

Screwdriver

## Step 3: The Control Circuit

This is the control circuit that I designed for this project. It is based around a 555 timer IC. This configuration is known as "Astable Mode." In this mode, the output a pin 3 is a series of HIGH and LOW pulse. This turns the relay off and on. The frequency and duration of these pulses is determined by the values of the resistors and the capacitor.

The time that the relay will be on is based on the formula:

t = 0.7 x R1 x C

The time that the relay will be off is based on the formula:

t = 0.7 x (R1 + R2 + R3) x C

In these formulas "t" is measured in seconds, "R"is measured in ohms and "C" is measured in Farads.

For my application, I used C = 0.000330F (or 330 micro farad), R1 = 22000 ohm (or 22 kohm), R2 = 000 ohm (or 1 kohm), and R3 = a 100000 ohm (or 100 kohm potentiometer). With these values the "ON" time was 5 seconds per cycle. Based on the setting of the potentiometer, the "OFF" time ranges between 5.3 seconds and 28.4 seconds.

You can change the timing of the on and off cycles by changing the values of the resistors and the capacitor. You can make the values whatever you want. But keep in mind that you always need to have at least 1 kohm of resistance between pin 7 and pin8 on the IC chip. If you don't, the IC can be damaged when the output turns on and off.

## Step 4: Prototype the Circuit on a Breadboard

With any electronics project, it is a good idea to prototype the circuit on a breadboard before soldering it together. This give you a chance to find any problems and make any necessary adjustments. For testing, I connected an LED to the output of the relay. This made it easy to see when the relay was on.

R2 from the circuit diagram is not shown in these pictures, I added that later when I was soldering the circuit onto the circuit board.

## Step 5: Prepare the Power Cord

In order to connect the heating element to the control circuit, we need to modify the power cord. First, I cut the power cord in half using a pair of wire cutters. Then I pulled apart the individual wires on each piece.Next I stripped the insulation off of the cut ends.

One wire from each side was connected together using an insulated twist on connector cap. This is effectively reconnected on of the cut wires. In hindsight, it would have been easier to simply leave this wire connected and never cut it in the first place.

Now you have one pair of cut wires that you can attach to the control circuit.

## Step 6: Solder the Circuit Together on a Perf Board

Once the circuit was working properly, I soldered all the components onto a piece of perf board. In the final configuration, I replaced the small potentiometer from the previous step with a larger potentiometer that could be mounted to the side of the housing. I also attached the wires from the power cord to the switch contacts on relay.

## Step 7: Mount the Circuit in an Insulated Project Enclosure

Because we are connecting to an AC power cord, it is absolutely essential that all the parts be mounted inside an insulated project enclosure.

I started with a generic project enclosure that I purchased from Radio Shack. I drilled a hole in the top so that I could mount the potentiometer. Then I cut slots in the sides so that the power cords cold go in and out of the box. I fit all the parts into the enclosure and closed it up. Then I added a knob on the shaft of the potentiometer to make it easier to turn.

## Step 8: Finished Temperature Controller

Now plug in the power cord of the appliance and plug in the 5 volt power supply that powers the control circuit. You can now use your control circuit to adjust the output of a heating element and control its temperature.

You can use this circuit for anything that uses a simple resistive heating element. You can use it on a soldering iron, a hot glue gun, a candle warmer/coffee warmer, a heat lamp or anything else.

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## Questions

0

Hi. May I ask what type of diode you have used in this project? Thanks

It is just a generic rectifier diode. It doesn't need to be anything specific.

Can this be replicated for an appliance that has a ground plug?

Yes. Just connect the ground wire of the input to the ground wire of the output directly.

Is there any reason R1 couldn't be a potentiometer?

How would I make the "on time" adjustable?

Use the formulas in step 3.

This is not a controller. If a lot of heat is used, the temperature will drop. A controller measures and compares with the value of the setpoint. Based on the difference between measured value and setpoint it will control the heat.

Most controllers for heating elements don't have feedback. They just set a certain output. Then the user adjusts the setting to compensate for the environment and working conditions.

can anyone tell me where to get the 5v relay, and how to wire the 4 pins to the circuit.

You can purchase them from Radio Shack or any online electronics parts distributor. I frequently use Mouser.com.

Just connect it according to the circuit diagram. If you need help reading the circuit diagram, check out this Instructable:

Have there been any issues with the contacts sticking? From my experience each time a contact opens there is a spark generated. This causes not only carbon build-up but also heats those contacts which will end up welding them shut...Unless there is as spark arrestor.

That can happen. It have only happened to me when it is turned on and off rapidly. If you keep a low duty cycle it should be a problem. You can also replace the relay with a solid state relay to avoid that problem.

This is a very documented & clean ible! Although I'm still trying to figure out if there is any functional difference between this & an off the shelf sliding lamp dimmer?

Also, just a thought, with some minor modifications and substitutiion of a thermistor for the potentiometer you could give this controller a feedback feature.

2 replies

This kind of circuit can be a lot more precise than an off the shelf dimmer. It also has a wider operating range. Dimmer only work in the range where a light bulb would glow.

Annnnnd I just thought about it and realized that the sliding lamp dimmer reduces the voltage going to the output while your device uses PWM.

Just an idea.. But you could open up the 5v transformer and wire it to the "lamp cord" (before the relay of course) and tuck everything inside a bigger project enclosure, that way you would only have to plug one cord in.