Regulating precision heating

Question

Whats the simplest electr(on)ic(al) method for heating a coil or similar element to a specific temperature?

Generating heat is easy; dictating and regulating a specific temperature is the hard part.

For example, a modern kettle might have a wax thermostat that opens/breaks the circuit once it reaches 100°C, but it doesn't maintain, hold or regulate that temperature. The water doesn't continue to boil indefinitely. It simply reaches boiling point and then the heat is removed. I'd like to reach the desired temperature, but not exceed it, and then just stay that temperature.

Some consumer products with this kind of capability:
Inductive cooktops, laboratory hotplates, heating mantles, crucibles, furnaces, climate control devices, etc.

Answer 1

What accuracy do you want for the control, for instance the peak to peak temperature excursions?

The very simplest would be to strap a bimetallic switch to the element. However, due to the time lag between the element heating and the strip heating, and the hysteresis present in the switch, you could expect possibly 10°C to 20°C cycling in the element temperature.

If you want tighter control of the excursion, then you would need some form of continuous sensing, with a PID control of the heat.

My definition of 'simple' was to go onto a well known online auction site and buy a cheap PID temperature controller, for a few 10s of GBP, which takes a thermocouple input, and drives a PWM relay output. It has worked with default tuning for every heater/load scenario I've thrown at it so far, from a sous vide water bath, to hot air ovens.

Depending on your heater, you could perhaps sense the temperature directly. Most resistors and resistance wires used in heaters have near zero tempco. However, if you can build the heater from a straight metal (iron, copper, stainless steel for example), then you can sense the resistance of the heater to control the input power. As this is very fast responding, you should be able to get relatively small excursions with a simple on/off controller. You can either sense while driving, with an asymmetric Wheatstone Bridge arrangement, or drive with PWM and measure the resistance during the 'off' times.

Answer 2

The absolute simplest is ON/OFF control where the heater is turned on when the temperature is too low and off when the temperature is at or higher than desired. For a setup to achieve better stability of the temperature you will want to actively control the average power delivered to the heater.

If you are using AC power to drive the heater then the simplest method is to use phase control of the heater power to control the heat level. This would work very similar to a light dimmer switch except that a compute element sets the turn on phase for the triac switching element. A temperature sensor feeds the compute element the detected temperature.

If the heater is being supplied power from a DC source then the heater would be setup with a MOSFET switching circuit so that a PWM signal can determine the amount of average power being delivered to the heater. From this point the control mechanism is still a compute element with a temperature sensor input.

There are multiple control algorithms available for implementation on your compute element. Some examples are proportional control where the PWM/PHASE setting are set proportional to the difference between the desired and detected temperature. Complexity can increase up include a full PID control loop that can give faster approach to the desired temperature while still maintaining a stable target value.

Hopefully this gives you something to think about so you can start research into just how you want to proceed. If it all seems too complex it is possible to purchase ready made temperature controller devices that implement the control algorithm and control the power switching device.