Using a PTTC PolySwitch fuse as a temperature controller

[OperaHouse]

In another discussion I mentioned how a PTTC fuse might be used as a simple temperature controller because it has an almost exponential rise in resistance at a certain temperature. This was met with some skepticism. Some of you might not be aware of these devices. I found this experiment interesting enough to spend over an hour with it and you might find it interesting too. Maybe someone will even find an application for this or a new product. A lot of articles can be found if you google PolySwitch. Who knows how stable this would be long term, but I bet it could be improved if they had a market for that specification. Enjoy.

THE EXPERIMENT
Six 4.7 ohm resistors were connected in series next to a RXE-300 PolySwitch PTTC with thermal compound. A thermocouple was also attached to monitor the temperature with a FLUKE 2167A. These components wwere all bound together with some tape and placed inside two sheets of 3" x 3" styrofoam that was 0.4" thick. The surface of this warmed up and the layers of styrofoam were doubled for better insulation.

At an applied voltage of 10V, the temperature reached 121.8C. Five minutes later it was 122.1C, in another five it was 122.2C. Three minutes later it was still 122.2 and then the voltage was changed.

At an applied voltage of 14.3V, the temperature reached 122.6C after 5 minutes. Five minutes later it was 122.7C. To prove that a sufficient amount of heat was being lost, the power was then removed. After 3 minutes, the temp was 96.4C and two minutes later it was 77.1C.

Power was adjusted to 7.5V, but this was too low to bring it up to full temperature. Voltage was adjusted to 9V and the temp went to 121.2C. Three minutes later it was121.4C.

Voltage then adjusted to 19.9V and the temp went to 123.6C. Eight minutes later it was 123.8C. Not bad for a single component! Anyone remember how much the power increases when the voltage is doubled?

Power was removed and five minutes later the temp was 83C. 19.9V was then applied again and two minutes later, the temp reached a peak of 138C. This cycled down to 117C , two minutes later climbing to 121.3C in 4 minutes (the reason temperature controllers should be smart). Even in this little unit, the effects of thermal mass can be seen. Just grabbing the wires from 3" away would cause the temperature to drop a couple tenths due to thermal loss. Designing a thermal chamber where two points remain at close to the same temperature is not a simple matter.

In conclusion, a PTTC device can act as a simple temperature controller. It certainly performed much better than I expected since no effort was made to optimize the component values .

 

[itsmoked]

Opera; Fun experiment. I absolutely agree PTCs can be used as heater controllers as indeed thousands of them are in heat tracing.

The problems are the drift since this is a quasi-chemical reaction who's ingredients slowly change with use.

Setting of the temperature is hard since this stuff is only available in a few "trip points".

Certainly is a nice simple scheme!

Would be good in a situation as a backup/overtemp for a stuck-on heater that's supposed to run at a slightly lower temp.

 

[felixc]

If you want to use it this way, you will also have to think about a mounting or encapsulation method that removes any physical stress, including vibration, to the component. This device litterally fuses. When hot, its physical stregth goes down to nothing. It becomes two metal foils with a semi-liquid in between, becoming easy to damage.