Design circuit to hold temperature at 200 Celsius

Hello,

Im trying to design a circuit that would hold a small coil, about the size of a penny at 200 Celsius. Also trying to keep the size and cost way down. I believe I should be using a Thermistor but I'm not 100% sure, I checked out the temperature controlled fans but had some trouble, is it possible someone could put me in the right direction?

I'll guess a termistor is the best choice for measurement.

How are you planning to maintain a stable temperature? By regulating a fan or by enable/disable current flow through your coil?
Just because I'm curious, what's the purpose of the circuit?

trying to build a hand held crafting pencil.

I understand the concepts of a thermistor I think they react by changing the resistance according to the temperature and thus I would need to create a way to detect the resistance at x which would trigger the device to turn off?

Looking at thermistors on the net, how would I find one that could work at 200c?

For heating and thermostabilizing a small volume or device, I can recommend the simplest way using an iron thermostat contact and a power heating resistor, e.g. a 25W or 50W 5-Ohm resistor with a 12 VDC power supply.
The iron thermostat can be adjusted to 200 C set as a tripping point and operates extremely reliably.

I have also designed thermostats with thermistor sensors, a LM311 comparator driving an IRF520 Mosfet switch in heater circuit. Sometimes I added an optical power-mosfet coupler to switch a fan, so the heater and fan operation alternated to keep the system temperature to +/- 1 deg.C.

But the iron thermostat is easily available, no electronics involved, and cheap.

jiripolivka said:

For heating and thermostabilizing a small volume or device, I can recommend the simplest way using an iron thermostat contact and a power heating resistor, e.g. a 25W or 50W 5-Ohm resistor with a 12 VDC power supply.
The iron thermostat can be adjusted to 200 C set as a tripping point and operates extremely reliably.

I have also designed thermostats with thermistor sensors, a LM311 comparator driving an IRF520 Mosfet switch in heater circuit. Sometimes I added an optical power-mosfet coupler to switch a fan, so the heater and fan operation alternated to keep the system temperature to +/- 1 deg.C.

But the iron thermostat is easily available, no electronics involved, and cheap.

Click to expand...

Cool, Im pretty interested in the first example as my first attempt. Can you give me an example power heating resistor you have used before or maybe know of one you wanted to try?

There are several high-power resistors available, molded in aluminum bodies and rated 10 W , by Ohmite; you can choose a suitable resistance to match your DC power supply or AC transformer. Estimate the dissipated power according to the size of the body to be heated.
The mentioned resistors are specified to be mounted by two screws on a heat sink. My experience shows that by attaching a small aluminum sheet, 0.1" thick and 2"x2" square, 10 Watts will easily heat the object to 200 deg.C in 20 seconds; then you can attach the bimetal adjustable thermal switch next to it, and using a thermocouple or any suitable thermometer you can set the switch to keep the body at 200 +/- 10 deg.C .
The temperature will vary according to thermal loading of your controlled heater. Instead of the resistor by Ohmite, you can make your heater from a heating resistive wire. Old "open" tubular resistors with values < 200 Ohms offer a good source of such wire. Use an ohmmeter to adjust or estimate the length after unwinding the wire from an old resistor of a known value.

Hello there,

I just noticed in MCM Electronics catalog that without much tinkering you can buy for $13.88 an "adjustable solder station" by TENMA, order # 21-7945. It is a fine soldering iron with an adjustable temperature from 100 to 800 F, so setting 200 deg.C is pretty easy.
You can replace the soldering tip with a similar tool fitting your purpose. For more money you can find "better?" digital-display fancy soldering irons. Please look at "www.mcmelectronics. com".