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Using a rheostat in place of a Glass Encapsulated NTC Thermistor?

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sheepsblood

New Member
I have a flat iron hair straightener that I have pulled apart in the hope I may be able to control the temperature to a lower heat value. Currently, the lowest it reaches is around 330F. I would like to drop it to around 200-250F. I am wondering if anyone can help by telling me if I can replace the Glass Encapsulated NTC Thermistor with a Rheostat. I have chosen the idea of a Rheostat because I figured I could raise or lower resistance and trick the device into thinking it has already reached it's given limit. Though, I feel this Glass Encapsulated NTC Thermistor could be a thermal protection device and not what is regulating overall temperature.
The flat iron also has a potentiometer, which does allow temperature to be changed. IDK what the value of the potentiometer is, there are no Part Number markings that I can see. (At least without de-soldering it first maybe). I thought maybe if I got a potentiometer to replace the original one that had a different resistance that it may allow me to reduce the temperature also.

Please share your thoughts, ideas, and point me in a direction with some explanation if you will.

Thank you for reading and considering.
 

MikeMl

Well-Known Member
Most Helpful Member
Hi Blood,

Here is what I would do:

Modify the wiring around the thermistor (but leave the thermistor installed in the iron) to add a 100kΩ pot wired as a rheostat (only two wires) in SERIES with the thermistor. Initially set the pot to zeroΩ and the iron's built-in control to minimum temperature. Heat up the device and let it stabilize. Now turn the new pot to add resistance in-series with the thermistor, say about 10KΩ at a time. If the temperature drops, then keep increasing the resistance until the new temperature decreases to what you want. If the temperature increases as the added pot is increased, then go to plan B.

PlanB. Rewire the added pot in PARALLEL with the thermistor. Initially set the pot to maxΩ (>100KΩ) and the iron's built-in pot to minimum temperature. Heat up the device and let it stabilize. Now turn the pot to reduce the effective resistance of the thermistor. If the temperature drops, then keep decreasing the added pot resistance until the new temperature is what you want.

If either of these gets you the correct new minimum temperature, then remove the added pot, measure its final value, and add a fixed resistor of that value either in series or in parallel with the thermistor, which ever worked. The built-in control should still adjust the temperature, just from a new, lower starting point. If I guessed wrong on the thermistor resistance, you might have to repeat the tests with a 10kΩ added pot instead of a 100kΩ one.

Keep one hand in your pocket while working on this. The circuitry might be connected to the AC line with no isolation.
 
Last edited:

Tony Stewart

Well-Known Member
Most Helpful Member
If it is NTC to reduce temp and simulate a higher Temp, you need to use a Pot in Shunt not Series

ie. Lower R to simulate a higher T and thus get lower power out and lower T.



The other thing is if the Thermistor is poorly attached with a gap or loose, then it will give a low reading and result in a hotter iron.
 
Last edited:

sheepsblood

New Member
Hi Blood,

Here is what I would do:

Modify the wiring around the thermistor (but leave the thermistor installed in the iron) to add a 100kΩ pot wired as a rheostat (only two wires) in SERIES with the thermistor. Initially set the pot to zeroΩ and the iron's built-in control to minimum temperature. Heat up the device and let it stabilize. Now turn the new pot to add resistance in-series with the thermistor, say about 10KΩ at a time. If the temperature drops, then keep increasing the resistance until the new temperature decreases to what you want. If the temperature increases as the added pot is increased, then go to plan B.

PlanB. Rewire the added pot in PARALLEL with the thermistor. Initially set the pot to maxΩ (>100KΩ) and the iron's built-in pot to minimum temperature. Heat up the device and let it stabilize. Now turn the pot to reduce the effective resistance of the thermistor. If the temperature drops, then keep decreasing the added pot resistance until the new temperature is what you want.

If either of these gets you the correct new minimum temperature, then remove the added pot, measure its final value, and add a fixed resistor of that value either in series or in parallel with the thermistor, which ever worked. The built-in control should still adjust the temperature, just from a new, lower starting point. If I guessed wrong on the thermistor resistance, you might have to repeat the tests with a 10kΩ added pot instead of a 100kΩ one.

Keep one hand in your pocket while working on this. The circuitry might be connected to the AC line with no isolation.


Thank you for your response. Your last line made me laugh, maybe you should have suggested which hand :)
I know, don't let the voltage cross your heart. I spent 10 years as an Aircraft mechanic but spent little time in electronics. I ended up changing professions to Computer Science.
 

sheepsblood

New Member
If it is NTC to reduce temp and simulate a higher Temp, you need to use a Pot in Shunt not Series

ie. Lower R to simulate a higher T and thus get lower power out and lower T.



The other thing is if the Thermistor is poorly attached with a gap or loose, then it will give a low reading and result in a hotter iron.
Thank you for your response. Are you suggesting a gap inside the glass? or loose inside the glass? I am unfamiliar with NTC Thermistors. I can see this one, I removed the protective gold colored wrapping and I have some handheld microscopes and jewer glasses. Maybe I can get a closer look.
 

spec

Well-Known Member
Most Helpful Member
I have a flat iron hair straightener that I have pulled apart in the hope I may be able to control the temperature to a lower heat value. Currently, the lowest it reaches is around 330F. I would like to drop it to around 200-250F. I am wondering if anyone can help by telling me if I can replace the Glass Encapsulated NTC Thermistor with a Rheostat. I have chosen the idea of a Rheostat because I figured I could raise or lower resistance and trick the device into thinking it has already reached it's given limit. Though, I feel this Glass Encapsulated NTC Thermistor could be a thermal protection device and not what is regulating overall temperature.
The flat iron also has a potentiometer, which does allow temperature to be changed. IDK what the value of the potentiometer is, there are no Part Number markings that I can see. (At least without de-soldering it first maybe). I thought maybe if I got a potentiometer to replace the original one that had a different resistance that it may allow me to reduce the temperature also.

Please share your thoughts, ideas, and point me in a direction with some explanation if you will.

Thank you for reading and considering.
Hi SB,

If you could trace out the circuit of your flat iron and post a sketch; that would be a big help.

spec
 
Last edited:

Tony Stewart

Well-Known Member
Most Helpful Member
The temp sensors must be tightly attached to heater. the gold tape is called Kapton tape or polyamide is a high voltage insulation that can withstand high temp. A photo is best
 

MikeMl

Well-Known Member
Most Helpful Member
... I spent 10 years as an Aircraft mechanic but spent little time in electronics. I ended up changing professions to Computer Science.
Thats ok. I spent most of my working life as a Computer Scientist, and now dabble in restoring aircraft. I presently have the engine out of my C182 and will be reinstalling it after it comes back from overhaul.
 
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