Thermistor for Temperature Regulation

[Ads]

Hello Everyone

Does anybody have any experience with Thermistors?… I am planning to develop a circuit to regulate the temperature of a soldering iron and would like to use a Thermistor and a PIC to test what range the temperature is in:
<50 C
50 - 225
225 - 260
260 - 290
290 - 325

(resolution is not a major issue – precision could be as loose as plus or minus 10 degrees C).

Obviously I need a Thermistor with a temp range of at least 0 to +325 degrees C. and was recommended that to use with a PIC with no additional signal amp it should be around 1k at room temperature.

Does anybody know where I would find a Thermistor that can deal with these ranges? The most I can find only go up to 125 degrees C.

Also has anybody ever designed a temp testing circuit using Thermistors that could give me any pointers (this will be the first circuit I’ve designed as I have very little electronics experience).

Thanks everyone.

Adam.

 

[Nigel Goodwin]

You don't use a thermistor, you use a thermocouple!.

 

[Analog]

Thermistors need excitation voltage, whereas a TC generates its own voltage. If that doesn't bother you, then a thermistor or TC or RTD for that matter would be just fine.

 

[Nigel Goodwin]

Thermistors need excitation voltage, whereas a TC generates its own voltage. If that doesn't bother you, then a thermistor or TC or RTD for that matter would be just fine.

That's not the problem, the problem is the range required - most thermistors don't go high enough.

 

[Hero999]

A small wheatgrain light bulb can be used but the resistance is quite low.

Don't thermocouples need a cold junction?

 

[Nigel Goodwin]

A small wheatgrain light bulb can be used but the resistance is quite low.

Interesting idea! - I'd not heard of that!.

Don't thermocouples need a cold junction?

No, most temperature probes for multimeters are thermocouples.

 

[Sceadwian]

Sorry Nigel but yes, thermocouples require a cold junction. In the case of a multi meter with a temperature sensor the cold junction is inside the meter and closely coupled to a thermistor, once the temperature of the cold junction via the thermistor is known the voltage between it and the sensing end of the thermocouple will mean something. The type of material used for the cold junction depends on what type of thermocouple probe you're going to use. Wikipedia has an exaustive entry on exactly how thermocouples work.

 

[SPDCHK]

A small wheatgrain light bulb can be used but the resistance is quite low.

Don't thermocouples need a cold junction?

Actually, Yes, for industrial use they do need cold junctions. In local RKC thermometer equipment the cold junction is done on the PCB itself. (Usually a small thermistor).

But as for Ads requirements, you could get away without cold compensation. He says that accuracy (resolution) is not an issue.

 

[Sceadwian]

The cold junction isn't a thermistor, it's just closely coupled to a thermistor. Apparently no one here seems to understand that a thermocouple requires two very specific types of different metals in order to work. In the case of a type K thermocouple which is very common, one end is a nickel/chromium alloy, and the other is an nickel/aluminum/manganese alloy. When one end has a higher or lower temperature than the other a voltage differential exists between the two materials. In order for that voltage to mean anything the 'cold junction' has to be at a known temperature because the voltage is a differential measurement of the temperature, not an absolute. This can be done simple and cheaply with a thermistor attached closely to the cold junction. If you don't compensate for the cold junction temperature then the ambient temperature of the measuring device is going to affect the reading. If you calibrate it for room temperature you could be off in the winter by 70 degree's and in the summer +10-20 or more depending on the device enclosure. So basically if this is for a soldering Iron, cold compensation would be an absolute requirement.

Edit: In case you care, the exact names of the materials are Chromel and Alumel those are trademarks of some company, but if you can find a supplier of those two types of metals you can make your own thermocouples, although I'm not sure what kind of material is used for the wire connecting the two.

 

[philba]

don't most temp controlled soldering irons use the resistance of the heating element to determine temperature? I've seen that stated several places. kind of makes sense - cheaper

 

[Nigel Goodwin]

I've never seen one that does, all those I've used have an extra pair of wires from the sensor - with the exception of Weller, which use a crude curie effect system.

 

[Ayne]

may be another way, i think so.

measure the current flowing in the soldering iron, it will also indicate the heat.

 

[Sceadwian]

Only if the iron and it's tip have been designed properly and calibrated. A thermocouple is going to be significantly more accurate.

 

[Hero999]

Apparently no one here seems to understand that a thermocouple requires two very specific types of different metals in order to work.

I do, that's why I asked the question.

I suppose it makes sense to use a thermocouple for temperature measurement because it's operating temperature range is greater than a thermister.

 

[whiz115]

Also has anybody ever designed a temp testing circuit using Thermistors that could give me any pointers (this will be the first circuit I’ve designed as I have very little electronics experience).

one day i said: lets reinvent the wheel! and i came up with that...

**broken link removed**

i was so happy that it was my idea and i didn't copy it from somewhere else..so i was trying 2-3 hours to regulate it! but no luck! i don't know if it is helpful enough but i hope it can give you an idea!

btw if someone can help me improve this project i would be glad! the problem
is that the motor starts when the base is in higher voltage than when it stops...let's say it starts when the base is at 0,70V and stops when the base
is at 0,68V

 

[Ads]

Yeah, It looks like i'm going to go down the thermocouple route. I have a thermocouple to digital converter on order from Maxim-IC that will cope with the ranges so I guess it will be a case of tinkering around with it and a PIC when it arrives.

Thank you all for all your help.

Adam

 

[cadstarsucks]

one day i said: lets reinvent the wheel! and i came up with that...

**broken link removed**

i was so happy that it was my idea and i didn't copy it from somewhere else..so i was trying 2-3 hours to regulate it! but no luck! i don't know if it is helpful enough but i hope it can give you an idea!

btw if someone can help me improve this project i would be glad! the problem
is that the motor starts when the base is in higher voltage than when it stops...let's say it starts when the base is at 0,70V and stops when the base
is at 0,68V

Put the motor in the collector lead and a resistor in the emitter. The result is a crude current source that might help. The best way around it is to chop it to overcome the startup torque characteristic of the motor and maintain high torque at low speeds. The simplest, though not the most accurate circuit is a simple schmidt trigger oscillator with diode steering from the pot in the feedback loop to vary the duty cycle.

D.