Temperature:Speed Timer Circuit?

[MP1]

ante, a thermistor is defined as a thermally sensitive resistor whose primary function is to exhibit a change in electrical resistance with a change in body temperature. The NTC thermistor is a negative coefficient type and the PTC is a positive coefficient type. What this means is that the PTC will increase in resistance as the temperature goes up and the NTC will decrease in resistance when the temperature goes up. Although the resistance goes in opposite directions, they are both thermistors. A thermocouple is a different type of device. Is this what you were meaning? A thermocouple consists of two dissimilar metals joined together at one end. When the junction of the two metals is heated or cooled, a voltage is produced that can be correlated back to the temperature.

I use quite a few of the thermistor type temp sensors in the meteorological field. I use the NTC type since htey are the most common and readily available. I use the thermistor as the bottom leg of a voltage divider and run 2 VDC through the circuit. This voltage goes into the channel of a datalogger. The resistance of the thermistor determines how much of the voltage goes to ground. What you use as a resistor for the other leg of the voltage divider depends upon the range of the thermistor you are using. Whatever the value of this other resistor, I recommend a tight tolerance like .01%. Otherwise the drift will be more than the resolution.

It sounds like Twizted has a thermistor similar to a Radio Shack #271-110. This one has a spec of 27.28K ohms at 0 deg. C and 4.161K ohms at +50 deg C. If you are only looking at near room temp data, the device is rated 10K at 25 deg. C.

Hope this info is helpful.

MP

 

[ante1]

Hi MP,

Thanks for the info.Yes I am familiar with the difference between NTC and PTC resistors they are not very complicated. Here we use the PT-100 & PT-1000 for meteorological purposes not NTC:s. I use contact free measurement up to 100C the SMTIR9901-2 from Smartec. About thermocouples, I only use them for higher temperatures where other kinds don

 

[MP1]

Thanks for the link ante. I have mostly used the YSI types.
Hope you also have a Happy New Year!

MP

 

[Twizted]

Hi Twizted,
Your thermal probe has an enormous range to change its value from 50K when freezing to only 3K at body temperature. You just need to know its resistance at the temps that you are going to use it.

Try your circuit with the thermistor in series with a 1K resistor replacing the 47K timing resistor. Since the total resistance is now much lower, the timing capacitor's value should be increased so replace the 1uF cap to about 10uF.

Thanks audioguru, I will give that a go in the morning.. Right now I am about to crash.. Just need to make one more post looking for a supplier for a couple parts I need... thanks for the input I will let you know what happens..