Thermistor help ?

[curry87]

Ive got a 2 pin 100k ntc thermistor that i want to use to shutoff a mosfet if it were to overheat.How do i calculate the expected resistance of the thermistor when it is over 60c ?

 

[vne147]

The data sheet should have that information. What's the part number?

 

[curry87]

Its the same as this one, where i bought it from there was no datasheet for that thermistor.
http://i00.i.aliimg.com/photo/200327620/MF51E_NTC_thermistor.jpg

 

[Reloadron]

They look like pretty standard leaded epoxy coated thermistors. Without a data sheet there is no way to know the actual temperature to resistance curve. Your best bet would use a source of heat to heat it and at the same time compare it to a good standard thermometer or temperature measuring device. At 60 Degrees C note the resistance of it.

Ron

 

[crutschow]

If there's no part number or information then just heat it in water at 60C and measure its resistance.

Edit: Ron beat me to the punch.

 

[curry87]

Forgot to add it has B constant = 4850K if that helps ?

 

[ericgibbs]

Forgot to add it has B constant = 4850K if that helps ?

hi,
This formula will give you a close approximation.
Rdiff = R25 * Exp(-Beta * ((1 / (273 + 25)) - (1 / (273 + 60))))


Rdiff = 100000 * Exp(-4850 * ((1 / (273 + 25)) - (1 / (273 + 60)))) = 18K at 60C

28.37K at 50C
7.9K at 70C

 

[MrAl]

Hi,

Usually the B constant is given at 25 degrees C but you should check it if you can find it to make sure.

The formula given by Eric looks like the formula i've used in the past and it works pretty well. You might also want to note that when measuring the voltage it causes in a voltage divider made with upper resistor R fixed and lower resistor the thermistor (or vice versa) the highest dv/dT occurs when R=RT, which for your thermistor would be 18k at 60 degrees C. It also produces a rather convenient voltage point being half the supply voltage at 60 deg C.