# School Task with NTC

I need to solve the task shown on the picture.

The first part is to calculate the temperature (in °C) from the NTC when Uv = 2V is. This means Iv is infinite.

The given Values are: Uq = 12V, R1 = 100Ohm, R2 = 200Ohm, R3 = 300Ohm

NTC: Rn = 1kOhm, b = 2000Kelvin, Tn = 293K

I tried using the formula: Uv = Uq((R2/(R1+R2)) - (R3/(R3+NTC))) and got 600Ohm for the NTC. After using it in the forumla for calculating an NTC I got only 81.85 degrees Celsius.

Can someone help me?

## 2 Answers

Assuming E3 is greater than E2, here's another way to do it:

Leaving you do to the work but here's a clue where you have gone wrong.

Why did you subtract the resistance of the NTC.?

                                " R3/(R3-NTC) "


The resistance is POSITIVE it is the change in resistance (dR/dT) that is a negative slope.

ADDITIONAL EDIT:

Note the direction of the voltage source. Their will be 4V across R1 and as point A is more positive than point B by 2V that means the voltage across the thermistor will be 2V.

The thermistor and R3 also form a voltage divider so 2V must also equal 12Rx/(R3 + Rx). Gathering terms and simplifying this gives a value of 60 ohms for the thermistor not 600.

• Ah, I made a mistake there. It should be +. – Pascal Oct 2 '14 at 14:12
• But I did not make this mistake in my calculation. So I still have no clue how to solve this – Pascal Oct 2 '14 at 14:25