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[on:]

Hi,

I have a lot of thermistor to read and I am thinking using the internal pull-up resistor so i can connect directly the thermistor to the pin and the ground.
So i don't need to connect a lot of 10k resistor, one for each thermistor.

On the Arduino Uno description page :
http://arduino.cc/en/Main/ArduinoBoardUno
It says that the internal pull-up resistor is of 20-50 kOhms.

Is it a variable resistor value ?
Is there a way to know precisely the resistor value ?

Cordially,
Alex

[Reply #1 on:]

Is it a variable resistor value ?

No.

Is there a way to know precisely the resistor value ?

No.

I am thinking using the internal pull-up resistor so i can connect directly the thermistor to the pin and the ground.
So i don't need to connect a lot of 10k resistor, one for each thermistor.

Don't. 10k resistors are cheap and small. Use them.

[Reply #2 on:]

I was able to calculate the internal pullup resistor and the value is 34kOhm for my Arduino Uno board but what is wrong to use the internal resistor ?

[Reply #3 on:]

I was able to calculate the internal pullup resistor and the value is 34kOhm for my Arduino Uno board but what is wrong to use the internal resistor ?

The value of the internal resistors (each pin has its own) can be (and probably is) different. 

You would need to determine the value of each pin's resistor.  Then remember that those will differ from chip to chip / pin to pin.

[Reply #4 on:]

The internal pullup resistors are great for use on digital inputs, but I would not use them on analog inputs used with analogRead(), because of the variability of the value of the resistor as explained in previous replies.