NI9219 2 wire

[TomFTC]

Hi,

 I am trying to take thermistor emsurements using the NI9219 in the Automation explorer. As a first step, I am trying measure 1 120 Ohm and a 30 Ohm resistor.

 Device:  NI9219 ( with USB-9162 USB carrier)

 wiring:  Pin 3 and 5  

 As an ouput,  I am not getting the resistance value

 Settings:

 Channel 2"   120 Ohm resistor

 Max input range:  200 Ohm

 Min input range:  10 Ohms

 Internal current source set to 500 uA 

 channel 3:  30 Ohm resistor 

Max input range:  40 Ohm

 Min input range:  5 Ohms

 Internal current source set to 500 uA 

 Result:  I am getiing a signal reading which is not in Ohms.

 I  am not doing something not correctly. Any ideas where my error is ?

 Thanks. 

[mikeporter]

You are not measuring resistance. In fact if you are simply putting the resistors across an analog input, you aren't measuring anything.

What sort of background do you come from? Do you know what ohm's law is? You see ohmmeters don't really measure resistance at all. They measure voltage drop across a resistance and knowing that voltage and the current allows them to **calculate** resistance.

Taking this discussion one step further, reading a thermistor is just a very fancy resistance measurement where the calculated resistance is used to calculate a temperature.

Thermistors need an excitation current and a series load resistor (that is not temperature sensitive) so you can measure the voltage drop across the thermistor.

Mike...

[RavensFan]

Mike,  Actually it looks like he has it wired up the way the manual says to for a 2-wire resistance connection.

http://www.ni.com/pdf/manuals/374473e.pdf  page 18