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9237 built in excitation?

OK, I'm not sure if this goes here or not so please forgive me if it's in the wrong spot. I have a NI cDAQ-9178 with 5 NI 9237. I'm using NI 9949s to connect my potentiometers. 

I'm going to be using 10 pots and 10 straing guages eventually, but at the moment I just have one pot hooked up. It's a 10k pot and I have it hooked up with a 9V battery and 1.44 Mohms of resistors I'm trying to determine the degrees that the pot has been turned. So that's finally all hooked up and we've decided that there's no way we can use a seperate 9V for each of these things. We want the data we collect to be the same each time, so we can't have the battery running low affecting it. 

 

So, my question is, how do I use the built in excitation? I see there's this square at the bottom with 4 square holes and it has EX+ and EX- written on it, but I have no idea what plugs into that. 

 

Also, I know that the DAQ can handle up to 150 mW power, right? If I use the built in excitation, what size pots and strain guages should I use? I haven't bought any of them yet, I'm experimenting with the 10k pot now but I could get different sizes when I order all of them. 

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Hi Krickette,

 

The NI 9237 is a Bridge Measurement module, and what you want to do is just measure voltage across a pot, which is definitely not what this module was made for. Since a bridge is made for measuring really low changes in voltages due to small changes in resistance, it has a high precision, but because of that it will not have a high range of values it can measure. What you want to use is an Analog Input module such as the NI 9205, which can measure up to 10 V, or a multifunction DAQ card, e.g. the PCI-6251.

 

That being said, if you don't have another DAQ card at your disposal, you are correct in saying that the NI 9237 has a maximum power dissipation of 150 mW as you can see from the specifications on page 16. On page 17 the specifications say you can use external excitation if you connect an external voltage source to the EX+ and EX- (on the four-terminal connector you mentioned) you can then apply the excitation using the EX+ and EX- (pins 6 and 7 if using ai0 with the RJ50 connector, or pins 2 and 21 using ai0 with the DSUB connector). On page 29 you can see that we have a maximum of 10 V the recommended external excitation you will provide.

 

Regarding the value of the pot, you want to make sure that you have a separate resistor that is going to limit the amount of current you are going to draw from your excitation voltage. If you just have your pot directly connected to your excitation voltage source, and you lower the resistance to its minimum, you are going to demand a really high amount of current to your excitation voltage (short circuit) and you do not want that. You can have a resistor of 10K ohm and measure that resistor instead of the pot. The pot will be connected to the resistor, but you will be measuring the voltage of the resistor. As you change the value of the pot, the value of the voltage on the resistor will change.

 

Regards,
Daniel REDS
RF Systems Engineer

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