SCXI 1338 - input voltage range

[Parny]

Hello All,

I am using SCXI-1338 blocks in SCXI-1125 modules. Currently the channels are measuring currents of range 4milliamps to 20 milliamps.

Will I be able to measure VOLATGE signals by connecting to SCXI-1338, and what is the acceptable voltage range of AC or DC? And also do I need to do any

changes in the SCXI-1338 block in order to measure voltage signals? I want to measure 20V DC signal driven by a PLC.

Thanking you.

[Lars_L]

Hey Parny,

Because the SCXI-1125 is only rated for a voltage range of +/- 5V(peak) or VDC, you will need some sort of attenuation before trying to measure a signal as large as 20V and the SCXI-1338 only has a 249 precision resistor. To do this, you could look at the SCXI-1327 which is a SCXI High Voltage Attenuator. Here is the product page: http://sine.ni.com/nips/cds/view/p/lang/en/nid/1680

Lars

[Parny]

This means, though SCXI-1338 is capable of handling 20V or more, the barrier is at SCXI-1125(since it can handle only + or - 5v), and ofcourse 1338 hasn't got attenuation functionality, am I correct ??

Message Edited by Parny on 05-07-2009 06:50 PM

[Lars_L · Accepted solution]

Well it really comes down to Ohms law in the end. For V/R = I, you will have 20V/250Ohms = ~80mA which is far over the limit of the 1125 and the 1338. I would recommend either going with the 1327 Voltage Attenuator or you could possibly do some external wiring with a voltage divider. If you place a 1kOhm resistor in series then your V/R = I equation turns into 20V/1250 Ohms = ~16mA max. To find your voltage, you will then have to create a custom scale to account for this new resistor. However, since most resistors are going to have some error, it would be better if you measured the resistance in this circuit using a DMM or an equivalent to get a more accurate estimate of the actual resistance.

In the end, the 1327 is going to be a cleaner solution but you could also go the voltage divider route if this doesn't work for you.

Lars

[Parny]

Indeed, we planned to go for external logic circuit. But at the end the whole plan has been changed now, and we don't need to measure that high signal for now.

I still accept this as solution for my scenario, will make an external voltage attenuator incase needed in future.

Thanking you again for your time.