# What Resistance Ranges Can Be Used with the NI 9235 and 9236?

Updated Mar 22, 2019

• NI-9236
• NI-9235

## Issue Details

• I am using NI 9235 with 120Ω quarter bridge completion, or NI 9236 with 350Ω quarter bridge completion. The quarter bridge strain gage I would like to use isn't exactly 120Ω or 350Ω respectively. What resistance ranges can I use with the 9235 and 9236?
• Can I use the NI 9236 with a 120Ω strain gage?

## Solution

The NI 9235 and 9236 perform quarter bridge completion of a Wheatstone bridge. The 9235/9236 specifications linked below state that the full range voltage for these devices are ± 29.4 mV/Vexc. We can leverage that range along with the Wheatstone bridge formulas shown below to figure out the ranges of our devices.

Figure 1. Wheatstone bridge circuit

Figure 2. Wheatstone bridge formula

Figure 3. Simplified Wheatstone bridge formula for quarter bridge completion

In the formulas above Vr/Vexc is the ± 29.4 mV/Vex specified in the manual. R1, R2, and R3 will all be 120Ω for the NI 9235, and 350Ω for the NI 9236. Using these formulas, you can solve for Rx to be the values shown in the table below.

 NI 9235 Maximum Resistance 135Ω Nominal Resistance 120Ω Minimum Resistance 107Ω NI 9236 Maximum Resistance 394Ω Nominal Resistance 350Ω Minimum Resistance 311Ω