NI 9505 Motor Bridge Internal Resistance

Updated Jun 12, 2020

Reported In


  • NI-9505

Issue Details

I need to guarantee some performances for what concern the voltage drop between the power supply and the motor to test. Knowing the value of the internal resistance of the module NI 9505 I can calculate this voltage drop. It is possible to know this value?


Unfortunately there's no resistance specified for the motor bridge of the module and the reason is that it's a non-linear/non-ohmic circuit. It's basically 4 transistors arranged in such a way that you can choose to output No Voltage, Positive Voltage, or Negative Voltage. The images below replace the transistors with switches, but it shows how the part works.


Above: H Bridge in "off" configuration. Below: H Bridge with current flowing in both directions.

File:H bridge operating.svg


Transistors don't have a resistance, per se, so there will be negligible voltage drop across the transistors in this H-bridge. They will basically act as conductors when in the "on" position.