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Power and Current measurement using LM25056Eval board

Brennen DiRenzo
Prodigy 20 points
Other Parts Discussed in Thread: LM25056

Using the LM25056Eval board we are not sure how to determine which Rs to use in order to measure current. We are also not sure if we should put this resistor across Vs+ and Vs-, on the Rs2 pads, or across, Rs+ and Rs-. Our input voltage ranges between 0 and 15 V, with a load resistor of 270 Ohms. We currently have Vs set to the default 30mV. When taking readings over the I2C bus we only get 0000.

  • 0
    TI__Guru* 97775 points

    Brennen,

     

    I would start by measuring a larger current. I think your problems arise because your load is very small.

    First note that the sense resistor already mounted on the board is 0.004Ohms (This is just above the device with the copper leads). With 12V and a 270Ohm load, it means that you have a load current of 44mA.

    Across your sense resistor you would see: 44mA x 0.004Ohms or 17.8uV. On the 30mV scale this is just over one LSB.

    If you have some power resistors at your disposal or an electronic load, I would use a larger load current. If you would like to measure smaller currents, you would need to remove the existing sense resistor and replace it with something much larger. For example if you use a 0.1 Ohm resistor, the LM25056 would be able to measure about 300mA of current (assuming a 30mV full-scale range)

     

    Joseph Wu

  • 0 in reply to Joseph Wu
    Prodigy 20 points

    That all makes sense.  I'll start with reducing the size of the load resistor and let you know how that goes.