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How the mbed knows real current?

Topic last updated 01 Aug 2012, by Kevin Hobbs. 1 reply
Tejas Panchal
01 Aug 2012

I am sensing the current from the terminal of the induction motor with use of the hall effect sensor. As we know the output of the sensor in term of the voltage in range of 3.3Volt. now my question is that How the mbed know the real value of the current from the sense value? IS there any look up table is required? How can we scale it?

Kevin Hobbs
01 Aug 2012

You need to calibrate or know the sensor characteristics i.e. Sensor Const =Volts per Amp.

You then need to derive an equation, e.g.

current = volts * Sensor Const.

Now in practice the sensor will be bipolar so you really need an op amp to bias the sensor and provide gain to match the ADC 0 to 3.3V range.

The mBed code will then take 0 amps or the zero sensor volts as 3.3/2 i.e. 1.65V so the equation above will need to take this into account:

current = (volts - 1.65) * Sensor Const * Op Amp gain

Hope this points you in the right direction