March 16, 2018

How to measure AC/DC current with an hall effect sensor?

Filed under: Electronics — Tags: , , — mosioX @ 6:59 am

Hello Everyone!

In this article, I’ll talk about a current measurement technic for home appliances by using an hall effect sensor. You may ask why you are mentioning about ordinary current measurement topic although there are plenty amount of off-the-shelf digital multi-meters. There are two reasons, one of them is most of the DMMs do not have USB/UART interface to dump their measurement into the PC, and if you would like to measure the current from high voltage source they do not provide enough isolation for the equipment.

This project consists of two major components. First one is the front-end for the hall effect sensor. The sensor I used in the project provides with current measurements as a current source. Simply when we apply +/-12 volts to its supply terminals (it requires symmetrical power supply to be able to measure negative current), the sensor creates current, proportional to the measured current. In this case (for LEM, LA 100-P) this ratio is 2000:1. It means, when we apply 2 Amp through the sensor, it provides 1 mAmp as measurement output.
Second one is the acquisition unit. Main responsibility of this unit is to convert analog readings into 10 bits digital values. It also calculates DC and AC RMS values by using sampled current values.
As can be seen in the picture below, this acquisition unit creates a bridge between front-end and a regular PC via UART interface.

Fig.1: Measurement with validation setup.

In this measurement example, I am using a vacuum cleaner to simulate a load.  Let’s take a look at the measurement with results.  As can be seen on Fig.2, I am calculating both average and root mean square value of the current.  This technic helps me to measure AC and DC component of the signal, especially if I do not have any idea of the characteristics of the measured current.

Fig.2: Terminal output.

Let’s analyze the details of the analog measurement part of the system.  One important part in the design is Op-Amp section.  As I stated before LA 100-P hall effect sensor behaves as a current source.  This means if we want to measure the signal with an ordinary ADC, we would need to convert this current into voltage by using Transimpedance amplifier first.  Another important function of this module is to map converted voltage value to ADC measurement range.  Current configuration of the acquisition circuit uses dsPIC30F4012, and its ADC got configured for 0 – 5 Volts operating range. Also 2.5 Volts off-set has been added to create threshold to detect direction of the measured current.  Simply if ADC sees 2.5 Volts at the output of the front-end, calculation algorithm in the uC firmware assumes there is no current floating on the Line-Meas.

Fig.3: Sensor front-end schematics.

So far I wanted to give some insights for hall-effect measurement.  This project can be achieved in more economical and simple way using a current transformer, but I needed to use LA 100-P sensor for this project specifically.

I did not mention about the acquisition unit in details.  I believe this section deserves a dedicated article. I hope this article would give some clues for hall measurement.


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