Measuring the DC current with a Microcontroller

Basically the microcontrollers doesn’t have specific ports for measuring “dc” current or “ac”  current, but they do have ADC (Analog to Digital Converter) and we can take advantage from this point of view so we can measure analog voltages of a certain range (usually 0-5vcc). The way of doing this is to place a resistance in series with the current path and measure the voltage drop across it. For this trick we need to use a resistor with a very low value just to not affect the original current of the load.

Resistors with a value less than 1 ohm are necessary and can be found in electronic stores. For a proper resistor placed into the circuit we have to pay attention for maximum current used. So, let’s say that you pick 0,47Ohm and the maximum current in the circuit is about 5 A, then the resistor should have the capacity of dissipation equal with: I²xR => 25 x 0,47 = 12 Watts of heat.

The appropriate design for the resistor is to create by yourself a coil from an Cu wire. For  this test I have create one from 1,5 meter Ø 1,3 mm with enameled insulation on outer side, as shown below.

Coil

Now let’s measure its resistance, for example directly with an multimeter. My digital multimeter shows a value equal with 0,4 Ohm. Of course this measurement can have uncertainty, because of very small value which we measure and we don’t had to forget the fact that usually the digital multimeters does not show values beyond 1 decimal digit. Our resistance can be measured respecting the ohm’s law. We can connect in series with the coil resistance (Rs), a known resistor for example 47 ohm and supply a 5vcc as shown below. Next, measure the voltage across Rs and current through it separately using the multimeter. In the current case, I foud the measured voltage and current values to be 25.6mV and 88.6mA. This gives the resistance of the coil equal with 0.289 Ohm. (Rs = V/I).

Now, suppose that the range of current to be measured using this coil resistance is from 0-5A. Then the voltage drop across the coil resistance will be somewhere from 0 – 1.44 V. Because of its low range, this voltage signal may not be accurately measured with a microcontroller’s ADC module. In this case we will use an operational amplifier circuit for a voltage scaling mode. The entire schematic for this design is shown below.

As a description: in the above circuit Rs represent the low value current sensing resistor (our coil resistor) which is connected in series with the consumer (load resistor). Our purpose is to derive the load current (I). The low voltage drop across Rs is amplified by the non-inverting amplifier with 3.5. This is enough to linearly scale Vs (0 – 1.44 V) to Vo (0-5vcc).

At this point we have 0-5vcc signal that corresponds to 0-5A current through Rs. This voltage signal is now more appropriate for ADC conversion with Vref = 5vcc.

Vo= 3.5 x I x RS = 1I (Rs=0,289) => I = Vo/1.

For 10-bit ADC with Vref = 5vcc:

·         Our resolution will be equal with: resolution = 5/1024 = 0.0049 (5mA).

·         For input signal Vo, the ADC O/P will be Vo x 0,0049,

·         Then I = ADC O/P x 0,0049/1 = ADC O/P x 0.0049.

Practical pictures with the experiments.