Wednesday, 28 October 2015

INA117P Current Measurement

A blog reader asked me to help them design a current sensing circuit.  They weren't too specific about the requirements of the measurement output some I'm going to guess it was the ADC input to a microcontroller.  The measurement side was specified as being able to measure 5V at 30 mA from what is referred to as a low wind speed energy harvesting system...I'm guessing they are talking about some sort of simple wind powered generator.

They have asked me to design a circuit using the INA117P which is a difference amplifier made by Texas Instruments.  It has many applications including current measurement.  It's datasheet describes its function as a "precision unity gain difference amplifier".  That means that it measures the difference between it's inputs perfectly without adding any gain at the output.  The information about the device itself can be found from the datasheet:

INA117P datasheet from TI

If we read through the datasheet we can find some helpful application schematics which should provide all the information we need along with the standard specification and ratings information. The circuit in figure 5 is the one we need to apply to our requirements:

Let us first do some Mathematics applying Ohms Law:

V / I = R (Load)

So to calculate the load we have:

5 volts / 0.03 amps = 166 Ohms which is the load being applied.

We can now design the circuit:


The circuit is fairly standard.  The current to be measured from the voltage V1 (the output from the wind powered generator) is passed through a 1 Ohm 'sense' resistor which should be a high precision resistor.  The load is then connected after that which is as calculated 166 Ohms.  The sense resistor is also connected to the differential inputs of the INA117P - the differential unity gain amplifier.  This device measures the difference between the voltages on those pins and provides the output which because the sense resistor is 1 Ohm will be the current flowing in the load.  To simplify matters I used a voltmeter to show that the voltage at the output will be 0.03 volts which is the same value as 0.03 Amps - we have made a circuit which converts current in the load to voltage at the output of the differential amplifier.  The voltmeter can be removed and the output of the INA117P can be connected directly to a microcontroller ADC input and used to display or monitor current - whatever the designer requires...

The issue with the INA117P is that in order for it to work correctly it requires both a positive supply and a negative supply.  This is often difficult to achieve with standard microcontrollers which normally only use a positive 5 volt supply, particularly the arduino...however here is a trick which can be used to make a +/- 5V supply...if you are in a pinch....

Just make sure the GND connection of the power supply is not connected to chassis ground or earth - The circuit won't work and at the worst the power supply could go into current limit...which might damage the supply or cause it to shut down briefly.

Once we have our circuit and the +/- supply all that's needed is to connect it up to a microcontroller and presto we have a method of monitoring the current from the wind generator.

Here is the full circuit:


The parts list for this circuit is below:

1x Arduino R3 or similar microcontroller
1x INA117P Differential Unity gain amplifier
1x 1Ohm Precision current sense resistor
2x 100k Ohm resistors
2x 10 uF Capacitors
2x 5mm Screw terminal connectors

Once you have all of the components connected up and wired up as shown in the schematic we will need to write some code to control the arduino and get it to perform the function we want:

Read the voltage presented at pin A0, convert it from a raw bit value into volts and then display that voltage (labelled) as current via the serial terminal or on an LCD display or whatever we need - No problems.  To be correct and to ensure we get correct values lets display the average of 100 samples to reduce noise issues:

//Alex's INA117P Current Meter test code

const int analogIn = A0; //output of INA117P is connected to A0

int RawValue = 0;  // The raw analogue value

// Define the number of samples to keep track of.  The higher the number,
// the more the readings will be smoothed, but the slower the output will
// respond to the input.  Using a constant rather than a normal variable lets
// use this value to determine the size of the readings array.
const int numReadings = 100;

int readings[numReadings];      // the readings from the analog input
int index = 0;                  // the index of the current reading
int total = 0;                  // the running total
int average = 0;                // the average

float Amps = 0;

void setup()
{
  Serial.begin(9600);
  Serial.println("INA117P Current Meter Test");
  
  // initialize all the readings to 0: 
  for (int thisReading = 0; thisReading < numReadings; thisReading++)
  readings[thisReading] = 0;   
}

void loop()
{
  
  // subtract the last reading:
  total= total - readings[index];         
  // read from the sensor:  
  readings[index] = analogRead(analogIn); 
  // add the reading to the total:
  total= total + readings[index];       
  // advance to the next position in the array:  
  index = index + 1;                    

  // if we're at the end of the array...
  if (index >= numReadings)              
  // ...wrap around to the beginning: 
  index = 0;                           

  // calculate the average:
  average = total / numReadings;         
    
  RawValue = average;
  
  Amps = (RawValue / 1023.0) * 5; // convertaveraged raw value to mV 
  
  Serial.print("Raw Value = " ); // shows pre-scaled value
  Serial.print(RawValue);
  Serial.print("\t A = "); // shows the current measured
  Serial.print(Amps, 3); // the '3' after voltage allows you to display 3 digits after decimal point
  delay(100);

}

I haven't actually got an INA117P so I can't test this circuit...But don't worry I know it will work - the simulation perform came out perfectly and I have built and used many similar circuits.  

Let me know if you try this and it doesn't work - Cheers for now - Langster!