PWM output voltage too high

Question

I'm experimenting with PWM with the code below, but (according to my analog multimeter) the voltage on the output pins reach up to 5V whilst the largest analogRead(IN1) value is only around 700:

const int EN1 = 5;
const int EN2 = 6;
const int EN3 = 7;

const int IN1 = 9;
const int IN2 = 10;
const int IN3 = 11;

// SPWM (Sine Wave)
// const int pwmSin[] = {127, 138, 149, 160, 170, 181, 191, 200, 209, 217, 224, 231, 237, 242, 246, 250, 252, 254, 254, 254, 252, 250, 246, 242, 237, 231, 224, 217, 209, 200, 191, 181, 170, 160, 149, 138, 127, 116, 105, 94, 84, 73, 64, 54, 45, 37, 30, 23, 17, 12, 8, 4, 2, 0, 0, 0, 2, 4, 8, 12, 17, 23, 30, 37, 45, 54, 64, 73, 84, 94, 105, 116 };


/// SVPWM (Space Vector Wave)
//const int pwmSin[] = {128, 147, 166, 185, 203, 221, 238, 243, 248, 251, 253, 255, 255, 255, 253, 251, 248, 243, 238, 243, 248, 251, 253, 255, 255, 255, 253, 251, 248, 243, 238, 221, 203, 185, 166, 147, 128, 109, 90, 71, 53, 35, 18, 13, 8, 5, 3, 1, 1, 1, 3, 5, 8, 13, 18, 13, 8, 5, 3, 1, 1, 1, 3, 5, 8, 13, 18, 35, 53, 71, 90, 109};
const int pwmSin[] = {128, 132, 136, 140, 143, 147, 151, 155, 159, 162, 166, 170, 174, 178, 181, 185, 189, 192, 196, 200, 203, 207, 211, 214, 218, 221, 225, 228, 232, 235, 238, 239, 240, 241, 242, 243, 244, 245, 246, 247, 248, 248, 249, 250, 250, 251, 252, 252, 253, 253, 253, 254, 254, 254, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 254, 254, 254, 253, 253, 253, 252, 252, 251, 250, 250, 249, 248, 248, 247, 246, 245, 244, 243, 242, 241, 240, 239, 238, 239, 240, 241, 242, 243, 244, 245, 246, 247, 248, 248, 249, 250, 250, 251, 252, 252, 253, 253, 253, 254, 254, 254, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 254, 254, 254, 253, 253, 253, 252, 252, 251, 250, 250, 249, 248, 248, 247, 246, 245, 244, 243, 242, 241, 240, 239, 238, 235, 232, 228, 225, 221, 218, 214, 211, 207, 203, 200, 196, 192, 189, 185, 181, 178, 174, 170, 166, 162, 159, 155, 151, 147, 143, 140, 136, 132, 128, 124, 120, 116, 113, 109, 105, 101, 97, 94, 90, 86, 82, 78, 75, 71, 67, 64, 60, 56, 53, 49, 45, 42, 38, 35, 31, 28, 24, 21, 18, 17, 16, 15, 14, 13, 12, 11, 10, 9, 8, 8, 7, 6, 6, 5, 4, 4, 3, 3, 3, 2, 2, 2, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 2, 2, 2, 3, 3, 3, 4, 4, 5, 6, 6, 7, 8, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 17, 16, 15, 14, 13, 12, 11, 10, 9, 8, 8, 7, 6, 6, 5, 4, 4, 3, 3, 3, 2, 2, 2, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 2, 2, 2, 3, 3, 3, 4, 4, 5, 6, 6, 7, 8, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 21, 24, 28, 31, 35, 38, 42, 45, 49, 53, 56, 60, 64, 67, 71, 75, 78, 82, 86, 90, 94, 97, 101, 105, 109, 113, 116, 120, 124};

int currentStepA;
int currentStepB;
int currentStepC;
int sineArraySize;
int increment = 0;
boolean direct = 1; // direction true=forward, false=backward

//////////////////////////////////////////////////////////////////////////////

void setup() {
  Serial.begin(9600);

  setPwmFrequency(IN1); // Increase PWM frequency to 32 kHz  (make unaudible)
  setPwmFrequency(IN2);
  setPwmFrequency(IN3);

  pinMode(IN1, OUTPUT); 
  pinMode(IN2, OUTPUT); 
  pinMode(IN3, OUTPUT); 

  pinMode(EN1, OUTPUT); 
  pinMode(EN2, OUTPUT); 
  pinMode(EN3, OUTPUT); 


  digitalWrite(EN1, HIGH);
  digitalWrite(EN2, HIGH);
  digitalWrite(EN3, HIGH);


  sineArraySize = sizeof(pwmSin)/sizeof(int); // Find lookup table size
  int phaseShift = sineArraySize / 3;         // Find phase shift and initial A, B C phase values
  currentStepA = 0;
  currentStepB = currentStepA + phaseShift;
  currentStepC = currentStepB + phaseShift;

  sineArraySize--; // Convert from array Size to last PWM array number
}

//////////////////////////////////////////////////////////////////////////////

void loop() {

  analogWrite(IN1, pwmSin[currentStepA]);
  analogWrite(IN2, pwmSin[currentStepB]);
  analogWrite(IN3, pwmSin[currentStepC]);  

  if (direct==true) increment = 1;
  else increment = -1;     

  currentStepA = currentStepA + increment;
  currentStepB = currentStepB + increment;
  currentStepC = currentStepC + increment;

  //Check for lookup table overflow and return to opposite end if necessary
  if(currentStepA > sineArraySize)  currentStepA = 0;
  if(currentStepA < 0)  currentStepA = sineArraySize;

  if(currentStepB > sineArraySize)  currentStepB = 0;
  if(currentStepB < 0)  currentStepB = sineArraySize;

  if(currentStepC > sineArraySize)  currentStepC = 0;
  if(currentStepC < 0) currentStepC = sineArraySize; 

  /// Control speed by this delay
  delay(10);
  Serial.println(analogRead(IN1));
}


void setPwmFrequency(int pin) {
  if(pin == 5 || pin == 6 || pin == 9 || pin == 10) {
    if(pin == 5 || pin == 6) {
      TCCR0B = TCCR0B & 0b11111000 | 0x01;
    } else {
      TCCR1B = TCCR1B & 0b11111000 | 0x01;
    }
  }
  else if(pin == 3 || pin == 11) {
    TCCR2B = TCCR2B & 0b11111000 | 0x01;
  }
}

If analogRead(IN1) is just around 700 max, shouldn't the voltage be around 3.5V (700/1023 * 5V)? How come the output voltage is so high?

Any help would be appreciated.

Answer 1

It seems that you misunderstand how pwm works. Pwm is the modulation of duty cycle, so the pin are outputting square waves 0-5v at the same frequency, but varying the time it's low and the time it's high. Your multimeter probably is only displaying high intervals, instead your analog read probably has filters and calculates the efficient voltage of the sine wave (divide max for sqr(2)).