PWM and DC motor frequency control

Other Hardware Motors, Mechanics, Power and CNC
1

HI, I am an IE and I have this project where I would like to control the speed of a 12v DC (fan) motor from a Nano pin 5.

I control it with PWM from pin 5 and via a FET on the negative site of the DC fan, I have the code for controlling the speed working but my problem is that the fan is very noisy at all levels so I tried to raise the frequency of the PWM on timer 0 by changing the prescalar and the noise disappeared (from 980 Hz. to 7812,50 Hz. with this code:

TCCR0B = TCCR0B & B11111000 | B00000010; // for PWM frequency of 7812.50 Hz

The problem with this is that pin 5 is HW connected to timer 0 and all other timers and timing are affected by this so this is not the solution for me.

I can't change the HW in the project so I need pin 5 for this, a solution could be to put in a Pro Mini as a frequency converter but if there were a SW solution for this, I would prefer this, any of you adults in here have any good suggestions it would be appreciated hence I am a rookie programmer.

2

Do you also have a freewheel diode across the motor?

1 Like
3

Apart from DPin-5, can you use one of the following DPins for your PWM signal generation:?

9, 10, 11, 3
4

Yes, thanks for a quick reply

5

No all pins are in use and the HW is made, the original idea was to use a PWM controlled fans but hence I had a bunch of ordinary DC fans I chose to use them.

6

Hi @mriisj

If your hardware is fixed then you'll have to resort to using another timer (either 1 or 2) to generate the 7812.50Hz timing, and interrupts to bit bang the digital pin D5 output:

// Set up timer 2 to output 7812.50Hz, 8-bit resolution PWM and bit bang the output on digital pin D5
void setup() {
  pinMode(5, OUTPUT);                   // Set digital pin D5 to an output
  TCCR2A =  _BV(WGM21) | _BV(WGM20);    // Enable fast PWM output with duty-cycle controlled by OCR2A 
  TIMSK2 = _BV(OCIE2A) | _BV(TOIE2);    // Enable timer 2 match compare channel A and overflow interrupts 
  OCR2A = 128;                          // Set the duty-cycle to 50%
  TCCR2B = _BV(CS21);                   // Kick-off timer 2 with the prescaler set to 8
}

void loop() {}                          // Loop function

ISR(TIMER2_COMPA_vect)                  // The timer 2 compare A (COMPA) interrupt service routine
{
  PORTD &= ~_BV(PORTD5);                // Clear the output on D5
}

ISR(TIMER2_OVF_vect)                    // The timer 2 overflow (OVF) interrupt service routine
{
  PORTD |= _BV(PORTD5);                 // Set the output on D5
}
7

Is this is a 2-pin or 3-pin computer fan.
If so, then you should use a PWM frequency below our hearing range.
30Hz is commonly used for a 2-pin or 3-pin PC fan.

4-pin fans don't need additional electronics.
They should be controlled directly, with ~25kHz PWM.
Leo..

8

Hi Leo
Thanks for your reply, it is a 3 wire fan where the third wire is for freq counting/speed

I still can't change the freq on pin dp5 without changing the complete timing on all timers...
Michael

9

Hi MartinL
Both timer 1 and 2 are dedicated to other tasks, is it possible to make an interrupt on timer 0 dp5 so the fan control on pin 5 lives it own life with a dedicated freq ?
Michael

10

If you intend to use that, then you must PWM the fan high-side (the + wire),
with a PNP transistor or P-channel fet.
Third diagram here.

Why not use millis() timing for that 30Hz PWM.
Leo..

11

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