====== PWM amp cooler ======

~~META:description abstract=Fan driver for the PWM amplifier~~

Fan driver for the PWM amplifier

===== Math =====

Max ADC input voltage is 1.1V (higher means saturated).

Output of TMP36 in volts is <m>V(T) = 0.01 * T + 0.5</m>, targeted temperature range is at least [20 .. 100]°C, that is [0.7 .. 1.5] V.

Theorical response is 0% up to 50°C, from 0% to 100% between 50°C and 75°C and 100% above 75°C.

Needed multiplier before ADC is at most <m>1.1 / 1.5 = 0.73333</m>, R1=33k and R2=82k fits (0.731).

<m>ADC(T) = floor(1023 / 1.1) * U(T)</m>

<m>PWM(T) = {T / 25} - 2</m> (min 0, max 1).

<m>OCR0A(ADC) = 255 * ({{{1.1 / 1023} * {ADC / {R2 / {R1 + R2}}} - 0.5} / {0.01 x 25}} - 2)</m>

Using <m>OCR0A(ADC) = 49 * (ADC - 665) / 32</m> (fits an int 16) gives an average of 3.6% deviation from theorical response.

{{:projects:pwm_amp_cooler:amp_fan_pwm_reg.ods|Calculation file}}
===== Electronics =====

{{schematic>schematic.svg}}


===== Source code =====

<file c main.c>
// CPU frequency 9.6MHz
// Temperature sensor (TMP36) on ADC3 (PB3)
// MosFet output on OC0A (PB0)

int main(void) {
	int16_t t;
	
	ADMUX = 0x43;	// internal reference, right adjust, ADC3
	ADCSRA = 0x87;	// ADC enable, prescaler to 128 (175us/conv.)
	
	TCCR0A = 0x83;	// fast pwm on OC0A
	TCCR0B = 0x01;	// prescaler to 1 => f=37.5kHz
	
	while(1) {
		ADCSRA |= 0x40;			// start conversion
		while(ADCSRA & 0x40);		// wait for completion
		
		t = ADC - 665;
		t *= 49;			// approx.
		t /= 32;			// 1.6451
		
		if(t < 0) t = 0;		// limits
		if(t > 255) t = 255;		//
		
		OCR0A = t;
	}
	
	return 0;
}
</file>

{{tag>analog}}