I have added some code in the "temperature.cpp" for have the possibility of control the time of the states with the heater drived by relays, with this feature now it is possible use PID function in conjunction with relay to control the temperature. I have made some tests and the temperature stay always in the range of +-1°C from the setted temp.

There is the possibility of turn on this features adding "#define SLOW_PWM_HEATERS" in Configuration.h

Marlin/temperature.cpp

@@ -497,7 +497,7 @@ void manage_heater()
     SERIAL_ECHO(" iTerm ");
     SERIAL_ECHO(iTerm[e]);
     SERIAL_ECHO(" dTerm ");
-    SERIAL_ECHOLN(dTerm[e]);  
+    SERIAL_ECHOLN(dTerm[e]);
     #endif //PID_DEBUG
   #else /* PID off */
     pid_output = 0;
@@ -1221,68 +1221,311 @@ ISR(TIMER0_COMPB_vect)
   static unsigned char temp_state = 10;
   static unsigned char pwm_count = (1 << SOFT_PWM_SCALE);
   static unsigned char soft_pwm_0;
-  #if (EXTRUDERS > 1) || defined(HEATERS_PARALLEL)
+#ifdef SLOW_PWM_HEATERS
+  static unsigned char slow_pwm_count = 0;
+  static unsigned char state_heater_0 = 0;
+  static unsigned char state_timer_heater_0 = 0;
+#endif 
+#if (EXTRUDERS > 1) || defined(HEATERS_PARALLEL)
   static unsigned char soft_pwm_1;
-  #endif
+#ifdef SLOW_PWM_HEATERS
-  #if EXTRUDERS > 2
+  static unsigned char state_heater_1 = 0;
+  static unsigned char state_timer_heater_1 = 0;
+#endif 
+#endif
+#if EXTRUDERS > 2
   static unsigned char soft_pwm_2;
-  #endif
+#ifdef SLOW_PWM_HEATERS
-  #if HEATER_BED_PIN > -1
+  static unsigned char state_heater_2 = 0;
+  static unsigned char state_timer_heater_2 = 0;
+#endif 
+#endif
+#if HEATER_BED_PIN > -1
   static unsigned char soft_pwm_b;
-  #endif
+#ifdef SLOW_PWM_HEATERS
+  static unsigned char state_heater_b = 0;
+  static unsigned char state_timer_heater_b = 0;
+#endif 
+#endif
   
-  #if defined(FILWIDTH_PIN) &&(FILWIDTH_PIN > -1)
+#if defined(FILWIDTH_PIN) &&(FILWIDTH_PIN > -1)
-   static unsigned long raw_filwidth_value = 0;  //added for filament width sensor
+  static unsigned long raw_filwidth_value = 0;  //added for filament width sensor
-  #endif
+#endif
   
+#ifndef SLOW_PWM_HEATERS
+  /*
+   * standard PWM modulation
+   */
   if(pwm_count == 0){
     soft_pwm_0 = soft_pwm[0];
     if(soft_pwm_0 > 0) { 
       WRITE(HEATER_0_PIN,1);
-      #ifdef HEATERS_PARALLEL
+#ifdef HEATERS_PARALLEL
       WRITE(HEATER_1_PIN,1);
-      #endif
+#endif
     } else WRITE(HEATER_0_PIN,0);
-	
+    
-    #if EXTRUDERS > 1
+#if EXTRUDERS > 1
     soft_pwm_1 = soft_pwm[1];
     if(soft_pwm_1 > 0) WRITE(HEATER_1_PIN,1); else WRITE(HEATER_1_PIN,0);
-    #endif
+#endif
-    #if EXTRUDERS > 2
+#if EXTRUDERS > 2
     soft_pwm_2 = soft_pwm[2];
     if(soft_pwm_2 > 0) WRITE(HEATER_2_PIN,1); else WRITE(HEATER_2_PIN,0);
-    #endif
+#endif
-    #if defined(HEATER_BED_PIN) && HEATER_BED_PIN > -1
+#if defined(HEATER_BED_PIN) && HEATER_BED_PIN > -1
     soft_pwm_b = soft_pwm_bed;
     if(soft_pwm_b > 0) WRITE(HEATER_BED_PIN,1); else WRITE(HEATER_BED_PIN,0);
-    #endif
+#endif
-    #ifdef FAN_SOFT_PWM
+#ifdef FAN_SOFT_PWM
     soft_pwm_fan = fanSpeedSoftPwm / 2;
     if(soft_pwm_fan > 0) WRITE(FAN_PIN,1); else WRITE(FAN_PIN,0);
-    #endif
+#endif
   }
   if(soft_pwm_0 < pwm_count) { 
-      WRITE(HEATER_0_PIN,0);
+    WRITE(HEATER_0_PIN,0);
-      #ifdef HEATERS_PARALLEL
+#ifdef HEATERS_PARALLEL
-      WRITE(HEATER_1_PIN,0);
+    WRITE(HEATER_1_PIN,0);
-      #endif
+#endif
-    }
+  }
-  #if EXTRUDERS > 1
+#if EXTRUDERS > 1
   if(soft_pwm_1 < pwm_count) WRITE(HEATER_1_PIN,0);
-  #endif
+#endif
-  #if EXTRUDERS > 2
+#if EXTRUDERS > 2
   if(soft_pwm_2 < pwm_count) WRITE(HEATER_2_PIN,0);
-  #endif
+#endif
-  #if defined(HEATER_BED_PIN) && HEATER_BED_PIN > -1
+#if defined(HEATER_BED_PIN) && HEATER_BED_PIN > -1
   if(soft_pwm_b < pwm_count) WRITE(HEATER_BED_PIN,0);
-  #endif
+#endif
-  #ifdef FAN_SOFT_PWM
+#ifdef FAN_SOFT_PWM
   if(soft_pwm_fan < pwm_count) WRITE(FAN_PIN,0);
-  #endif
+#endif
+  
+  pwm_count += (1 << SOFT_PWM_SCALE);
+  pwm_count &= 0x7f;
+  
+#else //ifndef SLOW_PWM_HEATERS
+  /*
+   * SLOW PWM HEATERS
+   *
+   * for heaters drived by relay
+   */
+#ifndef MIN_STATE_TIME
+#define MIN_STATE_TIME 16 // MIN_STATE_TIME * 65.5 = time in milliseconds
+#endif
+  if (slow_pwm_count == 0) {
+    // EXTRUDER 0 
+    soft_pwm_0 = soft_pwm[0];
+    if (soft_pwm_0 > 0) {
+      // turn ON heather only if the minimum time is up 
+      if (state_timer_heater_0 == 0) { 
+	// if change state set timer 
+	if (state_heater_0 == 0) {
+	  state_timer_heater_0 = MIN_STATE_TIME;
+	}
+	state_heater_0 = 1;
+	WRITE(HEATER_0_PIN, 1);
+#ifdef HEATERS_PARALLEL
+	WRITE(HEATER_1_PIN, 1);
+#endif
+      }
+    } else {
+      // turn OFF heather only if the minimum time is up 
+      if (state_timer_heater_0 == 0) {
+	// if change state set timer 
+	if (state_heater_0 == 1) {
+	  state_timer_heater_0 = MIN_STATE_TIME;
+	}
+	state_heater_0 = 0;
+	WRITE(HEATER_0_PIN, 0);
+#ifdef HEATERS_PARALLEL
+	WRITE(HEATER_1_PIN, 0);
+#endif
+      }
+    }
+    
+#if EXTRUDERS > 1
+    // EXTRUDER 1
+    soft_pwm_1 = soft_pwm[1];
+    if (soft_pwm_1 > 0) {
+      // turn ON heather only if the minimum time is up 
+      if (state_timer_heater_1 == 0) { 
+	// if change state set timer 
+	if (state_heater_1 == 0) {
+	  state_timer_heater_1 = MIN_STATE_TIME;
+	}
+	state_heater_1 = 1;
+	WRITE(HEATER_1_PIN, 1);
+      }
+    } else {
+      // turn OFF heather only if the minimum time is up 
+      if (state_timer_heater_1 == 0) {
+	// if change state set timer 
+	if (state_heater_1 == 1) {
+	  state_timer_heater_1 = MIN_STATE_TIME;
+	}
+	state_heater_1 = 0;
+	WRITE(HEATER_1_PIN, 0);
+      }
+    }
+#endif
+    
+#if EXTRUDERS > 2
+    // EXTRUDER 2
+    soft_pwm_2 = soft_pwm[2];
+    if (soft_pwm_2 > 0) {
+      // turn ON heather only if the minimum time is up 
+      if (state_timer_heater_2 == 0) { 
+	// if change state set timer 
+	if (state_heater_2 == 0) {
+	  state_timer_heater_2 = MIN_STATE_TIME;
+	}
+	state_heater_2 = 1;
+	WRITE(HEATER_2_PIN, 1);
+      }
+    } else {
+      // turn OFF heather only if the minimum time is up 
+      if (state_timer_heater_2 == 0) {
+	// if change state set timer 
+	if (state_heater_2 == 1) {
+	  state_timer_heater_2 = MIN_STATE_TIME;
+	}
+	state_heater_2 = 0;
+	WRITE(HEATER_2_PIN, 0);
+      }
+    }
+#endif
+    
+#if defined(HEATER_BED_PIN) && HEATER_BED_PIN > -1
+    // BED
+    soft_pwm_b = soft_pwm_bed;
+    if (soft_pwm_b > 0) {
+      // turn ON heather only if the minimum time is up 
+      if (state_timer_heater_b == 0) { 
+	// if change state set timer 
+	if (state_heater_b == 0) {
+	  state_timer_heater_b = MIN_STATE_TIME;
+	}
+	state_heater_b = 1;
+	WRITE(HEATER_BED_PIN, 1);
+      }
+    } else {
+      // turn OFF heather only if the minimum time is up 
+      if (state_timer_heater_b == 0) {
+	// if change state set timer 
+	if (state_heater_b == 1) {
+	  state_timer_heater_b = MIN_STATE_TIME;
+	}
+	state_heater_b = 0;
+	WRITE(HEATER_BED_PIN, 0);
+      }
+    }
+#endif
+  } // if (slow_pwm_count == 0)
+  
+  // EXTRUDER 0 
+  if (soft_pwm_0 < slow_pwm_count) {
+    // turn OFF heather only if the minimum time is up 
+    if (state_timer_heater_0 == 0) { 
+      // if change state set timer 
+      if (state_heater_0 == 1) {
+	state_timer_heater_0 = MIN_STATE_TIME;
+      }
+      state_heater_0 = 0;
+      WRITE(HEATER_0_PIN, 0);
+#ifdef HEATERS_PARALLEL
+      WRITE(HEATER_1_PIN, 0);
+#endif
+    }
+  }
+    
+#if EXTRUDERS > 1
+  // EXTRUDER 1 
+  if (soft_pwm_1 < slow_pwm_count) {
+    // turn OFF heather only if the minimum time is up 
+    if (state_timer_heater_1 == 0) { 
+      // if change state set timer 
+      if (state_heater_1 == 1) {
+	state_timer_heater_1 = MIN_STATE_TIME;
+      }
+      state_heater_1 = 0;
+      WRITE(HEATER_1_PIN, 0);
+    }
+  }
+#endif
+  
+#if EXTRUDERS > 2
+  // EXTRUDER 2
+  if (soft_pwm_2 < slow_pwm_count) {
+    // turn OFF heather only if the minimum time is up 
+    if (state_timer_heater_2 == 0) { 
+      // if change state set timer 
+      if (state_heater_2 == 1) {
+	state_timer_heater_2 = MIN_STATE_TIME;
+      }
+      state_heater_2 = 0;
+      WRITE(HEATER_2_PIN, 0);
+    }
+  }
+#endif
+  
+#if defined(HEATER_BED_PIN) && HEATER_BED_PIN > -1
+  // BED
+  if (soft_pwm_b < slow_pwm_count) {
+    // turn OFF heather only if the minimum time is up 
+    if (state_timer_heater_b == 0) { 
+      // if change state set timer 
+      if (state_heater_b == 1) {
+	state_timer_heater_b = MIN_STATE_TIME;
+      }
+      state_heater_b = 0;
+      WRITE(HEATER_BED_PIN, 0);
+    }
+  }
+#endif
+  
+#ifdef FAN_SOFT_PWM
+  if (pwm_count == 0){
+    soft_pwm_fan = fanSpeedSoftPwm / 2;
+    if (soft_pwm_fan > 0) WRITE(FAN_PIN,1); else WRITE(FAN_PIN,0);
+  }
+  if (soft_pwm_fan < pwm_count) WRITE(FAN_PIN,0);
+#endif
   
   pwm_count += (1 << SOFT_PWM_SCALE);
   pwm_count &= 0x7f;
   
+  // increment slow_pwm_count only every 64 pwm_count circa 65.5ms
+  if ((pwm_count % 64) == 0) {
+    slow_pwm_count++;
+    slow_pwm_count &= 0x7f;
+    
+    // Extruder 0
+    if (state_timer_heater_0 > 0) {
+      state_timer_heater_0--;
+    } 
+  
+#if EXTRUDERS > 1
+    // Extruder 1
+    if (state_timer_heater_1 > 0) 
+      state_timer_heater_1--;
+#endif
+    
+#if EXTRUDERS > 2
+    // Extruder 2
+    if (state_timer_heater_2 > 0) 
+      state_timer_heater_2--;
+#endif
+    
+#if defined(HEATER_BED_PIN) && HEATER_BED_PIN > -1
+    // Bed   
+    if (state_timer_heater_b > 0) 
+      state_timer_heater_b--;
+#endif
+  } //if ((pwm_count % 64) == 0) {
+  
+#endif //ifndef SLOW_PWM_HEATERS
+  
   switch(temp_state) {
     case 0: // Prepare TEMP_0
       #if defined(TEMP_0_PIN) && (TEMP_0_PIN > -1)