So, here is a new algorithm:
> There are 1024 levels of power desired.
> There are four 1000 watt elements
> There are four zero-crossing solid state relays to control power to the elements. Such a relay passes one or more half-cycles of voltage to an element.
> 1024/4=256. So assign 256 possible ON half-cycles to each element. If 1000 watts were desired, for example, each element would be on for 256/4=64 half-cycles.
> 256 half-cycles takes 256/120 = 2.1333 seconds
> Space the four elements apart in time by 64 half cycles = 0.5333 seconds
This is all controlled by a Zilog eZ8 Encore! development board (cost about $43)
In the main() function, we read the pot from the analogue-to-digital converter, getting a value between 0 and 1023:
int power_setting;
unsigned char elements_123, element_4;
power_setting= (store_adc_values[0]); //read pot from ADC
elements_123 = power_setting/4;
element_4 = power_setting - elements_123*3;
That last element_4 just makes up for power_settings that are not multiples of 4.
Now in the file interrupt.c, do an interrupt every half-cycle:
extern unsigned char elements_123, element_4; //these were defined in file main.c
#pragma interrupt
void isr_PA7(void) //the half-cycle is pulsed on port A pin 7
{
//This interrupt will turn on and off the four elements
//The first 3 elements all stay on for the same elements_123 1/2 cycles
//The fourth element stays on for the time of the first three plus 0 to 3 extra 1/2 cycles
static unsigned char counter1 = 0,counter2 = 64,counter3 = 128,counter4 = 192 ; //stagger the elements by 64 pulses
if(++counter1 < elements_123)
SETBITPE(0x10); //turn on element 1
else
RESETBITPE(0x10); // turn off element 1
if(++counter2 < elements_123)
SETBITPE(0x20); //turn on element 2
else
RESETBITPE(0x20); // turn off element 2
if(++counter3 < elements_123)
SETBITPE(0x40);
else
RESETBITPE(0x40);
if(++counter4 < element_4)
SETBITPE(0x80);
else
RESETBITPE(0x80);
}
By measuring the temperture difference across the Liebig on a pot still, the power is read directly by the temperture difference.
Here is a single 1000 W element. The trace is the temperture difference of coolant on the Liebig:
