| ??? 09/20/10 11:42 Read: times Msg Score: +1 +1 Good Answer/Helpful |
#178599 - Why Pulses? Responding to: ???'s previous message |
Kiran,
Why do you use pulses for the triac. I would instead gate the triac with a level control pulse that stays active as long as you want the triac to be on. This should eliminate any concern about the triac not accepting a narrow pulse. If you go along with the idea I suggested in the other posting to only change the level control to the triac at the time of zero crossings then you will have greatly simplified the means to which you switch the control signal on and off. If you get an interrupt to the microcontroller for every half cycle you can devise the "period" for the control to be across half cycle times as opposed to full cycle times. I sincerely believe that you will reduce the stress on the heater element and it's connection terminals / wires if you stop using variable phase control of the triac and instead use the full zero crossing method. Note that the requirement that you specified to limit the power to less then 30% duty cycle does not have to be implemented as a phase control duty cycle either. It can easily be handled by limiting the number of zero to zero half cycles that are enabled in each period. ----- Last comment....if you use the lookup table scheme you could change the concept of table content from being a count of cycles per period to a bit map of what cycles to activate. So if you had 16 half cycles in the control period you could have each table entry be a 16-bit value. The benefit for this as that as you select more and more half cycles to be active you can assign then across the period instead of as a count. Here is for example: 0 half cycles active 0b0000 0000 0000 0000 1 half cycle active 0b0000 0000 0000 0001 2 half cycles active 0b0000 0001 0000 0001 3 half cycles active 0b0000 0001 0001 0001 4 half cycles active 0b0001 0001 0001 0001 5 half cycles active 0b0001 0001 0001 0101 6 half cycled active 0b0001 0101 0001 0101 Michael Karas |
