Heater control help needed

[UroBoros]

I am doing a PID controller with PIC18.
PT100 is the sensor.

I need to drive a cartridge heater of 500W power.

What are the methods available for the heater control.

One I know is Phase control (I think it is the same as one called as 'PWM' in AC. right?)

There should be something else. Because somewhere I read this phse control is not allowed in some countries beacuse of its interference problems.

What is the other method? Is it practical to do from a Micro for 500W?
Thanks and regards

 

[pranam77]

PWM is a technique used for DC power control and for AC power, Phase angle control is highly used. You need to detect the zero crossing of the AC waveform and switch the Triac at different angles so as to control the heat/ power.
Cheers

 

[UroBoros]

Got it.
But my question is which is the good and practical method to control a heater.
Some type of devices seems to control the number of AC cycles on and off. Some use phase control!
There should be a more used and best method for heater control.
Thanks

 

[Tahmid]

Hi,
There is another method called PSM - Pulse Skipping Modulation. You can use this as it's quite easy. This is how it works:
You want 90% power. So, turn on your switch (let's say this is thyristor) for 9 complete cycles and keep it off for 1.
You want 20% power. So, keep the thyristor on for 2 complete cycles and keep it off for 8.

That's how it works. You can quite easily use PSM or you can use Phase Angle Control. Quite a few heaters here use TinyAVR controlled Pulse Skipping Method based circuits.

Hope this helps.
Tahmid.

 

[UroBoros]

I plan to use a zero crossing detector built in opto coupler to implement pulse skipping modulation.But to keep track of the wave form timings(which may vary slightly) should I use a zero crossing detector circuit and divide on time and off time strictly based on the periods measured from zero crossing circuit pulses? Or Simply set the on off timings in multiples of 20 milliseconds?(My 50 hz AC period). In this case the AC frequency changes will not be compensated and on off ratio may be changed in the AC?
Is it something to be considered or just a zero crossing opto and a internally gerated 20 millisecond timing count is all that needed?
Thanks