Accurate Thyristor control, phase or cycle control

[AlfBaz]

Hi all

I want to build my own heater controller for a toaster oven using a triac or scr's.
It will be micro controlled as I want to implement the full reflow cycle of temperature ramp up, saturation, reflow, cool down.

I don't know the proper terminology for the two output methods I have so a quick description instead

The first is controlling the angle at which the thyristor fires, essentially chopping up the sine wave

Second is zero crossing activation for multiple cycles.
I can see two ways to control output in this second method. Choose a time period of say, several seconds and vary the on/off duty cycle (pseudo PWM) or simply just turn the heater elements on/off according to PID algorithm output.

I understand that changing methods is merely a change in software, but was curious if anybody had experience in which method (or others not described here) is the  most accurate.


The other question I had is whether or not to employ burst triggering in the phase angle method.
I have used this successfully before in a design using 4 quadrant SCR's with RC snubbers driving an inductive load where current control was critical.
With the advent of high commutation, 3 quadrant SCR's and triacs and that a stray cycle here and there in a lagging system such as this, coupled with the fact that most heater elements would be mostly resistive, would it be over kill or should I consider CCT design re-use?

[AlfBaz]

I've had a little think about it and the more I do the answere seems obvious

The smaller the output steps the finer the control duh!

[Bloch]

The smaller the output steps the finer the control duh!

Well to a point. How slow/big mass are going to "controlled" ?


One extreme example 1 watt heater in a living room. It will be very hard to measure room temperature differences if it is on 100% or 1%


My guess is 10mS/8,33mS (one half sinus wave) will be just fine.