Can PWM be used to control heating?

[Magen]

Hello Everybody,
Is it possible to use PWM to control the temperature of a heating element? I would like to make a circuit that will (using a pic) sense temperature via a DS18S20 sensor and adjust the PWM output accordingly for the desired temperature on a heating pad (similar to the ones used for keeping reptiles as pets in colder regions) the pad will be used for seedlings though. Also will this reduce energy consumption over traditional On/Off systems?

Any help/feedback appreciated.

 

[Nigel Goodwin]

You 'could', but it would be pretty pointless, adding extra complication (and expense) for zero benefit.

The thermal inertia of an element doesn't require PWM, and as it's presumably fed from an AC supply?, 'burst fire' power control is all that's needed.

 

[Magen]

Hi Nigel,
Txs for the reply. I think it is fed via ac supply. I just read up on burst fire control. This is what I was thinking of. Will read more on it.

Txs again
Regards

Magen

 

[Speakerguy]

What is this 'burst fire' power control? I probably know it by another name but have never heard of that particular term.

 

[smanches]

I was just reading about it. It looks like burst fire is the concept of turning on an SCR for a number of AC cycles, then turning it off. Instead of clipping the top of the sinewave, it just limits by changing the duty of the ac cycles going into the heater. That is, 3 cycles on, 7 cycles off, 30% duty.

Not sure I explained that real well. Here is the page I got it from...

Heater Control

 

[dknguyen]

Burst control or AC phase chopping. Either way, both work. You can either chop X% of the AC half cycle or simply apply power for X% of the time and off for the remainder of the period (although the period must be significantly greater than the period of AC half cycles for things to be stable). Both work.

You'd probably get more linear response if you did the bursting rather than the phase chopping since 30% of a AC half cycle is not 30% of the energy in the half cycle since it is a bell-shape curve.

 

[crutschow]

If you turn the SCR on at the AC zero crossing (it will automatically turn off at the zero crossing) then the control circuit generates minimal EMI and has a high power factor, especially as compared to a phase-control dimmer type of SCR control which delays the turn-on of the SCR during each AC cycle.

 

[Speakerguy]

Gotcha. All very simple enough. I guess with Burst you also get the advantage of ZVS.