SCR heater control with 0-5V?

[Oznog]

I was thinking of modifying a toaster over to do SMD reflow boards automatically, with controlled temp rise, dwell times, all that. I'd use a PIC microcontroller and probably a convection oven to mix the heat more thoroughly. I might put a thermocouple inside that would measure board temp more directly, also had a neat idea to use an infrared thermometer thermopile to measure the temp optically (ideal!).

So some of the specs I looked at give me the idea I might not get a well controlled temp with just on/off switching of the heater. It might be switching on/off way too often to get smooth temp control. So I was thinking maybe I want to use an SCR to control how of the waveform it conducts across for a smooth, continuous power control.

OK, so I know the SCR needs to trigger at a certain part of the positive and negative swings and by its nature will stay conducting until the next zero crossing. But, I don't know how to make this happen with a 0-5V signal off a PIC. I assume this should be isolated via optoisolators from the main's ground. Does one optoisolator go from the power main voltage back to the PIC to give zero point detection or what? Anyone got applicable schematics?

Even if the heater might work ok with on/off modulation, I'd still like to know how to do this.

 

[ante]

You could try the MOC3063 or similar, the zero crossing is all built in and the LED can easily be controlled by the PIC.

 

[ante]

You could try the MOC3063 or similar, the zero crossing is all built in and the LED can easily be controlled by the PIC.

 

[Roff]

You need a triac, not an SCR. The MOC3063 can be used to trigger it.

 

[ante]

Hi Ron,

Yes that’s right, if you take a look at figure9 in the data sheet you can see the SCR circuit. However a triac circuit is simpler to build. Here is the data sheet: http://www.atnet1000.com/pdf/MOC3061.pdf