Mods to control a fan with the second relay?

[JohnP]

#3583

Before I jump in and make a hash of things, I thought I’d see if anyone has done anything similar already….

In a reflow toaster oven application, I’d like to have one relay control the heaters and the other control a fan, such that if the oven temp is greater than, say, 30C/100F, the fan turns on,otherwise it turns off.

I don’t need any of the fancier PID or LF-PWM stuff for the fan, so it looks like I could simply sample “input” in the main loop and digitalWrite the 2nd relay directly from there….

Unfortunately, this feels very much like a bull-in-china-shop hack 🙂

Thoughts?

-John

[rocketscream]

#3969

John,

Your intention of using the fan is to circulate the air within the oven or to lower down the temperature (during cooling stage)?

I have personally tried this before. What I have notice is, during the stage preheat, soak, and reflow, the temperature on the target PCB climbs slower compared to that without the fan running. Only during cooling stage, the fan helps to basically cool the PCB faster (as long as it not faster than 6 C/s).
But, I also believe it depends very much on the volumetric size of the oven itself.

[JohnP]

#3970

I noticed this as well – the fan actually slows the heating cycle, probably based on the air cooling the radiant heat gained from the lamps.

At this point, I am more concerned with cool-down times in a well insulated oven – see my post over under tuning for details. My conceptual hurdle is that the PID stuff seems to be aimed at a single control mechanism – a valve, a heater, etc. Can the notion of “up” and “down” be managed within a single instance (rather than globally setting the “Direct” setting)? That is, turn on heating coils to increase heat, turn off to stop raising and start slowly lowering it, open door (with a servo…) or turn on fan to lower it more/faster.

I could imagine this being simple if the output of the PID algorithm went from +100 to -100, but it only goes from 100 to 0.

BTW, many many thanks for everything you have done with this topic/product! I’m learning a lot, albeit slowly.

-John

[Brett]

#3971

John, what you’re referring to is known as “Split range control.” where there is a cooling output and a heating output. the 0-100% pid output is then split, with 0-50% becoming 100%-0% cooling, and 50-100% becoming 0-100% heating. the output card on the osPID does provision for 2 outputs, but currently the firmware doesn’t suppose split-range control. I’ve added an issue to the github. if there’s enough interest I’ll see what I can do about adding it.

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