DIY Automatic Temp Control on a MiniWSM - PID VS On/Off Fan Control


 

A Lee

TVWBB Super Fan
Hi Everyone,

I built my first smoker a couple weeks ago and am contemplating installing a DIY temperature control system on my little smoker. My goal is to build something effective for a minimal cost. So far I have a Smokey Joe Gold that was handed down from family. It was converted to a MiniWSM using surplus 304 stainless steel that I got for free. Aside from some minor cleaning, the stainless plopped right onto the Smokey Joe with very minimal modification.

I'm familiar with industrial/hvac controls and have access to a variety of surplus equipment to automate the temperature of the MiniWSM but I need to get a better understanding of what sort of sequences would be good for controlling a charcoal fire. There appears to be a couple of basic common strategies for controlling smoker temperature:

PID Control of Fan Speed
It looks like many off the shelf BBQ contollers perform a PID calculation then convert the output a PWM voltage to modulate air volume going into the fire. I have a fully custom programmable PID controller but it's analog output range is only 0-10Vdc and is also not capable of driving a load, so in order to modulate a little computer fan, I'll need a circuit to convert the 0-10V signal into some sort of PWM. Anyone aware of an off the shelf board that does this? I'm thinking such a device has to already exist and costs very little if ordered from Ebay or Aliexpress. Any downsides of using PID control for modulating fan speed?

ON/Off Control of Fan
Until I figure out a decent way to convert 0-10v to PWM, I'm thinking On/Off control would be a very simple method for implementing automatic controls on the little smoker. Anyone know how well this method would work on a WSM? What kind of temperature swing would I get if I cycled a fan on/off based on deviation of oven temperature from setpoint? I believe at least one of the commercially available controllers use this method. Anyone know what brand/model uses this and what sort of sequence of operation they use? When burning charcoal, is it a problem to have the fan shut off when setpoint is met? How much overshoot/undershoot should I expect? How much deadband would I need in my program? I'm thinking I can use the digital output of my controller to switch DC power to a little 12V fan based on deviation of oven temperature from setpoint. If the fan spins too fast, i could manually decrease the DC power supply voltage to reduce the fan CFM. Anyone think it's worthwhile to use more than one fan? To get smoother control, I could in theory I could use two fans, cut the CFM of each fan by half, then stage the fans to turn on/off at different deviations from setpoint.

Damper Control
I've seen some pictures of projects where a damper was controlled. I don't see any benefit of modulating a damper if the fan is also modulating. Are people using a modulating damper with an on/off fan? What are they trying to achieve with damper control?
 
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