Blower controller on a budget
When I got my own Medium BGE one of the first things I considered was purchasing a temperature controlled fan system. After considering multiple options, I settled on the Thermomart (Product Code DIN300A-BBQ-PP-12V). Everything else was, IMHO, ridiculously over priced.
It is basically a pre-assembled unit with an industrial temperature controller mounted in a (mostly) water-proof box and is complete with a probe, fan, and power supply. This controller allows you to adjust the hysteresis which is the difference in temperature between when it shuts off the blower and when it turns it back on again. This should prove useful to tweak. Available here: https://www.thermomart.com/barbecue-...ent-accesories
Once I purchased that, two obstacles conspired to cause a long pause before I completed the project. First of all I wanted a fan speed control. Secondly, I needed to make a mount for the small fan included with the system. It is just a little fan and doesn’t even have a tube that protrudes from it. I considered long and hard buying a third party fan and mount, but decided to postpone that.
In the meanwhile, I found that temperature regulation on the BGE is not all that difficult, though there is definitely some art to it and weather does come into play. Low and slow while working at home during COVID got me up to the top of that learning curve.
So the project sat for years. Recently I got back to this project because I wanted to clean up the pile of parts. Here is what I came up with (everything on the top surface is added by me):
Because I didn’t have a Double Pole Double Throw ON-ON switch for the MODE control, there’s an unneeded “OFF” position on that switch. It really should have only ON and AUTO. The speed control was meant to have MAX, PAUSE, and AUTO settings but I mislabeled it as as OFF, though the meaning is the same. There is no power switch in that plugging it in powers on the unit.
The ZK-MG Pulse Width Modulation* motor controller adjusts the negative not the positive power lead, which is odd, but not an impediment. It has programmable frequency as well as speed/duty cycle. I found the lowest frequency, 1 Kilohertz, works best, though as expected it makes a slight whine. It requires a roughly 60% duty cycle to start reliably. To ensure reliable starting I programmed it, easily done from the front panel, to allow speeds no lower than 65 and up to 100%. I don’t have a tachometer but it varies air flow considerably.
I added a waterproof connector on the temperature probe while I was at it. I have not decided, but expect to mount the probe in the egg permanently and don’t want to leave the controller outside.
The two remaining challenges were a system mount the fan to the egg, and a damper so I can adjust air flow when the fan isn’t running.
I mounted the fan to a piece of aluminum sheet metal cut to fit in the sliding door track on the egg's lower damper. Easily removable when cleaning out ashes. Some high temperature RTV Silicone keeps it from slipping out of the hole in the sheet metal.
Would I have preferred to have the fan further from the firebox? Yes. However I don’t expect the fan to get melted. If I am doing high temperature searing I will remove the fan once the fire gets going. If it does get damaged they are not expensive and I will replace it with a better quality name brand version and come up with a different mounting system. Or more likely buy a replacement fan and mount from one of the other companies, though the cost for the fan will raise the total cost significantly.
When opening the egg I switch the SPEED to OFF (AKA pause). Once the EGG is closed again I can run it at Max for a few seconds and then switch back to VARiable.
I need to do some careful analysis over the next few low and slow cooks to determine the best “usual” settings for the speed control and thermostat control hysteresis settings.
I have a two Combustion Inc predictive thermometers (thanks Chris, that thing is a marvel of both product design as well as engineering). So I will have some graphs to show eventually.
Of course Dr Blonder (I think it was him) has debunked the necessity for maintaining perfectly even temperatures during a cook (sorry, I can't find the article, but it showed fast temperature recovery and some other factors that add up to to it doesn't much matter) but it still nice to automate the tweaking needed over the course of a cook.
Total cost was under $100.
Here’s the wiring diagram in case anyone wants to build one. It is fairly easy to understand how it works, just keep in mind the positive side is wired directly and the negative side is switched, and when set to variable, is modulated by the speed controller.
Fun project. Now I just need to find the time to do more smoking - I have not been feeling well.
* What's PWM/Pulse Width Modulation?
Imagine any ordinary fan running, and picture yourself switching it off, then back on, then off on off on etc to lower the speed. Each on cycle is a "Pulse" of power. The duration of the pulse is the "width". The ratio (expressed as a percentage) of time on to off is the "duty cycle". And the time spent for each series of on-off cycles is the "frequency" ( expressed as cycles per second or Hertz).
A PWM controller works the same way, but extremely rapidly. In this controller the frequency is adjustable. By experimentation I determined this fan works best at the lowest available frequency - 1000 cycles per second. I also experimented with determining the lowest duty cycle required for the fan to actually get going (60%) and then set the adjustable range so that I am always above that. I want to be sure the fan cycles on reliably or the fire might die :0 !
Why not just lower the voltage? Many DC motors won't start turning at all if the voltage is too low. Also, with modern brushless DC motors there are considerations about what minimum voltage is required for the internal control circuitry. With PWM the voltage is always the same - it is either full voltage or no voltage at all.
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