So, in my last couple of posts I was doing some research around coming up with the perfect Electric Counter-Top Brewing System. I found what I thought would be a perfect Mini-Brew-in-a-Bag Electric System with the Cajun Injector Electric Fryer, however the Temperature Controller that comes with it could not maintain the temperature accuracy nor the range needed to brew the variety of beers I wanted to make.
So as any industrious DIYer would do, I took apart the Temperature Controller to see if I could modify it. There are six screws in the back of the unit along with two under the face plate that hold the housing together, once removed the housing can be snapped apart to get to the components inside.
As you can see in the diagram, there are several components that can be reused to drive the heating element from an external controller, all I needed to do is figure out what connects to what and build out a schematic that I can use to build a controller interface that I can drive with a Microcontroller.
The Safety Switch acts as a master switch for the incoming power, If the unit is not seated in the Cajun Injector slots for it, no power will flow to the unit. Now this is a pretty nice feature that cuts power to the heating element should you pull it out of the pot and set it to the side. You also don’t have to worry about dry firing the unit if it is not seated in the pot. However, you still have the dry fire issue when the unit is properly seated in the pot.
The Unit also contains what looks to be a GFCI Breaker that is wired to a probe that runs along side the heating element.
There is also a 12VDC Relay that controls turning the heating element on and off based on the control signal from the control panel.
Finally to power the control panel there is what looks like a reduction transformer that should provide the 12V power needed to drive the relay.
The control panel has an LCD mounted on it and what looks like a microcontroller or Programmable Logic Array which is the brains of the Temperature Controller. From a quick inspection it really doesn’t look like it contains anything of use.
There were three connections from the control panel to the components in the housing. These look to be an input voltage, the Thermistor input connections and the Relay control outputs. I should be able to extend these to an external controller and drive the heating element.
I also performed a little research by hooking up the Thermistor to my Multi-meter and measured the resistance at several temperatures. What I got was a pretty broad range of resistance; 3.58 M ohms at 40 degrees F down to 80 K ohms at a rolling boil. Now without knowing the actual part number of the Thermistor I really didn’t think I could come up with a temperature curve that I could use to figure out the temperature based on the resistance. Luckily after a little searching on the Internets I came up with a program that would generate a set of temperature coefficients based on a couple of readings at known temperatures. With this, I plan on reverse engineering the temperature coefficients of the Thermistor so I can come up with a resistance curve that I can use to determine the temperature by using the Thermistor in a voltage divider and then calculating the temperature value based on a known voltage.
So my next steps are to do a little more poking around the components and determine the transformer’s output voltage is and start coming up with a plan to build out a controller board that I can use with a Netduino to drive the unit.
So stay sanitized and keep an eye on those fermentation temps and I’ll catch you for a pint around the keg.
Jim Lavin – Otaku Brewer