PID Espresso Coffee Machine Mod

PID Coffee

After playing around with an old G3 Ferrari espresso machine, I find that my barista skills are rather lacking. However it becomes difficult to improve them when the espresso machine has such a wide temperature range. Most machines have a simple thermostat that switches the boiler on at one temperature and off at another. This could be over 20 degrees when coffee purists say the best temperature is 92 – 94 degrees C!

Coffee enthusiasts have been modding their espresso machines with PID controllers for years. Essentially a PID is designed to intelligently control a heating element to a set temperature. They can be tricky to tune, but fortunately most new controllers have an auto-tune function, making it straight forward to get started. So diving headfirst, I bought a PID controller on eBay for $45, and prepared myself for some fine coffee making. However DO NOT try this at home as there are potentially lethal voltages inside. 

The ITC-100VH is the actual PID controller, plus a 40A solid state relay (SSR) is needed to switch the boiler element, and a K-type thermocouple to measure the temperature. I also designed and 3D printed a small enclosure for the PID unit. The 3D files are here: http://www.thingiverse.com/thing:1666028

Upon taking apart the machine, there is mess of wires. The boiler is the round metal thing on the right side. Screwed on top are a red and a black thermostat. The pump is the square thing on the bottom.

Expresso Wiring

Simplified down, the electrical diagram looks like this. I’ve omitted the lights in the switches for simplicity.

Original wiring

It’s a fairly simple circuit: when cold, the red and black thermostat are closed and supply power to the heating element. At about 100 degrees the red thermostat goes opens circuit and turns off the heater. If the steam switch is turned on, it bypasses the red thermostat until the black thermostat opens and stops heating, at about 130 degrees.

So we essentially want to replace the function of the red thermostat if we are installing the PID controller. A new circuit diagram is below.

PID Diagram

Thermocouple

Thermocouple end

Now the PID can directly control the heating element through the solid state relay, and the steam switch will still use the black thermostat. The K-type thermocouple should be attached to the metal boiler using some thermal paste. You will get a much faster measurement if you remove the metal sheath on the end and simply put the end of the wires against the boiler.  Then cross your fingers, turn it on and enter the auto-tuning mode on the PID. This takes a while as the controller works out the best control parameters. If you are really keen and have hours to spare, the control parameters can be set manually. The set point temperature also requires trial and error to get right, but now I have one less excuse for my bad coffee…

PID

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