I know that there are many sites out there describing how to make a coffee roaster out of a popcorn popper. Some of them even use an Arduino for controlling a portion of the popper/roaster. However none of them were done the way that I would have thought to. Since I need to roast some green coffee beans and since I have just started playing with the Arduino platform, this seemed like a good project the take on as my first Arduino adventure that wasn't simply following instructions in a book.
My Roasting History
My wife and I recently went to Ethiopia where we saw a lady roast green coffee beans in a small cast iron pan over some open coals. Her coffee tasted great and we brought some green beans home thinking "How hard is that".
About the same time my brother-in-law was trying to roast some green beans at home as well. He tried using a pan over the stove, over the barbecue, and even roasting in an old popcorn popper. His attempts were all ill fated and he eventually ended up with a commercial home roaster that broke after a couple of months.
The Start
After hearing my brother-in-law's problems, I looked up home coffee roasting and realized that the lady in Ethiopia had a very hot bed of coals and a lot of experience. I got the old popper from my brother-in-law. It was the Wear-Ever Popcorn Pumper 73000 that my mom used to make popcorn when I was a kid.
I took it apart and found that it was made basically the same way as the Poppy II and many other air poppers. It was in fact the correct kind for roasting coffee, with air louvers in a pattern around the bottom edge of cup that causes the beans to swirl. Since there are many web pages that detail how to modify a popcorn popper, I won't go into those details here.
The fan was in series with a 48.3 ohm (measured cold) heating element. On the bottom of the fan was a full wave bridge rectifier. A second heating element of 11.2 ohms (cold) was across the entire 120vac. That was basically all there was to it. I did some rough calculations assuming that the fan motor needed about 20 volts (based on other web site's figures). This meant that the fan drew about 2 amps and the other 100 volts was across the 48 ohm element. In this case the element in series with the fan would only be putting out about 200 watts of heat. The other element was putting out about 1,285 watts of heat. Since the entire popper is only rated at 1,250 watts, my guess is that the 48 ohm element is really only there as a cheap voltage regulator for the fan motor and isn't needed for heating.
I looked through my bins of junk and found a power supply from an old IBM 100mhz Thinkpad.
This power supply was rated at 16VDC and 4 amps. After removing the full wave bridge rectifier from the bottom of the fan motor, I hooked it up to the Thinkpad power supply with an amp meter in series. The fan spun up and the amp meter read 1.8 amps. That's pretty close to what I had figured.
Next was to test the heating elements. Some of the popper modifications I saw controlled the fan, while others controlled the heating element. My guess is that since these poppers tend to not get up to roasting temp without modifications, then the answer would be to control the fan. The heating element is already working at 100 percent. Bypassing the thermal safeties wouldn't make the popper get hotter, it would just let the popper melt and start a fire if the fan failed. I still may have to bypass the thermal safety if it trips with the fan on low speed, but my hope is that I can achieve roasting temp by slowing down the fan, and still leave all the safeties in place.
I took some precautions (made a safe place to get things really hot), disconnected the fan and plugged in the heating element. I wanted to see how hot it would get without the fan before the thermal safety would open. On this model, the thermal safety is a small heating element next to a bi-metallic switch. This makes for an automatically resetting switch that is good for more than just one use. In about 60 seconds or less the element got hot enough to start melting the plastic and open the thermal safety. I didn't even get a good temperature measurement, but it was well over 200 degrees F. This was without the 48 ohm element even hooked up. My conclusion from this is that the popper will get plenty hot without the second element. I'm still not sure if I will be able to leave the thermal safety switch in there though.
After hooking up the fan motor to the 16VDC, I plugged everything in and watched the temperature. It heated up fairly quickly to 200 degrees F and hovered between there and 195. This isn't really hot enough for roasting coffee, but it is a good indication that controlling the fan would allow me to control the temperature of the device between being to cold (195) and meltdown.
When I unplugged the fan and element, the temperature immediately started to raise and was quickly up to 220 degrees F. I plugged just the fan back in and it quickly dropped. It was down to room temp in about 60 seconds. This would be a huge advantage to having separate control of the fan and element. One problem with popcorn coffee roasters is getting the coffee out quickly and cooling it down. I could just leave the beans in the roaster, turn the element off and turn the fan on high.
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