As far as I know in the UK there is only one popcorn maker suitable for coffee roasting. This is made by Prima it has a white body, and a black top. They also make several novelty poppers, the most common being the duck. These novelty poppers don't have the power to lift the beans and are un suitable for coffee roasting.
The quality control in the manufacture of these poppers is not good. My first one would complete a roast in around 6 minutes. Almost respectable. The results were not bad, not as good as a commercial roaster, but definitely worthwhile. The elements in this popper lasted about 6 months. My next Prima could complete a roast in less than 4 minutes. This was not really roasting, it was incineration. The results were terrible, not even up to supermarket standards. It was at this point that I gave up home roasting for about 18 months.
To control the roast and extend the life of the popper, I decided to build a controller. This would control the power to the elements, and have separate control for fan speed. I brought all the parts from http://www.maplin.co.uk/ for a total cost of around £30. Before the popper can be controlled the fan and element supply need separating. The elements in a popper are very fragile, being made of resistance wire and connected directly to the mains. Another coil of resistance wire is used as a dropper resistance to provide approximately 20 V for the fan. The fan motor has a board containing 4 diodes soldered directly to it. I removed this and connected wires to the motor (the white one in the picture). Actually you could leave these diodes and supply the motor with ac. When I built my box I was thinking about providing fan speed control. I used the original mains lead complete with 13A plug to connect the elements to my box. Both wires are cable tied to the base of the popper so that they cannot be pulled out. British plugs are great in that not only are they cheap, robust, can handle high currants, relatively weather proof, but also carry fuses. I put a 5-amp fuse in to protect my power regulator in the event of a fault. I drilled a hole in the wall of the popper sloping down at around 45 degrees. This is for my temperature probe.
The box is very simple. The first switch is the main on/off and the second elements on off. The third switch is not connected at present. It was going to be fan on /off but this could lead to catastrophic operator error - elements on fan off. So now when the first switch is in the on position the fan runs. The first knob controls the element supply and the second is not connected. This was going to be fan speed control, but having used the unit I've decided that this is not necessary.
Disclaimer
Using 240V in homemade metal-boxed project outside in a damp environment could easily be lethal. I take no responsibility for injuries/damaged caused as a result of following instructions given on this web site. However if you do decide to build a control station then make sure that it and the popper are properly earthed and under no circumstances are operated without a residual current detector (less than £10 at Home Base)
I simply connected the original mains lead to the two black wires ( I think on the popper one wire is actually white) and cut the red wire short. I left thermal fuse in circuit but put a blob of solder on the thermostat. This is necessary to ensure that the popper will get up to a high enough temperature.
The SCR module from Maplin Electronics is quite a bargain. It cost about £12 and can handle up to 2.6KW. The Prima popper only uses about 1KW so it’s running well within its capabilities and barely gets warm even after 4 consecutive roasts. It also appears to have a good linear response to the control. The fan motor takes a surprising amount of current, I haven’t measured it but the first transformer I used from my junk box got very hot. The one fitted in the box can handle up to 2 amps at 24v and is easily up to the task.
