Controllable coffee roasting using light dimmers

Modifying a light dimmer to control a popper for coffee roasting

Caution: this article provides information about how to modify a mains powered electrical device. Electricity is dangerous and Too Much Coffee cannot accept any responsibility if you over-estimate your abilities in this area and hurt yourself or cause damage to your property. Proceed at your own risk.

Having decided to give home roasting a go, I concluded that the popcorn maker was the first stop for me. After a bit of searching I managed to pick up a bargain for £3.30 on eBay.

My first attempts at roasting were OK, but too quick. I tried the method of periodically turning the machine off, but this was fiddly and gave erratic results, so I concluded that I needed to control the heat output. The most popular method for doing this is to use a variac to drop the voltage from the mains. As most popcorn machines are rated at 1100-1400W a high current one is required (minimum 6A at 230V) but they are expensive, £100+ new, and second hand ones can be hard to find (in the UK at least) and still fetch £50+. I decided that another route was required.

Typical domestic light dimmer

Dimmer - rear view

There is another way of reducing the power from the mains .This method is used in light dimmers. Rather than reducing the voltage, you reduce the power by chopping off part of the mains cycle. These circuits use a device called a triac (rather like a controllable bi-directional valve) to turn the output on part way through the sinusoidal mains cycle, hence reducing the effective power. More information on how light dimmers work can be found here.

Light dimmers are cheap, but most readily available ones are only suitable for loads of 400W, which is way below that which is required for controlling the heater on a popcorn maker. However with a few simple modifications they can be easily turned to our purpose.

I purchased a 400W light dimmer from an electronics supplier for £6, and opened it up.

Inside the dimmer

Dimmer components (unmodified)

The first thing you need to do is make sure the triac can handle the current for a 1400W heater which means for safe operation it needs to be rated at 8A or more. In anticipation of finding a low current one in the dimmer, I had purchased a 10A triac. Upon opening the dimmer, I discovered that it was already fitted with an 8A device, so I used the existing one.

The second thing you need to do to make the dimmer cope with the higher wattage load is to fit a decent heatsink. The extra current flow required means that there will be more heat dissipated in the triac, and you need to deal with this. Most light dimmers use a small bent piece of aluminium sheet as a heatsink, which must be pushing things even at the lower wattage. This aluminium part must be removed. It is usually held on with a pop-rivet that must be carefully drilled out. Depending on the type of heatsink you use, and how you are going to mount it, you may wish to remove the triac, and use flying leads to connect it to the board. You will need a heatsink with a quoted temperature rise of less than 10°C/W, I used a 5°C/W one to keep the heat very low.

Replacement inductor

The uprated parts used

There are 2 other components on the board that will need upgrading. There is normally a thermal fuse fitted; the one on my dimmer was a 2A one. Fuses of this type seem to be hard to come by so I actually removed mine and put a wire link in its place. The thermal fuse is only really there for protection of the triac, and unlike in a lighting circuit which only has a fuse at the consumer unit, you will have one in your plug.

The second component is an inductor. This forms part of a filter circuit which stops the noise that can be generated by the triac switching from getting back to the mains supply. Again you will need one capable of carrying 8A. You can, technically, just link this part out too, but you may have to explain to your electricity supplier why your neighbour's television is getting interference!

Once you have done these modifications you will need to mount the dimmer circuit in a housing, which must be SAFE! If you are going to mount it in one of the popular diecast boxes then you MUST ensure that it is EARTHED.

Modified circuit (complete)

I actually mounted mine in the base station for an old Weller soldering iron, as I wanted the 24V from the transformer to run the fan motor on the popper.

The results from my setup have been very good, and it is very easy to control the heat and profile the roast now. All at a total cost of under £15!

This modification does require some skill with a soldering iron, and some appreciation of mains safety, but then you shouldn't be attempting to modify a popcorn maker at all if you have no understanding of the mains!!! Following the parts list takes away a lot of the electronics knowledge required. Unfortunately the suppliers listed are only UK ones, but I don't have any knowledge of ones elsewhere in the EU.

Parts list

400W 220/240V light dimmer

• Farnell - 389-3406 £6.31 (+VAT)
• Maplin - FQ10L £7.99 (incl VAT)

Inductor between 40-150µH 6A current capability - not essential but recommended to reduce interference, and to stay legal!

• Farnell - 482-584 £2.44 (+VAT)
• Maplin - JL72P £1.99 (incl VAT)

Heatsink 10 °C/W or lower - absolute essential!

• Farnell - 707-405 £1.37 (+VAT)
• Maplin - RN81C £1.49 (incl VAT)

A box of some sort.