Home Knowledge Base Home Coffee Roasting Using A Variac With Your Coffee Roaster....

I wish I could take credit for the idea of using a Variac for home roasting, but unfortunately many people have thought of doing that long before i did. My decision to use a variac was caused by the increase in population (infill) within my area, that has not been matched with an increase in capacity of the electricity supply. At Christmas, I saw lows of 212V presumably because of all the Christmas lights! and until midnight or 1:00am mains voltages were generally around 226V. now of course all that has changed and my mains voltage never goes that low. However, during the early evening especially between 5 and 8pm, there is low voltage and large variations in voltage (222-232 volts).

At night especially after 11;30pm, I sometimes see mains voltages above 244 and sometimes 255 volts. Usually though it’s always 240-242V, from 11:30pm onwards.

When roasting coffee you are at the mercy of your mains voltage and fluctuations, low voltage or even excessively high voltage will affect your roasting. The degree to which your roaster is affected will vary and many factors come in to play. Remember power alters as the square of the voltage so a 10% drop in voltage results in a much larger than 10% drop in heating power!

240V with a 47 Ohm (240V) heating element will generate 1225W of power

240V2/47 Ohm = 1225W (of power)

220V only generates 1029 watts of power (or 16% less)….BUT the voltage only dropped 8.4%

220V2/47 Ohm = 1029W (of power)

Ambient Temperature

If the temperature is quite high, you may well not notice the effects of low voltage as the roaster will more easily be able to maintain and increase temperatures as desired. once the temperature becomes quite low, then the reduction on power available to the element becomes more apparent and the roaster may begin to struggle. You will start to see slower ramp ups in temperature, longer times to 1st and second crack and increased overall roast times. Worse still the roaster may not be able to reach a sufficiently high temperature for a decent roast. With some roasters (Hottop) reducing batch size is often helpful, but with insufficient voltage, these can become so small as to not be worthwhile. With the Gene Cafe roaster (March 07 or later model), batch sizes can be increased slightly to compensate for low voltage, but again if the voltage is very low, then this won’t help either.

Type of Roaster

my Toper Cafemino was hardly affected at all by low mains voltage, but this is because the heating elements are much more powerful than home roasters and the machine has a huge thermal mass. The Gene Cafe and Hottop are both affected, in my personal view the Gene is not as badly affected as the Hottop.

Heating element fitted

We are a now 230V country under European Mains Voltage Harmonisation, but in reality nothing changed, the % limits we are allowed to be above and below 230V were simply changed….and as if by magic we complied, but without having to change anything! That’s right folks, NOTHING changed, we are still 240V. If you think about it, it makes sense, were we going to change out millions of electricity substations and plant all around the UK at a cost of Billions…nope, just change the rules, it’s easier. As far as I am aware this strategy applies to all EU countries…so if you want to know your real mains voltage, find out what it was before harmonisation.

You may find your roaster fitted with 230V elements, which might help your if the mains voltage is 230V, but when it’s 245 volts, your roaster will not thank you. The manufacturers have also designed the roaster to work within a certain voltage range and home roasters are on more of a knife edge of design than other appliances. certainly for the Gene Cafe, you should have a 240V element fitted if you are in the UK, as a 230V element will probably roast slower in normal voltage conditions, that’s right slower, it’s not a typo. If you read other articles in the Wiki, you will see that it’s all down to overheat protection and element switching.

Bean being roasted

Some beans don’t need as much heat as others (maximum temp), some don’t sink heat away from the roaster as much as others and also the beans size and density will play a significant effect. e.g. surface area to volume ratio, ability to trap heat etc.

The above are some of the factors affecting roasting, fortunately there is a simple way to overcome this….the Variac.


My 8 amp variac (above), manufactured by Zenith for Claud Lyons. it’s old it’s ugly, but it works real good. Variacs come in various power handling capacities, for coffee roasting in a Hottop or Gene Cafe, I would recommend a 6-8amp variac. You can use one with a capacity higher than 8 amp (but not lower than 6 amp), but it’s overkill and why have a much larger variac than you need, they are large enough already. You can tell how big my variac is by looking at it’s size relative to the size of the mains plug. I was very lucky to have a variac previously used by (and wired up by) Ferranti, more about this later.

Note how thick the incoming mains lead is, much thicker than the lead on your 3KW kettle (which draws about 12 amps), this is for a very good reason. When a variac is first plugged in, it has a large inductance and high initial inrush current. So high that depending on where in the AC cycle (ast or near the top) it is switched on it can trip a 16 amp circuit breaker (or even much higher) instantly, this is a matter of chance, so it might trip standard breakers only 10-20% of the time. You can buy special breakers which can actually handle these high currents briefly before tripping if they are sustained http://www.connectingindustry.com/story.asp?storycode=183771. In use variacs have almost no losses and consume very little extra current.

Note: I only use my Variac when the mains voltage is low (or of course if it’s too high e.g. above 244V), otherwise I recommend connecting directly to the mains. e.g. last night the mains voltage was around 241, so I directly connected and roasted a couple of 300g batches.

OK I have a variac, what else do i need?

I think it’s useful (if not essential) to have one of these


The voltage, current etc.. measuring plug above allows “real time” monitoring of the variacs output to the roaster. I have found that if your measuring plug allows it, that it’s better to measure how much power the roaster is consuming (if you know how much it should consume under load). With the Gene Cafe, it consumes approx 1200-1220W at 240V (when the heating element is on) and I adjust the variac to keep it within this range. I find this better than monitoring voltage, because the voltage drop when the appliance is running is slightly larger when running off the variac than when running off the mains (about 3V and 5V respectively). Important Note: These mains measuring plugs do vary and you should verify the accuracy of yours with a multimeter e.g. mine under reads by 6 or 7 volts, of course once you know, you will adjust your settings accordingly for you plug. It’s important to ensure you don’t overload your appliance, remember a variac does NOT manage mains voltage, or stabilise it. If the input voltage rises, so will the output voltage, if it falls, so will the output voltage.

Using a Variac

This section summarises the information above and adds any extra hints and tips I have found when using my variac.

  • Start with the variac dial below 30 or 40V before you plug it in
  • Raise the voltage to around 240V (or whatever is right for your country) before plugging the roaster in
  • Monitor the Voltage or power consumption of your appliance and be prepared to adjust the variacs output accordingly.
  • The measured voltage at the plug under load for say a Gene Cafe would be around 235-236 volts and NOT 240V, a voltage drop is normal when the appliance draws significant power (if you plug your roaster into the mains you will notice a similar drop when the heating element comes on) if you set to 240V, it’s like starting with a mains voltage of 244-245.
  • Unplug the appliance when finished and before lowering the voltage of the variac
  • lower the voltage to 40V and then unplug from the mains
  • The variac takes a moment to respond to a change in output voltage, so turn that dial slowly and wait a moment to see the result.



A few safety tips, now I’m no expert and I am sure there are other things you can do, but this is what I am careful about (use at your own risk).

  • Ensure that the earth has continuity all the way to the mains supply, I would recommend testing for this with a multimeter! The roaster, must be earthed as must the outer case of the Variac. If you don’t know how to do this, consult a qualified electrician.
  • An RCD in the mains supply will NOT protect you from electrocution when using a variac (equally it will not afford any protection against shorts causing a fire). The variac prevents the RCD from sensing a live/neutral imbalance. Standing on a rubber mat would be a sensible precaution when handling the variac or the roaster.
  • Don’t overload the variac, remember it’s power in watts and current rating are two different things. so a 2000W (or 8 amp) variac WON’T allow a 1600W appliance to be powered at 120V…because you would need about 14 amps
  • Only handle the variac by the insulated bakelite knob when switched on.
  • DON’T leave it alone and DON’T leave the variac plugged in when you are not using it.

Buying A Variac

You can buy them brand new, Clairtronic, Carroll & Meynell although I am not sure whether these are only 0 – line voltage, or whether they make 0 – above line voltage units. I did check with Clairtronic who said that their variacs go from about 0-270V (so suitable for coffee roasting applications), so the published information on their datasheets may be incorrect. certainly the Clairtronic units seem to be significantly cheaper than other manufacturers units.

Prices for new Clairetronic variacs are likely to be in the £134 + VAT range for a 8+ amp unit non enclosed variac and £157+VAT for an enclosed model, although you will still have to wire it up. Link below:


A supplier of Clairtronic Variacs


Claude Lyons still make suitable variacs (that go from 0 – above line voltage), and you can buy direct. They come enclosed or non enclosed for panel mounting. Unless your handy with DIY and metal (wood) work, or intend to panel mount it, then buy an enclosed variac. A new variac will need to have a mains lead fitted and an output plug wired, you will get instructions for this, but you need to be competent.

Prices for new Claude Lyons variacs are likely to be in the £250-300 + VAT range for a 8+ amp unit non enclosed variac and more for an enclosed model, although you will still have to wire it up. I could only see 15A units on their online cataloge for about £300 Link below:


http://www.claudelyons.co.uk/tech.pdf (useful information and how to wire up a variac).

Buying a used variac is always a good idea and an 8 amp unit (that works well) will go for around £100 on e-bay, plus postage ranging from £10-£25 depending on sale price. Overall you should be looking to around £100 (including postage) for a Variac that’s “ready to use” and a reduced price for those where you have to make an enclosure, or wire the thing up… remember how expensive they are new and how few companies make them!

There are variacs that go from 0-240V (not much use to the coffee roaster) and those that go from 0 – 250/260 or even 270 volts. the dials may be marked in volts or in % e.g. 0-100% or 0-115% etc.., ensure you are buying the right type of variac.

the things to look for are:

  • Does the knob turn smoothly
  • any burnt smell, do the windings or case look overheated
  • Is it wired with a proper earth
  • Does it already have wiring for mains and output 3 pin plug (a plus if you don’t have to do it)
  • Is the outer case badly damaged (It may have been dropped)
  • Is there any water damage or rust on the outer case…..variacs don’t like water
  • try it if you can