I hope someone more qualified than me answers the 220 volt question. I suspect there could be a problem. There are 220 volt dimmers which would work if they're rated for the right amperage. They would go between the transformer and the machine.

Is there an electrician in the house?

If I wanted one of these for my three-round-prongs-in-a-row 240V Zerowatt, I would sure put it on the 110V side of the transformer. But I'm no electrical whiz. What is it, a variable transformer?

I'm planning it for a while for my Zerowatt. The only problem, most of the light dimmers (at least here) for 3-400W only.

I have followed this post with some interest and spoke with our friendly electronics guru this evening about using this type of router speed control on a heating element and the overall repercussions of use, if any. He explained that this type of dimmer device operates with a Triac type circuit which functionally cuts off a part of the standing sine wave of the 110 volt cycle. When on 'full' the entire S wave is delivered, at 60 cycles per second. As one reduces the setting, small parts of the sine wave are removed as a function of RMS voltage. When set at 1/2 maximum, therefore, 1/2 of the wave is removed. The result is neither a change in voltage or in current (the resistance of the element if fixed) but in essence a sort of switching on and off of the element as the parts of the wave are dropped out. This device essentially operates as a switch, but it switches on an off 60 times per second.
The wattage of the element is unchanged on a per cycle basis but over time the effect is to reduce the duty cycle of the element....when set at 1/2 max the element is under power only half the time, though at full power during the time it is exposed to current.
For example, a 1000 watt element will still run at 1000 watts on a setting of 75% max but only 75% of the time giving it an effective wattage of 750 watts when factoring in total time.

As far as any negative effects, there are none really since the resistor is not shown any other potential or current than it is not exposed to under normal use. The only time there is any undue stress on a resistor (element) is when the element is cold, since when cold, the resistance is lower, therefore the current is higher, constituting a kind of a surge (light bulbs always burn out when first switched on...this is the point when the resistance is lowest and therefore there is a current spike, causing the filament to be exposed to current too high for the rating of the now somewhat worn filament wire....same with a heating element). Since the heating element is hot and is constantly switching on and off (60 times per second) there is no increased current or stress on the element.

The short answer then is....no harm to the element when using one of these to control temperature.

Did you ask him if it would work in series with one of the transformers you sell? This would explain why the box doesn't get hot. It isn't a resistor? While you're at it, does the fuse protect the machine or just the controller? You ought to OEM one with a Bakelite switch.

Dan