During the winter I use a radiant heater under my desk to supplement the house's furnace, something like this:


It has a high and low setting (800/400 watts). But often I want something less than low (200w? 300w?), and occasionally something in between high and low (600w?). Is it safe to use a variable AC transformer for such control? Note: The heater has an optional motor that swings the heating elements from left to right; I will NOT use that.

Assuming that it is safe to do so, would 10 amp be enough, and where's a good place to find a reliable and not-too-expensive ones? Here is one example from amazon


There are many similar ones and I can't tell them apart. And then there are ones like this


and even


What's the difference?

I will use these devices indoor and I'm not an electrician, so safety is my top concern. With that in mind, I would like to not spend more than necessary for a $40 heater :-)

Many thanks for your help. Please let me know if you need additional information.

  • Have you considered just selling the one you have and buying one that's fully adjustable over the range you want? I haven't run the numbers, but I bet it would be cheaper, and it certainly would be easier, than trying to hack some additional controls in the one you have.
    – Nate S.
    Jul 19, 2020 at 1:12
  • OTOH, if you're interested in doing it anyway as a project, the folks over on the Electrical Engineering stack can probably help you figure out how to do that in a way that's safe, and would be a more appropriate venue for this question.
    – Nate S.
    Jul 19, 2020 at 1:14
  • 1
    A Variac with that much power capacity is going to be expensive. Jul 19, 2020 at 1:49
  • 1
    @WayfaringStranger Not if you get the Chinese to ignore every safety standard. Jul 19, 2020 at 17:25

3 Answers 3


I wouldn't mess with stuff like you are proposing. Electrical resistance heating always requires a lot of power. There's too much junk out there to be sure it's safe to use with a constant load like an electric heater. Like Harp has "harped on" (pun intended) often is stuff sold on Amazon often doesn't meet UL listing and other USA safety testing. There are a number of good alternatives to the setup you are proposing, like under desk warming panels that are safe. Do some research, but don't try to power down a heater like you asked, it's asking for trouble and potential fire.


Another option would be a small, but safe hack. A 12V car heating seat cover can be fed by a power supply, which should be variable between at least 8V to 14V, maximal current 2.5 or 3A. Advantages:

  • Saving much energy, ca. 40W (Power supply with low efficiency) vs. >300W
  • No noise, if the power supply does not have a fan
  • No dust will be whirled around
  • The main body will be warmer from back to thigh, a heater under the desk does only reach the legs, or must warm up the whole air in the room
  • No other person will be affected by higher room air temperature
  • Most likely less expensive
  • The power supply can be used for other purposes like properly charging batteries, at least in the summer time
  • The heating car seat cover can be used also in cars, other then the 120V/230V radiant heater

Stop. Buying. Electrical gear. From. Amazon.

It’s all nasty cheap junk that will kill you. It doesn’t even begin to comply with US safety codes, and is not certified by an independent lab, as required by law. (Any state which as adopted NEC).

Here’s a newsflash about Amazon: Jeff Bezos believes in open markets at all costs. Amazon has internal server farms, warehousing, shipping and of course a retail web site. Bezos has opened all of them to public use - via AWS, Amazon Warehusing, Amazon Fulfillment, and your undoing: the Amazon Marketplace.

You already know to avoid some channels, because what they sell is complete and total junk... right? It’s mail order, and it takes advantage of poor coverage by US Customs to sneak things through that don’t meet US safety standards. Well, all those same sellers jumped onto the Amazon Marketplace. Everything that does not say “Ships from and sold by Amazon.com” under the Buy button, that is eBay-tier trash-ola. It ships via Prime because Amazon opened that up too.

Even things sold by Amazon are corrupt due to (one guess) “open inventory bins”, where any third party seller can inject counterfeits into the inventory bins Amazon ships “their own” items from. /facepalm

The upshot is that Buying from Amazon is so problematic that it is simply not a safe place to buy anything that connects to AC mains power.

No more of that. For ordinary stuff, your local electrical supply house. For exotica like variacs, Digi-Key, Mouser, Grainger, Galco, or McMaster-Carr.

This is a $100 solution to a $10 problem

I don’t know why you’re brain-blocked on just buying another heater. But electric heaters are the cheapest (by which I mean inherently inexpensive due to their nature, not junk) appliances in the world. Resistors are cheap.

So if you want an “intermediate” heater, simply get another one of intermediate size, or with an adjustment knob. This is the point where I note that a 240V heater, run on 120V, will give 1/4 the heat.

Get rid of the fan. Now you can use $10 solutions.

Further, removing the “fan/rotate” component will be a huge step in the right direction. (That is a motor load, that requires particular regulation - heck a variac isn’t even the right thing; you’d want a VFD - and if you think variacs are expensive, lookout!) With no motor loads, the thing is simply a resistor, and will behave in a docile manner using a variety of energy-control devices (including a VFD), not least, a plain old cheapie leading-edge triac incandescent “dimmer”. (Incandescent lights act like resistors once they’re lit, so you’re copacetic.)

Or hack it for separate control

If you feel a huge, compelling need to keep the fan but regulate the heater, then simple matter: hack the device to completely separate fan control from heater control (so no wires at all connect them), and add a second line cord, one feeding fan, one feeding heater. Two power cords is allowed as long as there is 100% separation of the wires, so nothing connects power cord 1 to power cord 2. (The third-prong safety ground is allowed to connect) Now you can put the heater section on any of a variety of dimming devices.

Another example of “2 line cords” is a 4-lamp fluorescent light, wired so there are 2 switches and each pair of tubes can be turned on separately for different light levels. They are often cord-and-plug connected in the ceiling.

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