Adjusting IMUSA hotplate watttage with a router speed control

[Note: collecting/reworking info from different posts]

I’ve been cooking with solar-generated electricity for the last year. Works surprisingly well with only two hiccups:

  1. I don’t have quite enough solar grunt in December to cook 100% with electric. I typically have to cook one meal a day over propane in that month.
  2. I really wanted to run a IMUSA 1100w hotplate.

I’ve been dragging that hotplate around since I built the van but only used it once and that was on shore power.1 It’s slightly too large, power-wise, to run offgrid with my setup.

thinking about solutions

  • thermostatically-controlled hotplate in the 500w range do exist, but they all have 4” burner heads. Boo.
  • I found one thermostatically-controlled 750w hotplate with a 6” head and had it sitting in my cart…
  • “What I need,” I thought to myself, “is one of those $$$ variac things so I can dial down the power on my existing hotplate”

Last time I looked the cheapest I found was $100. But now my searching is finding some triac-based controllers that seem to do what I want in the $25 range. Pertinent reviews/comments:

this works by chopping the AC cycles down to half the cycle of a full AC cycle. This WILL NOT lower the voltage or current! Do not use this on any electronics as it could damage it or greatly lessen the life of your electronics!

No problem, I’m just running resistance loads.

I used it this morning to control the heater element on my coffee maker - the coffee maker element is rated at 1440 W at 120V, and a Kill-a-watt registers 1420 from it. If set to non-variable mode the kill-a-watt registered 1410. When switched to variable, the output was as follows: at the lowest setting it output 687 W, at the middle of L it output 710, at the top of L: 750, at mid-M 812, at low H 955, and at the max the Kill-a-watt showed 1374 W. My desire was to reduce the output of the coffee maker to slow the brewing and it worked perfectly for that. The unit heats up whether in variable or non-variable mode, in variable mode it does get hotter, but not excessively in my opinion. Your mileage will likely vary..

That sounds really promising. Interpolating from the above, On the larger hotplate it appears wattage can be dropped as low as 449w. I’m thinking the IMUSA’s 5.5” coils running at ~500w-600w would be verrah nice. Leave the thermostat cranked all the way up and use the controller to vary the heat.

Another review suggets there’s a pot to turn down the minimum even further:

I read another review that indicated there is a hidden adjustment potentiometer inside for minimum power setting. Wall dimmer switches have these as well. I was able to adjust it, but I’m waiting for my tachometer to get it where it needs to be. By the sound of the motor, I can tell it will go down enough. They really should disclose this adjustment in the instructions… Note: The hidden adjustment is soldered onto the top side of the PCB inside but you need to use a small flathead screwdriver to adjust it from the back after removing the back cover.

Down to 25% power would be nice.

Other possible uses: run the 300w hotplate at 150w for simmering. Run crockpot at varying power to control temp more precisely than HI/LO allows. Oh boy oh boy oh boy

I added this one to my cart;p it has >2,000 reviews and some clueful reviewers. I’ll let you know what we find…

mini triac testing

Turns out Harbor Freight had an identical-looking one in stock, although only 8A (ie, 1000w). Since I want to run things at <1000w I think it will work. Removed the one from my Amazon cart.

I do not know if either the MSW, PSW, or Triac will explode in various configurations, so I tested thusly:

  1. I dug out an old backup 400w MSW to test with.
  2. I ran the crockpot off the Triac on the MSW and it worked fine. I was annoyed because I couldn’t find my Kill-A-Watt, but watched the battery monitor fluctuate as I operated the Triac from low to high. Nothing exploded, made weird noises, got hot, etc.
  3. I removed the 400w MSW and installed the 1000w Amazon MSW
  4. I dug out the “1100w” hotplate (928w observed) and ran it on the triac at lowest position. Moved it up a bit from low and watched the current change on the monitor. Coolio.
  5. At this point I realized the new MSW has a “watts output” display, so I will use that until I find the KAW. It showed the hotplate running 550w on lowest.

The latter point aligns with the only post I found on the net about running a triac off an inverter; a dude tried it and found the current wasn’t reduced as much as when on PSW. He reported ~48% on PSW but did not give a specific figure for MSW. I’m seeing ~59% (550w/928w) which meets my needs just fine as long as it holds up. .

update actual use

Brewed coffee using the new 1000w msw (straight, no triac), worked fine.

Hooked up the big hotplate to the triac and warmed up sausage & egg leftovers for bf burritos. Wider coils much more stable. I put the hotplate between Low and Medium and it cycled on/off to hold that temp. The inverter reported ~525w when it was in an ON cycle. The triac’s backplate was maybe body temperature after cooking was done.

After that was done I heated the naan pan for tortilla wrappers. The wider coils aren’t just more stable but the pan was heated more evenly. Win-win. I really like this setup.

update: internal pot adjustment

I turned the internal pot CW a bit, as shown in this video. There is a point where it turns off if go too far so I didn’t fight for every watt. I used the 928w hotplate as the guinea pig and could stably get it down to 110w, or ~12% of the normal wattage.

Heating element resistance changes with their temperature so the numbers aren’t rock solid, but I annotated the image to the right with the values I saw from teh 928w hotplate at various settings.


Early days, but I think this could be a winner. I’ve started drafting a RVwiki page on the subject.


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  1. I was camped in my father’s driveway while I assisted with his hospice. I ran an outdoor extension cord from his patio to the van’s shore power port