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Potentiometer/Variable resistor with AC advice Answered

I'm a computer store tech who knows how to solder some(but DC stuff only) and who also enjoys cooking, so my ? is: What's the best choice of pot. for use with US household current to actually control the cooking temp of my hotplate? End result is to eliminate the annoying "heat spikes" you get due to lack of a pot. in the design, IE literally everything sold these days turns on at full current/temp until it reaches X temp, basically it's just temporally defined by how far you turn the knob, and your pancakes wind up crispy black around the edges and gooey in the middle  instead of golden brown and amazing all over. I want it to turn on at X current until X temp (like electric frypans/burners did if you're old enough to remember.) All I need to do ( I think) is add a pot/VR into my burner's element  "line" the right way, it already shuts off at X temp just fine. Incidentally, that's the "click" you hear when any home electric cooking device (sold in North America, at least) hits temp or turns on, the temp. control switch thingee connecting. It's NOT a VR/pot, just a pair of electrodes JUST barely touching strapped very tightly to some ceramic discs. It uses thermal expansion of the discs/electrodes to make things just a tiny bit bigger and separate the tiny little electrode spoons  But all that boring geekspeek affects in this configuration as basically just temporal heating, IE no reduction of the amount of heat in my frypan, just the cooking time. Science says if I reduce the amount of electrons flowing thru it just slightly as well,  it'll be colder but still hot and still shut off when it's hot enough. A pot/VR wired in the right way will do exactly that, won't it?

After some research I'm thinking something like a 30-50K "B"-taper pot. wired with 2 of 3 traces (https://www.instructables.com/id/Wire-a-Potentiometer-as-a-Variable-Resistor/), I'm looking for a "broad" response range while turning with a tight  "pinch" effect at low end for amazing banana-brown-sugar-pinch-of-nutmeg pancakes with coffee on the tiny balcony/fire escape in the morning but I have no idea how the numbers work for AC currents. Does 120vAC mean a different pot or a diff config/approach altogether? Or should I be looking into AC variable resistors, instead? I know an awful lot about "base" science, enough to get me this far, right, but AC throws me for a loop a lot, and there's a gap in my knowledge/experience, between the basic stuff and how it applies to stuff like my application, I don't get a lot of the numbers/formulas and how to use them. 
A VR would the ideal tool here, but they are hard to get in the right form with the electrical qualities I need, whereas pots are usually "turning switches", right?

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Downunder35m

2 years ago

IMHO this problem comes down to how good the entire thing is designed and how much the designer cared.
This is where modern induction cooktops come in place - they do regulate the way you like - and as a single you can have them for around 50 bucks these days.
For a standard hotplate all we get is so called bang bang regulation based on whatever the tempswitch provides, which usually is not perfect.
But you also have to keep in mind that this type of heat regulation is based on an average and the fact that most things in a pot don't really care too much.
If you can accept the full power switching but want much more control then have a look at PIT controllers - the REX100 clones with 30A SSR go for under 50 bucks and do just that.
It works basically the same way as your old control just with far more precision.
Where your switch cools down and heats with the hotplate the PIT control works about like this:
1. Always full power no matter what.
2. Once you get close to the target range the element is switched off for a short period of time to check how much further the temp will rise.
3. The closer you get to the set temp the faster this on-off switching happens.
4. Once target temp is reached the switching happens fraktions of a second so no overshooting of the temperature despite the fact it is all full on, full off.

This type of control is used in 3D printers, soldering oven and basically everything that needs to be accurate within reason.
Best of all is that you can tune it as well, or better said: You can train the controller to your heating needs.
It also eliminates the problem of you always adjusting for pot size, fill level and actual cooking content.

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MattH68Downunder35m

Reply 2 years ago

Full power switching is exactly what I am attempting to eliminate. random IE: crystals only form when a random solution is kept at precisely 250 Kelvin for 26.75 minutes. You could do the math and figure out how to apply that much thermal energy to your solution in 0.1 seconds (thusly the same "average heat over time"), but you will NOT get any crystals outta the result, simply vaporized solution and a cloud of gas. Exactly that with cooking for me, I was trained as a Cordon Bleu chef many years ago. So I know all about induction cooking, and want one (induction cooker) mad bad. But my income is very fixed atm, Provincial Disability in BC, Canada, $50 is literally half of my grocery $ for a month. Looking for a simple DIY solution, I'm just hesitant to apply the VR-Pot mod without knowing how AC is gonna affect things as it's my only means of cooking and I don't wanna burn my house down. I'm going to try Toga_Dan's SCR dimmer idea with a circuit-breaker "real" powerbar I use for my "workbench (NOT a garbage GFI-based "power bar" from Wal-Mart that's really just an expensive glorified extension cord, a heavy-duty, 25-year-old, replaceable-breaker, will-actually-save-your-device-from-a-surge POWERBAR that goes everywhere with me)," lots of renos going on near me and I'm sure I'll be able to scrounge one easy.

If everything works ok, I'll eventually make a 'Strucible showing what I did, gotta be other "old folks" looking for the same info.

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Toga_DanMattH68

Reply 2 years ago

b sure to check watt rating (probably obvious). Most light switch dimmers are about 600 w. Halogen floor lamps about 1000w IIRC (I find a lot of these lamps for free, and pull dimmers from em). Average stovetop burner is, I think 1500 w. And the sears scr dimmer I found online : 2000w.

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Downunder35mMattH68

Reply 2 years ago

PID controlled heating can be with on-of in very long intervals but also down to literally milliseconds.
You also have "stored" heat depending on the size of vessel involved, so keep steady temp with just a dimmer is next to impossible - at least for the accuracy you seek.

How do you regulate the power down once you reached the target temp?
How do you adjust for airflow, evaporation and other factors affecting the temp?
Without a fast and precise temp sensor or thermocouple you won't go nowhere and no microcontroller means going back to a dedicated pid controlled regulator...
Pid controllers for incubation puposes regulated within a fraction of a degree.

Might be easier to save the 50 bucks for an induction cooktop from Fleabuy...

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Toga_Dan

2 years ago

a better option is an SCR dimmer. Sears has a 2000 w one online for $8.

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Toga_Dan

2 years ago

a quick n dirty solution: wire a second burner in series to your main one. Altho not adjustable, it will get u in the ballpark. After learning if it is too hot or cold, u can test it's resistance.