Cooking is based on applying heat to food, ideally in a consistent, controlled way. Electric frypans suck at consistent heating. This is because they use a cheap thermistor, and dump all their power into the element until it reaches the set temperature, and then *turn off* completely, so your steak stops sizzling, until the temp drops enough and it turns the element back on again.
For that matter, some electric stoves also suffer the same problem, but because of their thermal mass the temperature doesn't drop so much.
Gas stoves are highly regarded by cooks, because you can finely control the amount of heating, and it doesn't turn itself on and off randomly at inconvenient times! But gas stoves are more complicated, expensive, require maintenance (replacing gas bottles), and some people are scared of using gas.
Why should electric heating work so badly?? Don't we control electricity? Isn't it supposed to make things more flexible?
Well there is a simple, well-known way to control electric power for motors, heating, etc: Pulse Width Modulation. I don't know why frypan makers don't use it, except that it's a bit more complicated and maybe more expensive. But you vary the % of time you are heating, and you can do it very fast. It changes the control mechanism from desired temperature to % of power to output. And you can do this pretty easily with a 555 timer IC, make the dial control the pulse width, and pass the AC current with a bigass MOSFET, SCR, or relay - solid state if possible, or there will be lots of clicking.
My friend Dylan points out that changing the dial to increase the power just means the % is increased, which is different from the existing functionality where it will dump all power until it reaches that temperature. In short, this means it will take a lot longer to reach the desired increase in temp (or a drop). I'm not sure how to approach this problem. Perhaps a hybrid thermistor and PWM solution, where some fuzzy logic figures out what PWM value to use to maintain the temperature, and nudges it up and down if the temp changes. That immediately brings to mind the idea of using a microcontroller like AVR (eg Arduino), instead of 555 Timer and so on, but I'd like to avoid that, at least in the final product. Still, it could be an interesting project!
3 comments:
Use a NTC Thermistor to vary the pwm circuit.
As the heat increases the resistance increases and slows down the PWM.
Cool, sounds good. I realised though, that it shouldn't try to replicate the same function as the electric frypan, lest it end up with much the same problem.
Gas stoves are operated differently - you change the amount of gas when you need more heat. The same thing can be done with electricity. So I want to do away with temperature control entirely and just adapt to controlling the power.
Seppo suggested a light dimmer would work there - I don't know if that's PWM or current control but either should work. Fan speed control is probably much the same. I can get parts from Bunnings and create a passthrough with a dial and a regular wall socket for the frypan.
A PID (propotional, integral, differential) controller is the best way to give a fast response time and accurate temp: it basically creates a response curve based on current temp and set temp and applies appropriate power (based on how far away it is (P) how far away on average it is (I) and how fast it's changing (D). They quite commonly have a digital/cmos IO. We use them in our furnaces at uni, and get stabilities down to less than 0.1 degC over a temp range of RT to 900degC.
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