Smart crock pot -looking for fan cooled heat sink.

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JR.
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Re: looking for fan cooled heat sink.

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I figured out how to sense temperature of my hot-plate. I have some old medium power transistors with large thin heat-sink tabs. I will use two of these under the fan attachment legs/screws, then I'll wire the two base-emitter junctions in series, and read the voltage with an A/D input. Tempco is around -2mV per degree C per diode drop so around 4mV per degree C total. If room temp is 25C and max rated temp for Peltier is 80'C I should have 55'C or over 200mV of voltage change with temperature range to detect and regulate. Since I will probably build this on one of my tuner boards, I'll probably provide 4'C steps from 80' down to 36' or so.

In fact this is pretty similar to my room heater thermostat but simpler. I don't need to switch AC mains and worry about zero crossings, in fact I can chop the voltage sent to the Peltier device with PWM so I don't actually need a full buck voltage regulation stage,

I don't need a precision time base but who knows I may program in some temperature profiles for cooking like first bringing it up to 80'C for 15 minutes to kill bacteria, than drop back to 130-140F for long low-slow cooking.

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Re: looking for fan cooled heat sink.

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The Peltier device and chip cooler was too clever to be effective. At the end of the day by closing the air inside a box I kind of defeat the heat transfer to ambient and the Peltier device is making more self-heat than pulling anything useful from ambient. A second critical limitation is that the Peltier device is spec sheet limited to only 80'C so marginally hot enough but just barely for safe cooking.

I have the parts and 15V 6A PS to start building my PWM control of the Peltier chip, but again I worry about the complexity.

I ran another experiment with this weeks meal, still cooked on my stove top. Yesterday I picked up a 4'x8'x1/2" polystyrene rigid insulation. It wasn't very rigid as I tried to carry it across the parking lot in a stiff breeze. :lol: I built a heat capture igloo or hot box to sit on top of the sauce pot on my stove's smallest burner at lowest heat. Letting my full pot of veggies and meat simmer overnight on the lowest heat by this morning the food was completely cooked and the stew was boiling, so too hot even on the smallest burner and lowest heat. Note: I typically cook once a week and this pot makes 8-10 servings that a freeze and reheat for daily meals.

This makes me optimistic about my smart crock pot approach where I regulate the temperature and keep the heat loss down to minimal amounts.

Now for my new improved KISS heater, I can buy 50W Image resistors that bolt to the bottom of my heat spreader plate for $3.65 each... The voltage breakdown on these is 3000V so I can drive these directly from mains voltage with a rig similar to my smart room heater thermostat... At 300 ohms these make 48W each so I can probably stack as few as two of these to heat my smart crock pot. At less than 1/2 A each I can stack several more of these in parallel if needed.

The good news is I don't really need to code up a dramatically different temp control... I just need to change it from a repeating daily 24 hour cycle to a serial one time heat profile, It already supports temp changes every 15 minutes. I will scale up the temps to 100-212'F range...more appropriate for cooking. If I make the hottest temp 212' I can calibrate that by boiling water and then scale down from there.

I can not cool the room while cooking but I am no longer heating it up, and will waste less energy as heat loss into the room. With superior time/temp cooking control. 8-)

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Re: looking for fan cooled heat sink.

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I am still waiting for my power resistors but I have started using my heat igloos for my normal stove top cooking and I am noticing more cooking from lower heat settings and less heat into the room.... Sweet.

It seems to me that we need a new kind of cooking pot that is built like one of those japanese steel thermoses with an air pocket between the inside and outside. Keep the heat inside the cook pot and not heating the room.

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mediatechnology
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Re: looking for fan cooled heat sink.

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I like the power resistor idea.

I saw this recently in Electronic Products and thought of you: http://www.ovenind.com/pdf/DS-5R7-573.pdf
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Re: looking for fan cooled heat sink.

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mediatechnology wrote:I like the power resistor idea.

I saw this recently in Electronic Products and thought of you: http://www.ovenind.com/pdf/DS-5R7-573.pdf
Yup that is a viable way to control them and I was headed in that direction. You can drive a Peltier device with PWM or duty cycled DC as long as the full scale DC does not exceed the max current for the Peltier device.

I have rejected the Peltier for my application for several reasons, it consumes several watts for every watt of heat that it moves, so more of a resistance heater even when working. For trying to pull heat from ambient (25'C) and put it out into 80'C hot side is 50'C+ thermal spread which is a lot. And 80'C (rated max temp) is marginal for cooking. I suspect I could get away with pushing the Peltier device up to 100'C (the solder inside doesn't melt until 138'C). But the whole thing to complex, and once I sealed up the thermal system, to reduce gradient across the Peltier device, it became a joke.

Another benefit of the power resistors is that I can drive them directly from 120VAC so no power supply needed.

The bottom line, or larger lesson that I am learning from this is that thermal insulation matters. I have noticed a huge benefit is putting less heat into my kitchen from stovetop cooking with my thermal igloos.

What the world needs is a smart Crock pot, with good insulation and precise temperature profiles. I can imagine providing a screen where you select beef, pork, or chicken, and it knows how hot to cook them for food safety and optimal texture on the meat. Some temp ranges help break down the connective tissue, while too much heat dries it out and makes it worse.

But I have no inclination to make one commercially. I might consider modifying my old soldier. Later. I already have worked out the isolated triac scheme from my smart room heater project.

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Re: looking for fan cooled heat sink.

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In light of the fact that this thread has gotten hundreds of views, I should mention that my thermal igloo idea is not recommended for use with a gas burner. The gas stove needs air turnover for the flame. I guess one could punch a hole in top for exhaust, and slots for fresh air inlet on the bottom, but this will degrade the thermal efficiency.

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Re: looking for fan cooled heat sink.

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I've seen a couple interesting infomercials. One shows a single burner induction cooktop, while I am not leaning that way. The second more interesting one is a powered pressure cooker, with programmable control... Now were getting closer. If the pressure cooker had good insulation and robust temperature detection that would almost be a slam dunk. While I am looking at slow and low cooking, not fast and higher temperature, I do recall my mom cooking with an old school pressure cooker and some really tender stews.

===

Oddly at the moment I am having trouble coming up with a 3.3V supply, for my one-off with AC so I can detect mains zero crossing for accurate timing, and precise triac control. I have several DC lumps but my AC lump is making too much voltage. It's always something.

----
Time to get out in the yard... so more later...

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Re: looking for fan cooled heat sink.

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I't alive
cooky.JPG
cooky.JPG (57.3 KiB) Viewed 22396 times
The wiring may look a little dodgy but it should be safe. :o :o

I use a 3 wire line cord with the aluminum plate grounded to the wall, all plugged into a GFCI outlet.

the two pairs of wires running from the processor board are the two power transistors in upper left hand corner that have their heatsinks mechanically connected to the resistors for temp sensing. Funny story about that, at first it wasn't working right because I neglected the serious reality that the collector were attached to the heat sink on old school transistors. But now it works with two base-emitter junctions in series and collectors at ground.

Bottom left is the line cord connection to a nice triac with an opto-isolated controller. That is the second pair of wires.

I re-tweaked the temperature scale from old 60-70'F to roughly 220F' full scale coming down 12'F per lamp (12 steps).

I only hooked up 3 of the 4 resistors for heat but that should be 150W... If i need more I can always hook up one more resotor.

Over night tonight I get to test it for real.

I lean toward starting out hot, then drop down to around 140' for long low slow cooking. But I expect to learn from doing.

Cooking with my heat igloo and the smallest stovetop burner, over-cooked the food last week. So I need less heat than the least the stove does. This puppy can be adjusted all the way down to warm enough to touch.... :D

I may fire it up first to see how hard it is to boil water and if my temp scale is close to reality.

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Re: looking for fan cooled heat sink.

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OK, first trial under fire was disappointing,, food undercooked. :(

I just spent a couple hours ASSuming my crude temp calibration was off, but in fact I was pretty damn close ... I can boil off a drop of water on the aluminum heat spreader at the top setting. Just to make it easier to control I offset the temp range one LED (12'F) hotter.

JR

[edit1] I am getting smarter about this, slowly. My end point temperature or how hot I want the food to get, will vary quite a bit below the hot plate temperature. While I am assuming a low thermal resistance or drop from the aluminum heat spreader and the pots thick aluminum base there will be some real series thermal resistance, and the food will have both resistance and thermal mass. Even 1'C/W thermal resistance means tens of degrees drop when pushing tens of watts into the food. Duh. :oops: [/edit]

{Edit2 made some more changes. Moved the triac heatsink down off one of the resistor screws and gave it it's own screw to the heat spreader plate that should keep the triac a little cooler. I put the temp sensing transistor on this same screw. I added the 4th resistor for 33% more power so now almost 200W at full grunt. I scaled up the temp display range to probably 250'F+

I just ordered some heat sink compound from rat shack (I didn't have any laying around) to put under the power resistors so they will thermally couple to the heat spreader plate better, for less heat rise in the resistors. Since each resistor is pushing near 50W even a few tenth's degree C/W of thermal resistance between the R and spreader will cause the resistors to run 5-10'C hotter. I also bought some higher temp wire (105'C) I was too cheap to buy teflon wire that would be another $20-$30.

I need to run this hotter to push more heat into the food. Still learning. [/edit2]
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Re: looking for fan cooled heat sink.

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Well I am about ready to stick a fork in this. I have added heat sink compound to the resistors and replaced the wiring to the hot inner ends of the power resistors with 105'C wire.

I have turned off the 24 hr timer so I don't get any stupid temperature targets, and added code so if the diode temp sensor opens up and reads too high or too low it will ignore it.

It works but a too much trouble for not cooling my kitchen. :roll:

In hindsight I could buy a brand new commercial crock pot with temp control for $50, less than I spent on parts. :oops: :oops: :oops:


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