Smart crock pot -looking for fan cooled heat sink.

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JR.
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Smart crock pot -looking for fan cooled heat sink.

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I thought I would ask around. I am starting toi put together my Peltier crock-pot cooker, and one one piece of the puzzle that I do not have an easy/cheap solution for is a forced air heat sink, actually a cold sink for the cold side.

I can source a nice peltier device that should work, and roll my own controller, but I need to deal with the cold side of the Peltier device. I would like to find something like one of those chip coolers added to hot rodded PC processors, but scaled up for a larger fan, since I may want to move tens of watts.

My plan is have a fan attach to fins side of the heat sink, and put that fan on the bottom, so use feet to space it up from the counter, to allow air flow.

Image something like this heat sink would be sweet, but I don't know if he'll sell me one.

Anybody know of sources for this extrusion or similar?

JR
Last edited by JR. on Mon Aug 11, 2014 3:40 pm, edited 1 time in total.
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Re: looking for fan cooled heat sink.

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OK that heatsink in picture is smaller than it looks... I contacted the guy and it's just a standard chip cooler. After doing some research I found some coolers they rate at 65-90W of heat dissipation,,, Worth a try since I don't expect to be moving that much power...

JR
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ricardo
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Re: looking for fan cooled heat sink.

Post by ricardo »

JR. wrote:After doing some research I found some coolers they rate at 65-90W of heat dissipation
That's a lot of H/S in a small space. Let us know what you end up using .. especialliy if it costs peanuts and isn't Unobtainium :D
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Re: looking for fan cooled heat sink.

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ricardo wrote:
JR. wrote:After doing some research I found some coolers they rate at 65-90W of heat dissipation
That's a lot of H/S in a small space. Let us know what you end up using .. especialliy if it costs peanuts and isn't Unobtainium :D
So far it looks like I will need to make a custom heat spreader for the hot side, but a standard chip cooler on the cold side should work... While you can buy chip coolers for $10 or less up around $20 they make some with larger fans and even copper center for finned heat sink.

I will need to roll my own micro control but that seems simple enough. Some of the fans even have a PWM input for speed control so I can control fan speed from the micro too.

I can get small 5A- 12V switcher for around $15-$20. I can add a mosfet switch and inductor to output to vary that supply voltage to control the Peltier cell. I might hack into the switching supply to vary it directly but it may be easier to just vary that externally.

I can throw a few diode junctions on the hot side to measure cooking temp to use to vary voltage to the Peltier supply, and diode on the cold side to vary fan speed based on cold side temp.

So far the parts cost to build this looks pretty reasonable $15-20 for Peltier cell, $15-20 for 5A PS, $15-20 for chip cooler, ??$ for aluminum heat spreaders, micro controller, assorted parts. Probably <$100 not counting my time.

JR
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Re: looking for fan cooled heat sink.

Post by JR. »

OK for a followup I placed a parts order today so I can get started on this.

I will need to roll my own custom temp controller.

The individual parts are not bad but I'm up to around $75 so far.

12V@ 5A AC/DC PS $27
Peltier device $19 http://www.cui.com/product/resource/cp60.pdf
6"x6"x.25" aluminum heat spreader $7
chip cooler $13.25 http://media.digikey.com/pdf/Data%20She ... 025S1x.pdf
Image
asst parts to make DC-DC buck convertor <$5
PCB processor and interface, TBD...

I still am mostly poking around in the dark, but the Peltier device should be pretty efficient for heating.

Before I design the control I may rig it up with max voltage and max fan speed to see if I can boil water.... If I determine I can get hot enough then I will worry about ramping down the drive voltage to the Peltier and fan speed (it has a separate PWM input)

I will use a PIC with several A/D inputs to measure hot side and cold side temp with diodes, then based on that I can vary voltage to Peltier with my crude buck convertor. I will use a PWM output from the PIC to drive the switching mosfet I need to use two mosfets first to level shift then to drive the high side switch. I will use a shotkey catch diode for the low side to KISS.

I need to work out a few details,,, I plan to mount some wooden legs to the 6"x6" heat spreader plate, then clamp the Peltier device to the bottom of the heat spreader with the chip cooler fan/heat sink assembly below that. I need some kind of a heat barrier right at the Peltier to keep the hot side and cold side separate. If i pull air down the heatsink it my be less effective (forced air works best blowing onto the HS), but any heat it pulls down will get absorbed by the cold side heat sink and pumped back up into the cooking surface.

JR.
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ricardo
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Re: looking for fan cooled heat sink.

Post by ricardo »

JR. wrote:Before I design the control I may rig it up with max voltage and max fan speed to see if I can boil water.... If I determine I can get hot enough then I will worry about ramping down the drive voltage to the Peltier and fan speed (it has a separate PWM input)
Is there sufficient spare intelligence for it to make a cup of tea (white, no sugar) ?

Un cappucino might be too difficult. Maybe for the Mk2 :)
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Re: looking for fan cooled heat sink.

Post by JR. »

In fact with the Peltier heater/cooler If I wanted to I could keep the raw ingredients cold until I was ready to cook them, then heat them up using a temp profile to slow cook while killing germs and even cool it down again after it finishes , if I want.

I don't actually plan to add cooling the food to it's trick box (I'd have to swap the polarity of the Peltier leads), but I might play games with temperature profiles, getting it just hot enough to kill the bugs but not over cook the food... the magic part is cooling my kitchen while I cook. 8-)

JR

PS: I hope i get it finished before winter. :lol:
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Re: looking for fan cooled heat sink... the eagle has landed

Post by JR. »

pel2.JPG
pel2.JPG (55.34 KiB) Viewed 29936 times
Top view of 6"x6" 1/4" aluminium heat spreader, sitting on wooden legs.
pel1.JPG
pel1.JPG (58.93 KiB) Viewed 29936 times
View from bottom... I may tweak air flow better.
===
My parts arrived yesterday so now I have it running on my kitchen counter finishing off a stew I just cooked.

I have it running wide open from the 12V ps which should be about 4.6A... So at a minimum it will put that 55W of heat into the food, + extract extra heat from the room.

As to plan, there is cool air coming out the bottom, and the hot plate is too hot to touch, I do not think it will burn the food whihc is pretty much done already... I'll leave it under power for a few more hours to see how hot it gets...

If it gets too hot, I will finish with the buck converter to drop the voltage to the Peltier device, and vary the fan...

But it has survived the first application of power with flying colors and a cool breeze.

:D :D :D :D

JR
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Re: looking for fan cooled heat sink.

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OK, after an overnight cooking test I am now smarter about Peltier effect... :roll:

My rig gets too hot to touch but does not put out enough heat to cook. It appears that I was over optimistic about how much heat this would pull from the room air, and miscalculated how much current the 12V supply would drive into it. When the hot side of the Peltier device gets hot the internal impedance rises, so my 12V supply was maybe putting only 3A into it, not 4.5A like I thought.

Another issue is max temperature, the Peltier data sheet specs a max hot side temp of 80'c so just hot enough to kill bacteria, but with that much temp differential it is not capable of pumping much heat from the environment.

I could stack two Peltier devices in series, this would reduce the temperature differential in half making them more effective at moving heat, and double the waste heat they develop (good for my app). But now I need a stronger PS. Instead of 12V/5A I need more like a 15V/10A supply.

At this point I am not enthusiastic about throwing another $100 at parts ($20 for another Peltier device and $90 for a PS), to get only 30W of room heat and some 125-150W of Peltier waste heat. I suspect the net result is to still heat up the room, just 20% less than using my stove.

[edit- bzzt no I can not stack these in series for heating, because the second Peltier device would need to pump the waste heat from the first device. Since the waste heat is more than the pumping heat capacity this is a non starter... never mind. /edit]

While it was fun to mess with this effect.

JR
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Re: looking for fan cooled heat sink.

Post by JR. »

OK, I have figured out why perhaps a Peltier device was not the optimal approach, but I am this far down the road, lets see how close I can get. At least we can all learn from my mistakes. :oops:

I just did another experiment, I figured if I could get the cold side hotter, the hot side would get hotter too... So I rigged up a way to capture the air flow and loop it though the heat exchanger. This way all the heat that I pump into the hot plate will stay in the system. I rigged up an inverted cardboard box set at the level to separate the cold side exhaust air from the fan inlet air. The exhaust air gets channeled up around the sauce pot, then a second larger cardboard box channels that exhaust air back down to the fan inlet on the bottom. It would be hipper if I could reverse the direction of the fan, but I looked and that is not practical.

This way the heat pumped into the system stays in the system...the air inside the box is allowed to heat up. It worked with some water in the sauce pot it got up to just before boiling, with small bubbles forming in the water.

I found a proper 15V/6A power supply on flea-bay for $13 so now I can put 25% more voltage into it and something like 50% more power.

Now I will need to finish my PWM temp controller since I am already pushing the envelop on how hot the hot side gets. The spec sheet says 80'C max and the heat transfer curves are shown at 50'C so at 80'+ the heat transfer will degrade but I am now using these as glorified heating elements. The solder inside the Peltier device melts at 137'C so I need to stay way below that. 80'C is hot enough to kill bacteria.Since I was just boiling water I was over 100' at the Peltier device.

If this keeps working I will make a proper air box out of rigid insulation. While I wouldn't be surprised if the Peltier device fails if I accidentally let it over-heat.
====
In hindsight I dusted off my old crock pot and it can boil water in the low heat setting so what I should have done (and may still) is switch that crock pot resistance element with an iso-triac to fine control the temperature and put that all inside an air box to keep the room air cool.

JR
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