Author Topic: A look at Peltier cooling  (Read 109059 times)

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Bruce S

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Re: A look at Peltier cooling
« Reply #99 on: November 04, 2014, 03:30:44 PM »
MB
I had heard of doing something like this, BUT as things do, work got in the way :(.
I tried stacking same sized units but you could only cool just so much, seemed like being the same size was wrong.
Might be time to re-ignite those O-2its :).
 
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Mary B

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Re: A look at Peltier cooling
« Reply #100 on: November 05, 2014, 01:38:00 AM »
The PDF I linked further back covers stacking. Well worth the read.

Nautilus1

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Re: A look at Peltier cooling
« Reply #101 on: November 07, 2014, 04:28:07 PM »
Steps one and two already done.

Covered the outer backside of the box in glued Styrofoam plates, 2 layers in some places and 1 layer where more convenient. This is to complete the inner insulation (3 layers on this side).

Covered the Styrofoam in a radiant barrier made from Aluband (thick gauge aluminum tape with sticky bitumen-rubber backing, used to insulate roofs). This is to reject radiated heat from the environment (back of the cabinet is towards the brick wall).

Strips of Aluband were matched edge to edge and not overlaid, to avoid trapping of humidity behind them. The layers were cut and molded around the heatsink to allow free exhaust of the cooling air.



Step three, the heatsink enlargement, is to be done if it can't cool enough in warm weather.

12V power brick is to be held away from the box, since it gets pretty warm in operation and all heat creep has to be avoided.

Bruce S

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Re: A look at Peltier cooling
« Reply #102 on: November 07, 2014, 04:49:14 PM »
Nautilus1  I have a complete unit scrapped working from a TEC someone in sales didn't want anymore. If you want it send me a PM and I'll mail it to you. No charge.

(Attachment Link)

If  Nautilus1 isn't interested in it .. i think i would be ...

And if kc7noa isn't I know I am  ;).
I have a nice little SFF Dell that bit the dust , I'm currently rebuilding it for desktop use and think I can use it to power these items as a good test base.
Cheers
Bruce S
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dnix71

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Re: A look at Peltier cooling
« Reply #103 on: November 07, 2014, 06:01:31 PM »
Bruce kc7noa got in the mail yesterday. The mechanic at work gets 'broken' cube fridges on a regular basis as sales people come and go. If I get another, I'll post it.

kc7noa

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Re: A look at Peltier cooling
« Reply #104 on: November 08, 2014, 12:26:05 AM »
Yep ... i thank you ...

I have an icebox up in the sticks im planning on putting this into .... along with a replacement heatsinks ....

the one for the hot side will be 3lbs ..... i used to work in an plant that did mostly aluminium rework ... ie , melted down couple tons a shift and reformed it ...
and one in a while there would be heatsinks that came through ... that i would buy some times -- wish i had bought alot more when i had the chance !!!!!!

its been over ten years since i worked in that foundry .....

ahhh .. the fond memories .... cept for the average death rate there was one person killed every year ..... YIKES !!!!!!

joestue

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Re: A look at Peltier cooling
« Reply #105 on: November 08, 2014, 08:48:35 PM »
peltier coolers have the problem of conducting heat even when they are turned off.

if the cold side were to be placed on top of the fridge, but instead of solid copper a heat pipe moved the heat vertically upwards from the fridge to the peltier, then when the TEC is off, it could warm up well in excess of the fridge internals, (provided there is no internal wick to pump the heat fluid up the pipe) then you could realize the convenience of a larger TEC, without the efficiency loss of conducting heat backwards.
yes, you can run a larger TEC at a lower current flow, but that isn't efficient either.

as it is now, you have to carefully size the TEC so that it runs 24/7 to get the best efficiency, and keep the fridge stocked with enough cold storage to absorb the daily load.
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Nautilus1

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Re: A look at Peltier cooling
« Reply #106 on: November 10, 2014, 02:49:35 AM »
joestue:

1. There wasn't enough space at the upper part of the cabinet to fit a heatsink / copper conductor / heat pipe and even if it was, it looked ugly.

2. The cold heatsink heats itself above room temps when current is turned off - the heat from the warm heatsink flows back.

3. The warm heatsink could not be run efficiently without a fan. In 13-14°C room temps, it heated quickly in the 50°C range and more without fan. The fan held it in the 20s.

4. Found out the shape of the GT440 heatsink gave warm spots (in the 29-30°C range) where there are less fins and the airflow was poor (right side, when you look at the back of the fridge).

Had some VRAM / MOSFET small heatsinks left over from an Arctic Cooling kit, fitted them with their thermal glue to the warm spots. They are 1.56 sq cm per fin side (15.6 sq cm per heatsink), plus a small one of 6 sq cm. That is, ~68 sq cm (10.54 sq inch) had been added to the total heatsink surface. The temps fell to normal throughout the heatsink.

Bruce S

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Re: A look at Peltier cooling
« Reply #107 on: November 10, 2014, 11:03:52 AM »
Bruce kc7noa got in the mail yesterday. The mechanic at work gets 'broken' cube fridges on a regular basis as sales people come and go. If I get another, I'll post it.
Thanks for the update.
Of the "Chef-mate" and other similar units, The only two thing I've found to be a problem with them is a blown fuse on the circuit board (usually hidden in shrink wrap, one is easy to find , the other is buried; they are of the 5A 250Vac verity) or the fans don't work.
I have only had one unit with a worn out TEC chip. The better units ( ones with dual external fans on the hot side) are able to keep the insides to 38F with the dial set to max cold.
Cheers
Bruce S

   
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MattM

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Re: A look at Peltier cooling
« Reply #108 on: November 10, 2014, 05:28:27 PM »
Heat spreaders on a laptop typically use 8 ounce/ft^2 or less.  Typical sheet metal is 16 ounce these days.  You only need a strip as wide as your biggest peltier unit.  If you bond aluminum heats inks to it use a nonmetallic and nonacidic adhesive.  JB Weld and epoxies just do not hold up.

Stainless

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Re: A look at Peltier cooling
« Reply #109 on: February 24, 2015, 12:59:08 AM »
I'd like to keep this interesting topic alive, so here's some additional information that I've learned about peltier units for a project of mine.

I am converting a cargo trailer to a small camper. I have an ancient Dometic RM2190 1.7 cubic foot propane/DC fridge that doesn't work very well on propane (and I don't want to cut the enormous holes that are required for venting anyway), so I'm going to gut the propane system and attempt to see how well I can make it work with a modulated voltage, low-current peltier setup.

This video (https://www.youtube.com/watch?v=JLmELheibss) suggests that a typical 12703 TEC chip is most efficient at around 9.5 volts and just under 2 amps. I read elsewhere that TECs can function properly using unfiltered PWM, provided that the frequency is above 3 kHz (I lost that link, unfortunately). So, those will be my starting points.

I'll be using an ATTiny85 as the controller platform, and a logic-level MOSFET to drive the TEC. I am going to try a closed-loop liquid CPU cooler on the TEC's hot side (http://www.memoryexpress.com/Products/MX48039), and I will plumb the hot exhaust air into the trailer's existing water heater ducting to the outside. On the cold side, I'm going to start with a passive heat sink with a fan blowing through it to circulate the cool air inside.

The pseudo code looks like this -- when the fridge's internal temperature > a setpoint, the TEC runs at 9.5 volts. When the temperature =< setpoint, the TEC throttles down to "maintenance mode" and the hot side fan slows down. I've added a switched light to the fridge, and I may wire that into the ATTiny as well to trigger a higher voltage whenever the door opens... it might be a little faster on the recovery than waiting for the temp sensor to figure out that the temperature has risen.

The TECs and other electronic bits are still on a slow boat from China, so I haven't fired up the soldering iron just yet, but that's the plan. We'll see if it works at all.

joestue

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Re: A look at Peltier cooling
« Reply #110 on: February 24, 2015, 02:10:53 AM »
The most efficient point is relative to what you are trying to do, that number of 9.5 volts and 2 amps sounds about right, you'd probably be pushing half the maximum heat flow, at about 30C temperature difference. yes the maximum is theoretically 60C but that's at zero heat flow and twice the amps.. waste heat being proportional to amps squared.

at some reasonable frequency the inductance of the wiring between the switch and the  TEC will average the current out, i'd expect it to be a bit higher than 3Khz but you shouldn't run into any switching losses until you get to 50 or more. at which point we might start picking pennies up off the sidewalk.

i made some comments earlier in this thread if i recall correctly that i think there is a significant energy cost improvment available if you use heat pipes correctly such that when the TEC is off, no heat is able to flow back into the minifridge by taking advantage of gravity here to keep the heat fluid at the lowest point within the heat pipe. Might not be much for a minifridge but for something larger it could be significant.
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Stainless

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Re: A look at Peltier cooling
« Reply #111 on: February 24, 2015, 12:39:11 PM »
Let's do some vague cocktail-napkin math to see what I might expect.

Assume an ambient trailer temperature of 25 C on a warm day. The CPU cooler that I intend to try is tested to maintain an 85 W load at 7.1 C over ambient (http://www.frostytech.com/articleview.cfm?articleid=2744&page=5), which seems similar enough for the sake of argument to what I'll be dealing with.

So, I can hope that the TEC's hot side hovers around the 32.5 - 33 C range when running at 9.5 volts. You've suggested a delta of around 30 C, which agrees with the video that I posted above, where the tester reached a 31.2 C delta before getting all nuts with the addition of frozen liquid insulation around the temp probe. So, I might see a TEC cold side that is ~ 0 C, and that will likely translate into a disappointing result in the fridge's actual internal temperature.

In practical reality, the 25 C trailer ambient is likely to be infrequent and short-lived (I camp in the mountains, and even mid-summer days start and end quite cool), so there might be some room for optimism in real world usage.

I don't expect to build an actual, meat-safe fridge here, but it will be fun to monkey with the thing to see how far I can take it. If I wind up with what's effectively a dry cooler, that's probably good enough for my purposes.

Mary B

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Re: A look at Peltier cooling
« Reply #112 on: February 24, 2015, 07:40:09 PM »
Look into stacking TEC's you can achieve much higher temperature differentials. But it is at the cost of double the current draw!

MattM

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Re: A look at Peltier cooling
« Reply #113 on: February 24, 2015, 10:26:44 PM »
Insulation is key to maintaining the cold.  If a warm room makes the cooler likewise warm, it may have less to do with the TEC and much more to do with exclusion of the heat from the cooler.

Bruce S

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Re: A look at Peltier cooling
« Reply #114 on: February 25, 2015, 08:46:17 AM »
I will certainly be keen to see how well the water cooled unit helps in internal cool down once the door has been open.
That seems to be one of the biggest drawbacks of the TEC fridges.
I have been amassing heat pipes from older HP Laptops for a try on a unit given to me with a burnt controller board , so your ATTiny85 project is also if interest  :).

I do like the fact the water cooler's total max consumption is only ~8 watts.

Keep us posted!
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Stainless

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Re: A look at Peltier cooling
« Reply #115 on: February 25, 2015, 12:09:39 PM »
"Bang for the buck" efficiency is fairly important in this project, as I only have a middle-aged 100 AH deep cycle lead acid battery supplying the trailer, and about 30 W of solar. I'm starting my reading on a LiFePo4 pack project, but that won't materialize until at least next summer.

So, TEC stacking isn't likely to happen. The trailer draws about 4 A @ 12 V with everything on, and I can only afford to spend about another 3 A on the "fridge." That's why I was also pleasantly surprised to see the ~ 8 W rating on the CPU cooler.

As for the insulation... this was a pretty nice little RV fridge back in 1990, so the walls are quite thick and the door seals well. However, it's rattled around a lot over 25 years, so there are a couple areas on the back that could use some attention. Since I'm pulling all the propane workings anyway, it shouldn't be a huge problem. But yes, I will be putting time toward ensuring good overall insulation.

The electronics should start to arrive in the next couple days (although the TECs themselves are still at least two weeks out), so I'll post updates as things progress.

Stainless

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Re: A look at Peltier cooling
« Reply #116 on: February 28, 2015, 01:19:15 PM »
Well, everything but the TECs has arrived. I've written the 0.01 version of the Arduino code, and tested it with a breadboard, some LEDs and an Arduino Uno on my bench (kitchen table).

So far, everything works well. I did decide to include the fridge light switch in the mix, and the TEC voltage rises whenever the door opens. I also decided to modulate the fridge's internal fan speed depending on the voltage supplied to the TEC, because it's probably wasteful to give the fan a full 12 V when the TEC is barely above idle.

This is my first meaningful circuit-level electronics project (and I am certainly not "classically trained" in programming or electronics engineering), so progress might not be as quick as I'd like. I bought multiples of all components, expecting that I'll release the smoke from a couple here and there, but it's been relatively smooth so far.

I just uploaded the program code to the ATTiny for the first time, and found that the PWM frequency adjustment code that worked on the Uno won't fly with the A85, so I guess I'll be doing some more reading this afternoon.

DamonHD

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Re: A look at Peltier cooling
« Reply #117 on: February 28, 2015, 02:50:01 PM »
Good for you!

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Stainless

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Re: A look at Peltier cooling
« Reply #118 on: March 06, 2015, 11:38:26 PM »
Well, the TECs arrived. They are model 12706, and I've been screwing with them in the garage this evening over a couple tall, cool glasses of milk.

I'm seeing a 30 C delta at 8 V and 2 A. And, that's just with the TEC's hot side resting on a cheap, salvaged heat pipe that's too small (with no thermal paste), and an old ~ 15 CFM muffin fan pointed at the pipe's cooling fins. Seems to be pretty easy to get a usable delta from these with minimal power input. I'm looking forward to trying the liquid cooler, which I intend to buy this weekend.

I also experimented to sniff out the optimal "maintenance mode" current and voltage by seeing how low I could go on the power supply while still seeing a delta between the hot and cold sides. At 200 mA and roughly 1 V, I saw a delta of 2 C.

Does anyone know if that's actually a reasonable method for hunting down the minimum power required to prevent heat "backflow?" Intuitively, it seems right, but I'm flying in dim light over here.

The ATTiny85 control prototype is complete and tested on a breadboard, and I'll be starting on the "real" protoboard version this weekend as well.

Stainless

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Re: A look at Peltier cooling
« Reply #119 on: March 11, 2015, 11:45:10 PM »
Alright, it appears that nobody is rapt with interest in this project. So, this will be my last update until it's finished.

I bought the liquid CPU cooler and did some more testing. All of the following measurements were at an ambient temperature of 19 C.

With the CPU cooler installed and no fan on the cooler's little radiator, I saw a TEC cold side measurement of -16 C (the radiator was barely warm to the touch). With the fan running on the cooler's radiator, I saw -21 C. So, a maximum delta of 40 C with the TEC running at 7.5 V and 2 A. Better than I expected.

Just for the hell of it, I tested the current draw from the cooler's liquid pump and found that it is 300 mA at 12 V. I won't be using the stock 120 mm fan because it feels pretty cheap, but I'll likely be looking at around the stated 8 W total cooler draw. So, I am on track for a ~ 3 A maximum draw from the entire "fridge" when running at full power.

Everybody likes pictures, so here are a few.


The TEC with thermo paste sitting atop the liquid cooler. I was concerned that the TEC might be too big, but there's piles of room.


The power supply during testing. It was running at 95% of its maximum, so I could only run for 10 minutes at a time before letting the poor thing cool down.


The lowest cold side reading that I was able to attain.


The fridge's original rear propane setup.


Removed all in one piece. In theory, I could put it back to original pretty easily.


Gutted. The rear insulation is 3 1/2-inches thick, and all other sides are 2-inches. Not too bad. The opening for the original propane cooling "arm" entry is going to work nicely for the TEC and the internal fan that I have planned.
« Last Edit: March 11, 2015, 11:50:11 PM by Stainless »

Bruce S

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Re: A look at Peltier cooling
« Reply #120 on: March 12, 2015, 10:06:44 AM »
Actually , I've waited with baited breath for these next installments!

I'll grab a few pics of the unit I'm working on and after seeing the numbers from the liquid cooler (cool master) , I'll be gathering the items in quick order.
We have a Micro-Center where I can purchase the cooler right off the shelf  :).
The Arduino programming is the important part as NO matter what I do the the controller on this unit, it simply will not work  :(.
I know the TEC works, I tested it directly.
SOooo I'm really interested in this project fur sure!!
Just the thing called work keeps getting in the way  :o
Cheers
Bruce S
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keithturtle

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Re: A look at Peltier cooling
« Reply #121 on: March 14, 2015, 01:57:02 AM »

Just the thing called work keeps getting in the way 


Just be glad you still have work <grin> Without it we couldn't support our habits.

Good to see the developments on this, Stainless

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kc7noa

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Re: A look at Peltier cooling
« Reply #122 on: March 14, 2015, 07:11:41 AM »
Iv been watching .... i kinda liked the heat pipe idea ...

Bruce S

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Re: A look at Peltier cooling
« Reply #123 on: March 14, 2015, 02:30:40 PM »
Here is the SS fridge that will be my test bed.

As you can see, there is this huge Aluminum heatsink followed by two 12Vdc 1.25A fans.
To the left is the controller that is shot.
The TEC is okay, I can attach an old computer PSU and get both cold/hot on it using 12Vdc.
 
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Stainless

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Re: A look at Peltier cooling
« Reply #124 on: March 17, 2015, 12:29:42 AM »
OK, fair enough... I guess there is some interest. I don't want to sound like a pouty forum baby, but I also don't want to waste my time and this forum's bandwidth writing longwinded posts if nobody's reading.

Since it appears that a few of you are reading, here's a longwinded post.

I finished making the protoboard version of the controller. I didn't think that this was the right time to learn how to etch traces, so I just used jumper wires. It's not THE most elegant thing in the world, but it worked on the first try. I was pretty chuffed, I must admit.

So, I've been trying different PWM duty cycles to see what the TEC likes versus an acceptable current draw. My power supply didn't like running the protoboard... as soon as the TEC's high duty cycle kicked in, the voltage dropped to about 6 VDC. That seems strange, because it ran the bare TEC just fine. I assume that it has to do with the PWM aspect that I've now added to the mix.

I lugged the real RV battery over to the bench and fired everything up. I managed to get my 40 C delta pretty easily after a couple reflashes of the ATTiny to nail down the duty cycle.

Here are some pictures. Be gentle.


The completed protoboard. I used a little buck converter that I had laying around to feed the ATTiny its 5 VDC, and fed the raw 12 VDC into the MOSFETs.

The blue wire heading north out of the picture is the temp sensor lead that I'll mount inside the fridge. The two small bare wires will be the door switch, but I'm just touching them together at this point to test the functionality (which works). The temp sensor and door switch are connected to the protoboard using recycled computer motherboard fan and LED plugs.

I used mini Dean's connectors from my local hobby shop as the TEC, fan and light connections. I knew that the 20 years of money I've thrown at R/C cars would come in handy eventually.


The test setup with RV battery. I won't lie to you; it wasn't pretty, but it worked.

To the right of the RV battery lies the hairdryer that I've been using to simulate a rising temperature on the temp sensor. If I had a wife, she'd kill me.

As you can see on the multimeter, the total system current draw at high duty cycle, including the liquid cooler, the TEC and both fans is 3.076 A. It continues to drop very slowly as the TEC attains its maximum delta, stopping at about 3.05 A. When I put my palm on the TEC to give it a heat load, current draw rises to about 3.15 A.

The fans have some pretty noticeable PWM whine, which will be as annoying as hell in the little trailer. I hoped that I'd eliminate that by increasing the PWM frequency to 3.8 kHz, but it doesn't seem to have worked. I don't have an oscilloscope to verify the frequency, but I'll keep testing to make sure that I used the right Arduino code for the frequency that I want.

After running this test setup for about 45 minutes, there were no surprises. The MOSFETs didn't warm up at all, the CPU cooler didn't heat soak in the slightest, and the current draw stayed consistent in both high-duty and maintenance modes.

So far, so good.



Stainless

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Re: A look at Peltier cooling
« Reply #125 on: March 17, 2015, 01:13:28 AM »
Here is the SS fridge that will be my test bed.
(Attachment Link)
As you can see, there is this huge Aluminum heatsink followed by two 12Vdc 1.25A fans.
To the left is the controller that is shot.
The TEC is okay, I can attach an old computer PSU and get both cold/hot on it using 12Vdc.
 

So far, I definitely recommend the CPU cooler if you're willing to throw ~$50 at a test project like this. Do you intend to run your fridge off mains power, or are you hoping to put it into an RV or something as well?

I tested the liquid cooler with the TEC running at about 90% (11 VDC, roughly 4.75A) and saw some pretty nice deltas. -34 C was the lowest TEC cold side temperature that I measured at a room ambient of 19 C. Even at that level, the cooler's radiator was barely warm after perhaps ten or fifteen minutes.

If you're not as concerned about current draw as I am, this cooler seems to eat up all the hot side energy that you can throw at it.

Bruce S

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Re: A look at Peltier cooling
« Reply #126 on: March 17, 2015, 09:14:18 AM »
For me ?
Actually both if or when possible.
Since I had to explain what was on my screen to my captain, she's bringing in a 2nd broken SS based fridge for me to have :-).
I have a small-ish bar in our basement and with the 100-odd watts of solar, I plan on running one as a hydration station  :o.
The other ( the smaller of the two) in our Grand Caravan's back area for camping (destination Arizona this year) as a possible chest fridge.
I've built PWM circuits for the 120mm 12Vdc fans before, they also had a noticeable whine to them, didn't bother me after say 10mins, but it could've been the satisfaction factor of NOT throwing away useable stuff.

Cheers!
Bruce S

 
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OperaHouse

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Re: A look at Peltier cooling
« Reply #127 on: March 22, 2015, 04:33:13 PM »
Just want to say I've been following this too.  Are you driving the fans directly with PWM output?   OR using a small inductor with diode and capacitor as in a buck converter?  Nice to see someone using a micro here.

Stainless

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Re: A look at Peltier cooling
« Reply #128 on: March 23, 2015, 01:24:42 PM »
Just want to say I've been following this too.  Are you driving the fans directly with PWM output?   OR using a small inductor with diode and capacitor as in a buck converter?  Nice to see someone using a micro here.

Yes, I'm driving the fans directly with PWM output (through a single MOSFET for both fans). I didn't bother smoothing the output because I hoped that the high-ish frequency would make it a non-issue. I haven't done anything since my last post to try to solve the whine, but I may have to add some smoothing in there if it continues to bug me.

I added a large recycled aluminum CPU heatsink and fan to the TEC's cold side this weekend and tested for actual air temperature that the TEC could produce. The results were not good... +12 C was the lowest air temp that I measured with a system draw of 3.35 A.

It seems pretty clear that the TEC running on restricted power doesn't have the "oomph" to handle a large heat load like an 80 mm x 80 mm finned block of aluminum. So, I'm going to pick up a much smaller heat sink from the local electronics store after work today and see if that makes a difference.

kc7noa

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Re: A look at Peltier cooling
« Reply #129 on: March 26, 2015, 02:28:13 PM »
I would of thought the larger mass of heatsink would help reduce fluctuations and help cool down any thing inside ....




Bruce S

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Re: A look at Peltier cooling
« Reply #130 on: March 27, 2015, 09:16:53 AM »
I would of thought the larger mass of heatsink would help reduce fluctuations and help cool down any thing inside ....
It will, eventually,,, the issue is the amount of time the TEC will take to first chill the Al then chill the air and surrounding items.
At least that is my way of thinking  :o
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Mary B

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Re: A look at Peltier cooling
« Reply #131 on: March 27, 2015, 06:24:30 PM »
That is the problem with TEC cooling, they are slow to cool any mass. Keep the big heatsink and be patient and wait for it to get cold. It will help moderate temps in the fridge as you add warm items.