Peltier question.

Quick question about peltiers.

I have a small polystyrene box (250mm x 200 x 250 tall) that I want to keep at aroud 8 deg C to keep earthworms alive in 'tupperware' and a bit of water, for up to three weeks for my axolotl. I know the worms will survive at that temp for that time and even empty out their guts which is an advantage.

I'm going to mount a 150 x 80 x 25mm fins aluminium heatsink in the lid, cut out a square of the lid big enough to fit a 40 x 40 peltier

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and the base of a CPU heatpipe heatsink (an all copper Thermaltake Mini Typhoon) with 90mm fan sucking up rather than blowing down. I have a few 40 x 40 x 5mm nickel plated copper spacers (scavenged from the bottom of AMD Athlon aluminium CPU heatsinks) which I can use to shim up to the thickness of the 20mm thick poly.

I have a 12v 5A power supply that came with an early LCD computer monitor and will control temperatues with one of these;

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I ordered three of the peltiers. My question is should I just use one peltier or would it be more efficient to stack two or more? I intend to use an adjustable LED driver to control the current to the pelteir/s. (I have more than one LED driver available.) I'd rather get input before I assemble it than have to modify it afterwards. My concerns are ability to maintain the required temperature in ~25 degree ambient and power consumption.

As the box will only be opened once a day I doubt I'll need to drive the peltier/s to the advertised amount. I have alternate, more powerful PSUs available of needed and thermal interface goop. Input appreciated, I've never used peltiers before.

Happy holidays etc etc.

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Shaun. 

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~misfit~
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With my limited experimentation using the same device and similar manner to keep a polystyrene box cold I found that a single 12706 wasn't very efficient at cooling and using more of them would have raised current consumption to the point where it wasn't viable.

I also used a CPU cooler and fan drawing through the fins rather than blowing and I found that to be more efficient. I also attached a smaller heatsink and fan on the cold side with the fan running at low speed (small 12V fan running at 5V) just to move the air around as I found that the peltier would get ice cold and freeze which reduced overall thermal transfer efficiency.

It may be better to use a larger cold plate on the inside of the box but my experimentation didn't get that far.

You can probably get it to cool down to 8C (from memory I achieved 11C) over a longer period of time, but I abandoned the project. Polystyrene boxes are usually available for free from some chemists so you may need to get a few and experiment.

You could even extend the project with an arduino to monitor the temperature and regulate the Peltier/fans automatically (MOSFET PWM controlled by the arduino to drive the Peltier/fans for example)

Reply to
Clocky

Thanks for the info Clocky, that helps quite a lot, I had zero info before. ;-)

I thought of a small interior fan but thought that, with the lavck of efficiency of peltiers in general having a fan producing even a small amount of heat inside the box wouldn't be a good idea. My 'cold plate' is quite large as I said above, 15cm x 8cm with 2.5cm fins 1cm apart, a bit big to 'fan'. I was planning to mount it on the side so that the air would cool and flow down through the fins (a reverse thermal siphon thingy) but it's going to be so much easier to mount everything in the lid.

I hadn't thought about chemists for the box. I've got them from the fish counter at the supermarket before but they're all too large for this project. Thanks, I'll ask my chemist when I go next (I have to go every 10 days for my morphine ...) but hopefully I'll do it once and do it right.

Even if I don't get it down to my desired 8C anything below ambient will be an improvement over my current method of storing live 'nightcrawlers'. (Polybox with no cooling, swapping the water for chilled water when I open it.) I tried the fridge for a while but it's too cold. I'll try it with a single 12706.

Thanks again,

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Shaun. 

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~misfit~

Peltiers are a bit of a pain as they don't like water but water condenses onto cold things. The "sealed" ones aren't really.

You might do better to put an insulated box (I suggest a $7 wide-mouth ALDI thermos flask) in the fridge (which is at maybe +4 deg C) and then heat the inside of the insulated box by 4 degrees with a resistor or length of resistance wire wound onto a convenient heat spreader. If the box is well insulated, a heated box in an existing fridge might even be more energy-efficient, as peltiers are not very efficient compared to a normal fridge and they are thermally conductive so they thermally short-circuit your insulated box to some extent. You could use a standard off-the-shelf PID temperature controller and thermocouple to control the heater. Please tune it without worms. (Reminds me of scratch-monkeys.)

I wish someone made a vacuum flask with a peltier built into the vacuum space beteween the walls. That would keep the water out of the peltier. I appreciate there would be two difficulties with this: one they bake the vacuum flasks very hot during evacuation and the peltier would not withstand that, and two, thermal expansion would probably stress the peltier if attached firmly to both inner and outer walls. I think the second problem could be solved by using a copper water block soldered to the hot side of the peltier with thin copper water pipes connecting the water block to the outside world, and the first problem might be solved by baking the stainless parts in a vacuum before installing the peltier, not after.

Reply to
Chris Jones

I think you'd like the performance of the 12715 for what it is worth. Very much better to the 12706/s I have on hand. A simple a test as between two fingers and hooked up to a 4.2 volt 18650 tells the tale the better of the two types.

Reply to
Wayne Chirnside

Thanks for the input. :)

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Shaun. 

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~misfit~

Thanks, I've ordered a couple. Probably be February before they arrive though.

I never test with an 18650 as the cells I have around can dump a shitload of amps into something fast. I tested my 12706 'collection' (I've bought a few but never used one yet) with an Eneloop AA cell and that's plenty of juice to find which is working and to find the hot and cold sides (I know it changes with polarity).

Maybe the 12715 simply has less internal resistance and so seems far better with your 18650 test? As the sites I use to buy are hard to decipher I haven't found anything about comparisons between the two types ...

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Shaun. 

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~misfit~

Ok so youtube tells me that; TEC1-12706 means:

TEC = Thermoelectric Cooler

1 = One layer 127 = 127 thermocouples 06 = 6 amps

So according to that the 12715 is the 15 amp version.

I would have thought the '12' part signified voltage. Perhaps more research required ...

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Shaun. 

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~misfit~

I ran the 12706 at 14.5V. (The power supply limiting current to 5A max)

The cold side generally has the writing on it when the polarity is matched to the wiring (red-positive).

Reply to
Clocky

Cheers for that mate, all info gratefully received. I may wait for the

12715s to arrive as (I assume) I can always use less current than they're capable of handling but crank then up if I need to.

If it starts using too much power there's a small fridge in the shed that I'm storing for a friend. I guess I can always reversably mod that to be run from a digital controller and at a higher temp than designed for. The problem is finding somewhere to put it where I can access it regularly though.

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Shaun. 

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~misfit~

One additional thing, the power supply must not have any ripple as that acts as a reversing voltage and impairs efficiency. Beefy linear supplies beat switchers in this case unless the switching supplies output is well filtered ( clamped below the ripple)

Reply to
Wayne Chirnside

Thanks Wayne. I don't have any beefy linear 12v supplies, only SMPS. Would it pay to put a big cap or two and maybe an inductor on the output? (I'm a relative newbie to electronics.) I have an Hitachi 50v / 10,000uF electro that I've been looking for a use for. ;-)

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Shaun. 

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~misfit~

TEC1-12706-12V-6A-

want/32517842372.html?

thermometer-

Try a capacitance multiplier. Regulate the output of the switcher throwing away the last 150 millivolts or so to loss, barring that yes large capacitance will help however what that does to the switching power supply is another matter.

I use switchers myself with a linear regulator afterwards taking the heat loss at the power supply rather than the peltier.

Reply to
Wayne Chirnside

I think a decent switcher would be good enough - as its ripple would be below a couple of percent. An unregulated linear supply would not be great, and PWMing the peltier to control the temperature would be awful (and against the advice in peltier app notes), though I have seen people do it (in spite of my advice) and it worked well enough and long enough for them.

Reply to
Chris Jones

Also beware that some fridge motors can't re-start when there is still pressure at the outlet of the compressor, so the motor stalls and overheats if it is switched off for a short period and then back on. i.e. if you switch off the motor, you have to leave it off for a guaranteed minimum time before turning it on again.

To deal with short power outages, it would be good to keep the motor off for a while after power is applied to the controller also.

Reply to
Chris Jones

Can be good enough however for my design I can well afford 4 watts heat at the linear regulator to gain the desired sub freezing temperature at the box without multi-staging peltiers.

Temperature control here is a $2 aliexpress temperature control module with hysteresis and led display.

Reply to
Wayne Chirnside

Thanks. The controllers I use have adjustable hysteresis and if I go this route I'd set it quite high. As an ex automotive engineer and general dabbler I'm aware that too many short duration stop / start cycles are bad for almost everything. ;-)

To achieve this I could add an extra layer of control with a delay timer but considering I've only been getting a couple outages a year (if that) I doubt that it's needed.

Thanks for the replies.

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Shaun. 

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~misfit~

I mentioned that I'm not a skilled electronics engineer yes? ;-) I dare say that, with some research and parts purchases I could add a another layer of regulation but it's not something I could do easilly.

Unless adding one of these

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and more smoothing capacitance to a laptop power supply would do the job? (I bought half a dozen of them a while back as I have a few 16v - 21v laptop PSUs spare and often need a high amp 12v supply.)

Cheers,

--
Shaun. 

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~misfit~

So far my only personal implementation of peltiers has been to cool the tip of a curious finger, however I noticed in this teardown video (at around 5min) a comment relating to "stacking" Peltiers:

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Basically because the lower peltier has to cope with the heat of that above, it has to be bigger or have more elements than the one at the top. Or you could transfer the heat from the top one to two others below, in parallel as it were. There are probably better references for this elsewhere online.

In your case, I'd be inclined to just use all three (or as many as you can power) on their own, one layer deep.

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Reply to
Computer Nerd Kev

Thanks Kev. I'm subscibed to Mikes channel but haven't investigated it to any extent yet.

Now that you mention it I recall seeing stacks of maybe five peltiers of increasing size on an overclocking site years ago. For the sake of this experiment I'm just going to use one of the high rated devices (when it arrives - and then I get my arse into gear ).

Cheers,

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Shaun. 

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~misfit~

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