[Noisebridge-discuss] N00b question - changing power supply voltage

Tue Jul 6 18:07:18 UTC 2010

7812s are pretty standard for this sort of thing, and you can get them in
beefy TO-2s, which take the silverpaste real good and have two places to
bolt them to a big heatsink.

That said, it's worth thinking through what jonfoote mentions about the
Carnot cycle.  That is: if you use linear regulators, you're dumping half of
your input power as heat right there, then dumping 90% of what gets through
as heat off the peltier device.

The exact math there is left an exercise for the reader.

(Hint: leave room in your enclosure for a little muffin tin, and consider
making some DIY versions of these:
http://www.amazon.com/Easy-Bake-Super-Pack-mixes/dp/B00005C32X )
--
/jbm

On Tue, Jul 6, 2010 at 10:57 AM, Corey McGuire <coreyfro at coreyfro.com>wrote:

> http://www.sparkfun.com/commerce/product_info.php?products_id=527 times
> 18?  That sounds like a bit much.  Maybe there's a better regulator.
>
>
> On Tue, Jul 6, 2010 at 10:14 AM, Corey McGuire <coreyfro at coreyfro.com>wrote:
>
>> This is the best back-of-napkin application I've seen for TEJ's, ever and
>> a 24v power supply is the perfect supply for it.
>>
>> There are two problems I see:
>>
>>    1. When you cycle them to maintain a temperature, the heat you moved
>>    to one side will quickly conduct back to the other.
>>    2. In this process, a charge will be created and sent down the wire.
>>
>> How you handle this is a mystery to me.
>>
>> Here's what I would do.  I would NOT wire them in series.  Instead, I
>> would implement switching power supplies and use them to keep the TEJ's
>> active at 16v when cooling or a lower voltage while maintaining the desired
>> temperature.  Then I would switch them relative to how far below the desired
>> temperature they are.  Any temp above desired, 16v; 1 degree below, 12v; 3
>> degrees below, 8v... or whatever.
>>
>> I would NOT overdrive them because I imagine they just get even LESS
>> efficient.
>>
>> 1 arduino
>> 1 temperature probe
>> a fist full of switching power supplies
>> a capacitor just to keep power going to the TEJ to help resist the heat
>> moving backward (I don't know if this is a problem, but it is a cheap
>> solution.)
>> whatever else (I am not even pretending to be an EE.)
>>
>> YMMV
>>
>>
>>
>>
>>
>>
>> On Tue, Jul 6, 2010 at 9:35 AM, Sean Cusack <sean.p.cusack at gmail.com>wrote:
>>
>>> Oh yes...I know they are terrible at efficiency...but they are also the
>>> only thing that I know of that can get you to sub-ambient temperatures
>>> without using a (comparatively) giant refrigeration system.
>>>
>>> I'm planning on using these to cool a few pieces of lab equipment.
>>> Typically, to get to sub ambient conditions, you have to use ice/water (gets
>>> you to 0C), or dry ice/acetone (gets you to -78C), or full on Liquid N2
>>> which gets you too cold for most practical applications. It would be
>>> *awesome* to hit like -20 or -10 or even 5C repeatedly and controllably for
>>> a million and one different chemical reactions.
>>>
>>> There is equipment that allows you to do this now, but pretty much its a
>>> standalone refrigeration system that pumps cooled silicon based oil through
>>> your reaction mixture. It takes up a ton of room on my bench, and since
>>> those refrigerators are on the order of $7k a pop, its tough to convince my
>>> boss to allow me to buy more than about 2 of them. In other words, longer
>>> hours for Sean in the lab = teh sux.
>>>
>>> So, I'm trying to use these doodads as a way to run a bunch of reactions
>>> at a controllably cold temperature. I agree there's problems, but given the
>>> application, it may just work!
>>>
>>> Sean
>>>
>>>
>>> On Tue, Jul 6, 2010 at 8:44 AM, Jonathan Foote <jtfoote at ieee.org> wrote:
>>>
>>>> On Tue, Jul 6, 2010 at 4:17 AM, Corey McGuire <coreyfro at coreyfro.com>
>>>> wrote:
>>>>
>>>> > Messy, messy stuff.  TEJ's are not efficient.  This is fine by
>>>> themselves.  When you stage them, their inefficiencies > become readily
>>>> apparent as they begin to compound.
>>>> >
>>>> > May I ask what you intend to do with them?
>>>>
>>>> Yeah, also curious. Corey is absolutely right: TEJs have terrible
>>>> Carnot efficiency --  way less than 10%. This means to move (not
>>>> remove) 5 watts of heat you have to put in 50+ watts of power, which
>>>> turns into heat you ALSO need to remove.
>>>>
>>>> So they are only useful in a few applications where the small temp
>>>> difference over a tiny scale is worth the waste. If they really were
>>>> the magic refrigerators people think they are, they would be in every
>>>> PC and laptop. And note that if you are trying to keep things cool,
>>>> there may be far better solutions.
>>>>
>>>> "In this house we obey the laws of thermodynamics!"
>>>>
>>>> -J
>>>> _______________________________________________
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>>>> Noisebridge-discuss at lists.noisebridge.net
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>>>>
>>>
>>>
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>>>
>>
>
>
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-- 
Josh Myer 650.248.3796
josh at joshisanerd.com
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