TO-3, yeah, typo. They'll go up to 1.5A, which is far more power than I've ever used =)<div><br></div><div>The LM338 looks great. I can't find them for less than $8, but that's still a really good deal. I needed exactly this for a project idea I had recently... maybe it's time to revisit that idea.</div>
<div>--</div><div>/jbm</div><div><br></div><div><br></div><div><br></div><div><div class="gmail_quote">On Tue, Jul 6, 2010 at 11:37 AM, jim <span dir="ltr"><<a href="mailto:jim@well.com">jim@well.com</a>></span> wrote:<br>
<blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex;"><br>
i'd tho't 7812s were only available in 1 Amp<br>
form; are there higher Amperages available for<br>
78XX regulators?<br>
i'm guessing you mean TO-3 rather than TO-2,<br>
yes?<br>
there's a variable voltage regulator, LM338,<br>
that comes in a steel TO-3 package, will output<br>
from about 5 to a little over 30 VDC, very good<br>
regulation, and is rated at 5 Amps; i love 'em.<br>
on the order of $5 each.<br>
<div class="im"><br>
<br>
On Tue, 2010-07-06 at 11:07 -0700, Josh Myer wrote:<br>
> 7812s are pretty standard for this sort of thing, and you can get them<br>
> in beefy TO-2s, which take the silverpaste real good and have two<br>
> places to bolt them to a big heatsink.<br>
><br>
><br>
> That said, it's worth thinking through what jonfoote mentions about<br>
> the Carnot cycle. That is: if you use linear regulators, you're<br>
> dumping half of your input power as heat right there, then dumping 90%<br>
> of what gets through as heat off the peltier device.<br>
><br>
><br>
> The exact math there is left an exercise for the reader.<br>
><br>
><br>
> (Hint: leave room in your enclosure for a little muffin tin, and<br>
> consider making some DIY versions of these:<br>
> <a href="http://www.amazon.com/Easy-Bake-Super-Pack-mixes/dp/B00005C32X" target="_blank">http://www.amazon.com/Easy-Bake-Super-Pack-mixes/dp/B00005C32X</a> )<br>
> --<br>
> /jbm<br>
><br>
> On Tue, Jul 6, 2010 at 10:57 AM, Corey McGuire <<a href="mailto:coreyfro@coreyfro.com">coreyfro@coreyfro.com</a>><br>
> wrote:<br>
> <a href="http://www.sparkfun.com/commerce/product_info.php?products_id=527" target="_blank">http://www.sparkfun.com/commerce/product_info.php?products_id=527</a> times 18? That sounds like a bit much. Maybe there's a better regulator.<br>
><br>
><br>
><br>
> On Tue, Jul 6, 2010 at 10:14 AM, Corey McGuire<br>
> <<a href="mailto:coreyfro@coreyfro.com">coreyfro@coreyfro.com</a>> wrote:<br>
> This is the best back-of-napkin application I've seen<br>
> for TEJ's, ever and a 24v power supply is the perfect<br>
> supply for it.<br>
><br>
> There are two problems I see:<br>
</div>> 1. When you cycle them to maintain a temperature,<br>
<div class="im">> the heat you moved to one side will quickly<br>
> conduct back to the other.<br>
</div>> 2. In this process, a charge will be created and<br>
<div><div></div><div class="h5">> sent down the wire.<br>
> How you handle this is a mystery to me.<br>
><br>
> Here's what I would do. I would NOT wire them in<br>
> series. Instead, I would implement switching power<br>
> supplies and use them to keep the TEJ's active at 16v<br>
> when cooling or a lower voltage while maintaining the<br>
> desired temperature. Then I would switch them<br>
> relative to how far below the desired temperature they<br>
> are. Any temp above desired, 16v; 1 degree below,<br>
> 12v; 3 degrees below, 8v... or whatever.<br>
><br>
> I would NOT overdrive them because I imagine they just<br>
> get even LESS efficient.<br>
><br>
> 1 arduino<br>
> 1 temperature probe<br>
> a fist full of switching power supplies<br>
> a capacitor just to keep power going to the TEJ to<br>
> help resist the heat moving backward (I don't know if<br>
> this is a problem, but it is a cheap solution.)<br>
> whatever else (I am not even pretending to be an EE.)<br>
><br>
> YMMV<br>
><br>
><br>
><br>
><br>
><br>
><br>
><br>
> On Tue, Jul 6, 2010 at 9:35 AM, Sean Cusack<br>
> <<a href="mailto:sean.p.cusack@gmail.com">sean.p.cusack@gmail.com</a>> wrote:<br>
> Oh yes...I know they are terrible at<br>
> efficiency...but they are also the only thing<br>
> that I know of that can get you to sub-ambient<br>
> temperatures without using a (comparatively)<br>
> giant refrigeration system.<br>
><br>
> I'm planning on using these to cool a few<br>
> pieces of lab equipment. Typically, to get to<br>
> sub ambient conditions, you have to use<br>
> ice/water (gets you to 0C), or dry ice/acetone<br>
> (gets you to -78C), or full on Liquid N2 which<br>
> gets you too cold for most practical<br>
> applications. It would be *awesome* to hit<br>
> like -20 or -10 or even 5C repeatedly and<br>
> controllably for a million and one different<br>
> chemical reactions.<br>
><br>
> There is equipment that allows you to do this<br>
> now, but pretty much its a standalone<br>
> refrigeration system that pumps cooled silicon<br>
> based oil through your reaction mixture. It<br>
> takes up a ton of room on my bench, and since<br>
> those refrigerators are on the order of $7k a<br>
> pop, its tough to convince my boss to allow me<br>
> to buy more than about 2 of them. In other<br>
> words, longer hours for Sean in the lab = teh<br>
> sux.<br>
><br>
> So, I'm trying to use these doodads as a way<br>
> to run a bunch of reactions at a controllably<br>
> cold temperature. I agree there's problems,<br>
> but given the application, it may just work!<br>
><br>
> Sean<br>
><br>
><br>
><br>
> On Tue, Jul 6, 2010 at 8:44 AM, Jonathan Foote<br>
> <<a href="mailto:jtfoote@ieee.org">jtfoote@ieee.org</a>> wrote:<br>
> On Tue, Jul 6, 2010 at 4:17 AM, Corey<br>
> McGuire <<a href="mailto:coreyfro@coreyfro.com">coreyfro@coreyfro.com</a>> wrote:<br>
><br>
> > Messy, messy stuff. TEJ's are not<br>
> efficient. This is fine by<br>
> themselves. When you stage them,<br>
> their inefficiencies > become readily<br>
> apparent as they begin to compound.<br>
> ><br>
> > May I ask what you intend to do with<br>
> them?<br>
><br>
><br>
> Yeah, also curious. Corey is<br>
> absolutely right: TEJs have terrible<br>
> Carnot efficiency -- way less than<br>
> 10%. This means to move (not<br>
> remove) 5 watts of heat you have to<br>
> put in 50+ watts of power, which<br>
> turns into heat you ALSO need to<br>
> remove.<br>
><br>
> So they are only useful in a few<br>
> applications where the small temp<br>
> difference over a tiny scale is worth<br>
> the waste. If they really were<br>
> the magic refrigerators people think<br>
> they are, they would be in every<br>
> PC and laptop. And note that if you<br>
> are trying to keep things cool,<br>
> there may be far better solutions.<br>
><br>
> "In this house we obey the laws of<br>
> thermodynamics!"<br>
><br>
> -J<br>
><br>
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> Josh Myer 650.248.3796<br>
> <a href="mailto:josh@joshisanerd.com">josh@joshisanerd.com</a><br>
><br>
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</div></div></blockquote></div><br><br clear="all"><br>-- <br>Josh Myer 650.248.3796<br> <a href="mailto:josh@joshisanerd.com">josh@joshisanerd.com</a><br>
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