[Noisebridge-discuss] N00b question - changing power supply voltage
jim
jim at well.com
Tue Jul 6 18:37:37 UTC 2010
i'd tho't 7812s were only available in 1 Amp
form; are there higher Amperages available for
78XX regulators?
i'm guessing you mean TO-3 rather than TO-2,
yes?
there's a variable voltage regulator, LM338,
that comes in a steel TO-3 package, will output
from about 5 to a little over 30 VDC, very good
regulation, and is rated at 5 Amps; i love 'em.
on the order of $5 each.
On Tue, 2010-07-06 at 11:07 -0700, Josh Myer wrote:
> 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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> --
> Josh Myer 650.248.3796
> josh at joshisanerd.com
>
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