[Noisebridge-discuss] N00b question - changing power supply voltage
Tymm Twillman
tymmothy at gmail.com
Wed Jul 7 03:15:31 UTC 2010
any reason for not just getting something like this:
http://www.allelectronics.com/make-a-store/item/PS-1512/12V-12A-FAN-COOLED-POWER-SUPPLY/1.html
and hooking up a power transistor (or better yet a mosfet) to an Arduino & PWM it for temperature control?
On Jul 6, 2010, at 3:03 PM, Josh Myer wrote:
> TO-3, yeah, typo. They'll go up to 1.5A, which is far more power than I've ever used =)
>
> 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.
> --
> /jbm
>
>
>
> On Tue, Jul 6, 2010 at 11:37 AM, jim <jim at well.com> wrote:
>
> 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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> >
> >
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> >
> >
> > --
> > Josh Myer 650.248.3796
> > josh at joshisanerd.com
> >
> > _______________________________________________
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> > https://www.noisebridge.net/mailman/listinfo/noisebridge-discuss
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>
>
>
>
> --
> Josh Myer 650.248.3796
> josh at joshisanerd.com
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