Thermal resistance of plywood

linear=20 ^^^^^ ??

NOT cubed, just area.

in=20

=20

Me thinks maybe you do not have much room sassing people over physics.

asked...

11"

Clean up your units please.

--=20 Transmitted with recycled bits. Damnly my frank, I don't give a dear

----------

Where?

John

John Larkin a écrit :

Thermal resistance is in K/W, while you're speaking of K/W/ohm.

I wonder how this should be called.

--
Thanks,
Fred.

Thermal resistance per ohm, just like I wrote it.

Most pure metals hit about the same value. Brass is about 2:1 higher, so use brass screws to conduct electricity with minimum heat transfer.

John

"Jim Thompson" wrote in message news: snipped-for-privacy@4ax.com...

From

about .13 W/mK is the thermal conductivity. So, about 7.7 mK/W resistivity.

I estimate about 86 or so watts.

Not much.

Cheers, John

...

...

Actually, the required wattage is probably between 20 and 40 watts. As I mentioned in a previous post, 40W maintains close to the desired temperature in a larger box:

"Here are some measurements from a light-bulb-heated food dryer with its vents closed: Turned on a 40W bulb at 3pm, 75F. At 4:20, with 90F near the middle of the box, turned off 40W bulb, turned on 60W bulb. At 5:20, with 108F near the middle, turned off 60W bulb, turned on 40W bulb. At

10pm, 94F near the middle. This box is about 10"x11"x28" inside, or twice the volume of your 8"x11"x18" box. 50% of its surface is 3/4" wood, 35% is 1/8" masonite, and the rest is window glass."

IIRC pure copper wins, but you have to use really really skinny screws. Generally the winning scheme for running wires to some thermally insulated thing is narrow trace Cu flex circuit, preferably with a thermal ground at each cooling stage.

Cheers

Phil Hobbs

(Getting back to my parts lists now)

Cheers

Phil Hobbs

[snip]

I swagged 63W... much more than I anticipated... another WAI (wild-ass-idea) down the tubes :-(

...Jim Thompson

--
| James E.Thompson, P.E.                           |    mens     |
| Analog Innovations, Inc.                         |     et      |
| Analog/Mixed-Signal ASIC\'s and Discrete Systems  |    manus    |
| Phoenix, Arizona  85048    Skype: Contacts Only  |             |
| Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  |
| E-mail Icon at http://www.analog-innovations.com |    1962     |
             
 I love to cook with wine     Sometimes I even put it in the food

OK, useful numbers, and right on-point... dryer application. I don't want the light, so I'll just use big wirewounds bolted to a finned heatsink/source ;-)

...Jim Thompson

--
| James E.Thompson, P.E.                           |    mens     |
| Analog Innovations, Inc.                         |     et      |
| Analog/Mixed-Signal ASIC\'s and Discrete Systems  |    manus    |
| Phoenix, Arizona  85048    Skype: Contacts Only  |             |
| Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  |
| E-mail Icon at http://www.analog-innovations.com |    1962     |
             
 I love to cook with wine     Sometimes I even put it in the food

At room temperature --- Wiedemann?Franz. The ratio is PTAT, right?

I stand corrected. I used the wrong coefficient for natural convection.

Thanks.

Cheers, John

asked...

11"

Use big nylon nuts and bolts. Or glass filled epoxy.

A rare moment of lucidity.

asked...

11"

Which don't conduct electricity very well.

John

You do not have to rely on the fasteners to provide the conduction path between two mating surfaces. In fact, you shouldn't.

I thought you were talking about thermal conduction anyway.

I was talking about the thermal conductivity of electrical conductors. The same electrons that transport current also transport heat - pretty much - so most metals are the same as regards how much current you can carry for a given heat loss. This matters for stuff like getting power into a crystal oven or a cryogenic gadget, where we really don't want the heat loss through the leads.

People use stainless and manganin wire to run to really cold (liquid helium) gadgets so as to not leak too much heat. Google "Garwin thermal integral" for some messy math.

John

That's because you've got all the smart people plonked. There were at least a half-dozen good responses, but apparently the sand got in your eyes.

Thompson is such an asshole, he doesn't really deserve an answer - why the hell should we do his googling for him?

Thanks, Rich

Spot on.