Re: Box filled with resin - Thermal resistance

What is better in term of heat dissipation: A TO220 in free air inside a closed box or a TO220 sunk in a box full of resin?

From inside, I wanted to say from the TO220 case surface! The box is rectangular, 2.25" x 1.75" x 0.75" and will contain a 2" x 1.5" PCB.

Thanks

"John Larkin" a écrit dans le message de news: snipped-for-privacy@4ax.com...

the

the box full of resin

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As a first approximation, you can take the resin filled box as all the same temperature and the thermal resistance from the case to the air will determine the box/resin temperature.

Achilles: I wish my wish would not be granted. < an undescribable event occurs >

Achilles: What happened? Where's my Genie? Tortoise: Our context got restored incorrectly. Achilles: What does that cryptic comment mean? Tortoise: The system crashed.

To email me send to :

rb AT ieee DOT org

There are special epoxy encapsulants that are thermally conductive. I've used them for potting temperature sensors in heatsinks with excellent results. Try these:

Electrolube ER2074 (thermal conductivity: 1.26 W/mK) Robnor RX700K/BK (thermal conductivity: 1.00 W/mK)

The first one is milky white colour, the second one is black.

Costas

On Sun, 27 Jul 2003 02:46:33 +0000 (UTC), "Costas Vlachos" Gave us:

Stycast 2850 Is fine, and available in pints. A higher conductivity version is available at a much higher cost, and contains huge grains of a crystalline substance.

Actually, just about any good, strong epoxy will work good as a conduction cooling design on a package that small with a single transistor device in it.

Be sure to optimize your design for minimal dissipation in the device first.

I did a similar experiment many years ago (too many!). 300mW plastic transistors in free air, and then potted in epoxy resin, in a plastic box about 1x1x0.4 inches.

Vbe measurements showed that the transistors required about 4x the power to reach the same junction temperature.

On Sun, 27 Jul 2003 17:42:53 +0800, Dave Baker Gave us:

Liquid steel is a good choice. Remember that even a good RTV can dissipate quickly enough to be good (not the tube/squeese kind, it must be able to be vacuumed). It would also be serviceable with RTV whereas epoxies usually mean you get one shot at the device, when it goes, the whole module is gone. It all depends on how hot you are getting it as well though. A poorer epoxy may do the job, but will be hotter in the overall, even though it would or may still be within operating specs for the device.

Look for the "stycast" stuff, it is pretty cheap. That same 50 bucks would probably get you a full pint of it. Enough for a couple dozen boxes that size at least. Most distributors want you to be a company, so you may have to worm your way to a "sample" purchase.

Emerson & Cummings is one.

On Sun, 27 Jul 2003 06:21:47 +0100, Tony Williams Gave us:

You failed to say which method produced the higher factor. Doh!

X-No-Archive: Yes

Costas Vlachos wrote:

There's also MG Chemicals stuff:

How good do you want the conductivity to be? You can also melt some tar in a soda can and pour it in. Many electronic ballasts and transformers are potted this way.

I was trying to do some analysis and for simplicity I used a sphere instead of a box to calculate the thermal resistance and I came to a strange conclusion.

The thermal resistance of a sphere is 1/(4*pi*k*r), where k is the thermal conductivity and r is the radius of the sphere. A you can see, the thermal resistance is inversely proportional to the radius. So it mean that if you increase the sphere, the delta temperature between the center and the surface of it will decrease . I would have sais the opposite, no?

"Roy McCamm> > I need to calculate the Inside to ambiant thermal resistance of a small

the

If I sink a TO220 in the middle of a 10cm sphere made of general poting epoxy (k = 0.682 W/m*K), it wil have a case to sphere's surface thermal resistance of only 1.2 °C/W (I considered the TO200 has a pontual heat source). Since the area of that sphere is 1000 time bigger than the TO220 case's surface, is it correct to say that the case to ambiant thermal resistance of that configuration will have a lot lot less thermal resistance than a TO220 in free air, no?

Thanks

"Ziguy" a écrit dans le message de news:sJXUa.746$ snipped-for-privacy@news20.bellglobal.com...

instead

small

what're

Run for long enough to heat the resin through?

You need the thermal conductivity of the resin, and possibly the heat capacity (if you'r not steady state)

If you are willing to spend a few moments, you can greatly improve the thermal conductivity by adding either silicon carbide, or aluminium oxide. There are many other possible additives. Ensure it's well mixed. This does make it somewhat harder to dissasemble.

The thermal resistance from "the center" of a sphere to its surface is infinite if "the center" is a point. For a finite source, say a smaller isothermal interior sphere of fixed size, clearly the thermal resistance from this source to the surface increases as the outside radius increases.

The thermal resistance from the surface of a sphere to ambient decreases as the radius increases; this theta is about inverse with surface area.

John

On Sun, 27 Jul 2003 16:00:10 GMT, AC/DCdude17 Gave us:

NO! Many electronic devices WERE potted that way.

Currently AFAIK, it is now a fire risk, and no longer approved.

On Sun, 27 Jul 2003 15:48:13 -0700, John Larkin Gave us:

Hahahahahahaha... Damned shame they didn't carry it out.

I owed you that, asshole. :] Go see "Pirates"...

Why is it infinite if the center is a point? That would mean the sphere as no thermal conductivity, no? Also, why is the Rth increase with r if the source is not a point? The relation Rth = 1/(4*pi*k*r) don't seam to explain neither of your affirmation...

By the way, do you have any equations for the thermal resistance from the surface of a sphere (or any abitrary shapes) to ambient?

I don't wan't to contradict you since I had no knowledge about thermodynamic before yesterday. I just try to understand! Thanks for you help!

Hey, speaking of nasty stuff, how are you these days?

John

On Sun, 27 Jul 2003 20:02:20 -0400, "Ziguy" Gave us:

It has an effect on the level of radiant dispersion, which effects overall capacity to conduct heat. A bit, but there nonetheless. So does the texture of the surface.