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Old 15th January 2013, 03:28 PM   #191
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Originally Posted by keantoken View Post
If we knew the thermal resistance to ambient of the altoids tin we could calculate our minimum bias. I would stand the tin up on its side with vertical vents, this way convection is maximized, and more surface area is in contact with the air.
the 1.76 oz (50g) Altoids tin has a surface area of over 17 inches! Sides included. Output devices should thus be placed on the bath-tub portion of the tin.

I placed a 6 ohm 50W dale resistor in the tin, thermal goo and attached via 4-40 screws. cooked it for 10 minutes @1.4A:

Click the image to open in full size.

Temperature rise after 10 minutes was all over the place -- from almost 65 degrees at the lip end of the cover to 40 degrees (C) where the cover is attached to the bath-tub. Thus you could fry an egg on this thing.
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Old 15th January 2013, 03:51 PM   #192
JMFahey is offline JMFahey  Argentina
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Wow !!
THANKS a lot for making this IMPORTANT experiment !!!
I didn't build mine yet, those pesky customers who demand what they've pre-paid for keep me busy!!!
But it sort of confirms my idea of adding a small CPU type fan .
That *besides* my original idea of pop riveting an aluminum rectangle to the case bottom, with thermal paste in between, of course.
Thin tin by itself is a very poor thermal spreader.

Just curious: what was the temperature just over the resistor?
Because what you mentioned ("lips" , "cover", is some distance away).
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Old 15th January 2013, 04:05 PM   #193
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So thermal resistance is somewhere around 3.2C/W. Very useful to know, thanks! Do you know the ambient temperature at the time of the test?

So it would take ~47W to fry a silicon output device at 150C. Although the lytics would be long gone by this time.

Lets identify the max internal temperature tolerable for preserving the lifespan of the components. Then we'll know how much we can dissipate in this tin with reasonable longevity.
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Old 15th January 2013, 05:01 PM   #194
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Thermal rise was 65 degrees -- it's a little warm in the workshop in the winter as the furnace is nearby -- so it started at 25 and ran up to 90. I think it can be improved by using a bit of 1/8" aluminum stock.

I will use lateral mosfets, lower over-head means less heating, no necessity of source resistors.
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Old 15th January 2013, 05:10 PM   #195
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So that's actually 5.5C/W, corresponding to about 27W at 150C.
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Old 15th January 2013, 05:25 PM   #196
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And that's about what i calculated prior to the experiment just using the surface area of the tin. It will be too hot to pick up or hold in your hand.
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Old 15th January 2013, 06:48 PM   #197
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The tin could be seen as a cooling tunnel into which an airflow would be forced.
Holes for in holes for out, both ends.
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Old 17th January 2013, 11:34 PM   #198
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Quote:
Originally Posted by jackinnj View Post
I will use lateral mosfets, lower over-head means less heating, no necessity of source resistors.
Lower cost versions of lateral mosfet include Power Mosfet and under-volted TDA7296.
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Old 18th January 2013, 07:25 AM   #199
bigwill is offline bigwill  United Kingdom
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Hi everyone,

I am still alive. Bear with me, I have been rather busy
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Old 19th January 2013, 02:06 PM   #200
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Plenty of room:

Click the image to open in full size.

The substrate is 3/16 aluminum and fits into an altoids tin.
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