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heatsinking

J

Jamie Morken

Jan 1, 1970
0
Hi all,

Would an 8mm thick large aluminum plate be as good a heat sink as a
~3degreeC/watt rated heatsink for a TO-220 package?

cheers,
Jamie Morken
 
M

Michael Clarke

Jan 1, 1970
0
Would an 8mm thick large aluminum plate be as good a heat sink as a
~3degreeC/watt rated heatsink for a TO-220 package?

The quick answer is yes in most circumstances. It will work if:
- the plate is not heated by some other source
- there is good thermal contact with your TO-220
- the plate has adequate ventilation for heat removal
A finger makes a good heatsink tester. If you can tolerate a few seconds
contact with the heatsink < ~ 50K, it is adequate. Make sure you test
during prolonged use, not just at startup to check for heat buildup.
Be aware of thermal mass is not the same as heat removal. A bulky metal
object will give the illusion of heat removal because it can absorb a
lot of heat, but if the heat is not being removed to the air, it will
eventually overheat.
 
L

Leon Heller

Jan 1, 1970
0
Jamie said:
Hi all,

Would an 8mm thick large aluminum plate be as good a heat sink as a
~3degreeC/watt rated heatsink for a TO-220 package?


It depends on the size of the Al plate. :cool:

Leon
 
J

John Woodgate

Jan 1, 1970
0
I read in sci.electronics.design that Michael Clarke <[email protected]>
wrote (in <[email protected]>) about 'heatsinking', on
Thu, 15 Jan 2004:
The quick answer is yes in most circumstances.

How large is 'large'?
It will work if:
- the plate is not heated by some other source
- there is good thermal contact with your TO-220
- the plate has adequate ventilation for heat removal
A finger makes a good heatsink tester. If you can tolerate a few seconds contact
with the heatsink < ~ 50K, it is adequate.

A few seconds at below 50 degrees Kelvin? Make sure your finger
penetrates the frozen air on the surface of the plate.

Seriously, DON'T use < and > outside mathematics. Even if you get it
right (the closed end points to the SMALLER quantity), other people will
misread it.

If by '50 K' you meant '50 kelvin above room temperature', that would be
a correct use of 'K', but I bet 99% of people wouldn't understand what
you meant, and I don't think you did mean that.
Make sure you test during prolonged
use, not just at startup to check for heat buildup.
Be aware of thermal mass is not the same as heat removal. A bulky metal object
will give the illusion of heat removal because it can absorb a lot of heat, but
if the heat is not being removed to the air, it will eventually overheat.

Quite right; a banana skin that should be more widely-known.
 
J

Jamie Morken

Jan 1, 1970
0
Hi,

It is about 14" by 6". I am not sure if the 8mm thickness will be able to
transfer heat as quickly as a high surface area (many finned) heatsink
would.

cheers,
Jamie Morken
 
J

John Smith

Jan 1, 1970
0
Jamie Morken said:
Hi all,

Would an 8mm thick large aluminum plate be as good a heat sink as a
~3degreeC/watt rated heatsink for a TO-220 package?

cheers,
Jamie Morken


According to this source http://www.datadart.com/al/sanyo/STK392_020.pdf,
you will need about 300 square cm to get 3 C/W from a flat, unfinished,
aluminum plate. The chart is for a 2 mm thick plate. Your 8 mm plate will be
better due to higher thermal conductance to the outer areas. Try to make
your plate square and put your device in the center.

HTH.

John
 
R

Rene Tschaggelar

Jan 1, 1970
0
Jamie said:
Hi all,

Would an 8mm thick large aluminum plate be as good a heat sink as a
~3degreeC/watt rated heatsink for a TO-220 package?

The heatsink is irrelevant compared to the surrounding.
Where and how does the heat go from the heatsink ?
Is it accessible to convection, is there forced cooling, and so on.

Rene
 
J

John Woodgate

Jan 1, 1970
0
I read in sci.electronics.design that Jamie Morken
Hi,

It is about 14" by 6". I am not sure if the 8mm thickness will be able to
transfer heat as quickly as a high surface area (many finned) heatsink
would.
Is it mounted vertically or horizontally?

Does anyone know of a supplier of software dental forceps for extracting
key facts from Usenet posters? (;-)
 
M

Michael A. Terrell

Jan 1, 1970
0
John said:
I read in sci.electronics.design that Jamie Morken

Is it mounted vertically or horizontally?

Does anyone know of a supplier of software dental forceps for extracting
key facts from Usenet posters? (;-)

Actually, what we need a software version of the "Spanish Inquisition"
"No one expects the Spanish Inquisition!"

--
We now return you to our normally scheduled programming.

Take a look at this little cutie! ;-)
http://home.earthlink.net/~mike.terrell/photos.html

Michael A. Terrell
Central Florida
 
M

Michael

Jan 1, 1970
0
I think (if my memory serves me right), the rule of thumb is ~70deg C
(or Kelvin for that matter) above ambient per 1sq inch per 1W
dissipation. This is "normal" atmospheric conditions, no forced air
flow.
One can check it by taking bunch of heat sink thermal_resistance/area
ratios from a heat sink catalog and averaging them.
 
J

Jamie Morken

Jan 1, 1970
0
John Woodgate said:
I read in sci.electronics.design that Jamie Morken

Is it mounted vertically or horizontally?

Does anyone know of a supplier of software dental forceps for extracting
key facts from Usenet posters? (;-)

It is mounted to the face of the plate. Thats funny my brother just got his
wisdom teeth out today! :)
 
J

John Woodgate

Jan 1, 1970
0
I read in sci.electronics.design that Jamie Morken
It is mounted to the face of the plate. Thats funny my brother just got his
wisdom teeth out today! :)

No, is the PLATE mounted vertically or horizontally? Can you not see
that that makes a BIG difference?

When did you have your wisdom teeth out? (;-)
 
R

Rene Tschaggelar

Jan 1, 1970
0
These data may be misleading. If the airflow is restricted, or not
in the way the manufacturer planed, you won't reach these values.
Notice that the finned heatsinks are intended to be mounted
on a vertical face with nothing below and above for 1 foot or so.
If the mounting surface is horizontal, or there are other
restrictions you won't reach the Watt/Kelvin specified.

Rene
 
J

Jamie Morken

Jan 1, 1970
0
Hi,

No, is the PLATE mounted vertically or horizontally? Can you not see
that that makes a BIG difference?

A picture is worth a thousand words:
http://www.rocketresearch.org/new/pics/c-channel.jpg

The red outline is the electronics board and the green outline is the TO-220
screwed to the plate (with isolation material). The aluminum is the large
grey piece (6" c-channel). It is about 8mm thick, 6 inches top to bottom,
and 14 inches left to right.
 
J

John Crighton

Jan 1, 1970
0
Hi all,

Would an 8mm thick large aluminum plate be as good a heat sink as a
~3degreeC/watt rated heatsink for a TO-220 package?

cheers,
Jamie Morken

Hello Jamie,
have a wee read here
http://w1.859.telia.com/~u85920178/begin/heat-0.htm#big
You will see a formula that you can use on your heatsink.
Your heatsink lacks "area."
Adding more fins to your aluminium plate as shown at that
website, will improve your heatsink greatly.
Regards,
John Crighton
Sydney
 
M

Mac

Jan 1, 1970
0
Hi,

It is about 14" by 6". I am not sure if the 8mm thickness will be able to
transfer heat as quickly as a high surface area (many finned) heatsink
would.

cheers,
Jamie Morken

It's much easier to follow the conversation if you don't top-post.

You can look up the thermal conductivity of Aluminum. It's pretty high. I
imagine an 8mm plate will pull heat away pretty effectively.

You can bound your problem by figuring out what the temperature rise over
ambient is for your dissipation with a flat plate of that size at uniform
temperature.

That is, if you are dissipating 10 Watts, you can find the formula for
flat plate dissipation, plug in 10 Watts, and see how much over ambient it
is.

In reality, the plate won't be at a uniform temperature. It will be hotter
near your part. But if the temperature rise isn't even close to your Tcase
danger zone, then you don't really have to worry about it.

Keep in mind that the flat plate formula only applies when the surface is
as described in the formula. You will need to specify horizontal or
vertical, and the plate has to be largely free and clear for air to
convect over it naturally. Of course, if you have a fan, then you are
almost certainly going to be OK. A fan blowing over a big aluminum plate
can take away a lot of heat.

Mac
 
M

Mac

Jan 1, 1970
0
Hi,



A picture is worth a thousand words:
http://www.rocketresearch.org/new/pics/c-channel.jpg

The red outline is the electronics board and the green outline is the TO-220
screwed to the plate (with isolation material). The aluminum is the large
grey piece (6" c-channel). It is about 8mm thick, 6 inches top to bottom,
and 14 inches left to right.

That's not plate, it's an extruded beam. The near side won't have ideal
convection because of the obstructing parts of the beam. But the far side
will have pretty good convection.

You still haven't said how much heat you need to dissipate, but I think
you will be OK, provided the far side of your beam is vertical and not
inside a small enclosure. My basis for saying this is that I have a 3deg
per Watt heatsink, and when I look at it, and compare it to your beam, my
gut just tells me that there is no way the beam is going to get hotter
than the heatsink, even in the region near your part.

Not very scientific. ;-)

Mac
--
 
J

John Woodgate

Jan 1, 1970
0
I read in sci.electronics.design that Jamie Morken
A picture is worth a thousand words:
http://www.rocketresearch.org/new/pics/c-channel.jpg

The red outline is the electronics board and the green outline is the TO-220
screwed to the plate (with isolation material). The aluminum is the large
grey piece (6" c-channel). It is about 8mm thick, 6 inches top to bottom,
and 14 inches left to right.

OK, a U-shaped extrusion makes a good heat sink, in my experience -
better than a flat plate, which tends to have laminar air-flow. And
yours is indeed quite large. The problem you might run into is that the
thermal conductivity of some of the alloys that are used for extrusions
is not very good at all. If you find a very hot area surrounding the
transistor, you may find another alloy that has similar mechanical
properties but better thermal conductivity. The manufacturer of the
extrusion can tell you what the alloy is, and British Aluminium can tell
you about thermal conductivities.
 
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