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'[EE] Heat Sinking'
2009\03\19@161416 by Jeff Anno

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I have a pressure transducer design that uses a 4-20 mA current loop.  The
current output converter (XTR117) that I am using needs to dissipate 0.8W
(when supplied with high loop voltage (40V is the high end of the 4-20
industry standard current loop)).

Since the sensor is in a (fully enclosed) circular tube I am very worried
about heat.  The XTR117 datasheet states that the heat produced by the
transistor (necessary for the design) will hurt the performance of the
sensor.  They suggest mounting it so the heat conducts to the housing.  

The problem I have is that the housing (the tube that the circuit lives in)
is curved and the transistor is flat (obviously), so there isn't any real
contact between the transistor and the tube.  I got samples of a gap-filler
made by Fujipoly. It was too sticky and doesn't appear to be able to work
well in a production environment when sliding against a surface (difficult
to slide the boards into the tube with the sticky stuff rubbing on the
tube).

Has anyone encountered a similar problem with a curved surface?  Any
suggestions?

Here's a link to the datasheet...

http://focus.ti.com/lit/ds/symlink/xtr117.pdf

Thanks in advance.

-Jeff



2009\03\19@163206 by Vitaliy

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Jeff Anno wrote:
> Since the sensor is in a (fully enclosed) circular tube I am very worried
> about heat.  The XTR117 datasheet states that the heat produced by the
> transistor (necessary for the design) will hurt the performance of the
> sensor.  They suggest mounting it so the heat conducts to the housing.  

What is the material of the tube?


2009\03\19@163640 by Jeff Anno

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-----Original Message-----
From: spam_OUTpiclist-bouncesTakeThisOuTspammit.edu [.....piclist-bouncesKILLspamspam@spam@mit.edu] On Behalf Of
Vitaliy
Sent: Thursday, March 19, 2009 4:31 PM
To: Microcontroller discussion list - Public.
Subject: Re: [EE] Heat Sinking

Jeff Anno wrote:
> Since the sensor is in a (fully enclosed) circular tube I am very worried
> about heat.  The XTR117 datasheet states that the heat produced by the
> transistor (necessary for the design) will hurt the performance of the
> sensor.  They suggest mounting it so the heat conducts to the housing.  

What is the material of the tube?


The tube is made out of Stainless Steel and is 0.800" internal diameter.


2009\03\19@174419 by Dr Skip

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A couple of things come to mind. Cast a lead (solder if temps permit)
semi-cylindrical piece that's flat on the side opposite the curve. Transistor
mounts on the flat and curve is radius 0.4" on curved side. If casting
yourself, you could use a tube section to make a long one and cut into squares
for each transistor.

Or, perhaps use a solid 0.8" diameter steel rod as an anvil with a flat, insert
into tube and press a flat into the tube for mounting.

Or, mount the transistor in a tube section of aluminum with a flat spot, tab,
or whatever for the transistor, OD=0.8", and press into stainless tube.

Or mount to a long rectangle (as long as you can, with width near the inside
circumference) of thin aluminum (you're making a thin, split, aluminum inner
sliding tube. Curve the rectangle to slide into the tube. Works for the
straight part of your tube).

Or combine the above with epoxy as gap filler heavily doped with aluminum
filings or powder.

Other ideas, but would need to see more exacting pics and restrictions (like
the tube needs to be watertight, etc.)... ;)

-Skip


Jeff Anno wrote:
> The problem I have is that the housing (the tube that the circuit lives in)
> is curved and the transistor is flat (obviously), so there isn't any real
> contact between the transistor and the tube.  I got samples of a gap-filler
> made by Fujipoly. It was too sticky and doesn't appear to be able to work
> well in a production environment when sliding against a surface (difficult
> to slide the boards into the tube with the sticky stuff rubbing on the
> tube).
>
> Has anyone encountered a similar problem with a curved surface?  Any
> suggestions?

2009\03\19@184101 by Vitaliy

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Dr Skip wrote:
> Or, mount the transistor in a tube section of aluminum with a flat spot,
> tab,
> or whatever for the transistor, OD=0.8", and press into stainless tube.
>
> Or combine the above with epoxy as gap filler heavily doped with aluminum
> filings or powder.

I like this idea. You can probably have a sort of half-moon heatsink
precision machined from aluminum, and use heat conducting glue to fix it to
the tube, and heatsink paste where it comes in contact with the transistor
(if board mechanics allow).

Can you make holes in the tube? If so, you could attach the heatsink with a
couple of screws, and use heatsink paste on both sides.

OTOH, maybe you just need to run some tests to see if this is a real problem
that you need to solve.

Vitaliy

2009\03\19@184428 by Vitaliy

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Jeff Anno wrote:
> The tube is made out of Stainless Steel and is 0.800" internal diameter.

Wait a minute.. you're putting your electronics in a steel tube... Are you
making an IED?


2009\03\19@225340 by Jake Anderson

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Jeff Anno wrote:
{Quote hidden}

If the tube is fully enclosed I'd think about filling it with oil.
If you really want to get fancy turn the tube into a heat pipe ;->

2009\03\19@235003 by Vitaliy
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Jake Anderson wrote:
> If the tube is fully enclosed I'd think about filling it with oil.
> If you really want to get fancy turn the tube into a heat pipe ;->

Way to think outside the box! :-)

Vitaliy

2009\03\20@051113 by Xiaofan Chen

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On Fri, Mar 20, 2009 at 4:14 AM, Jeff Anno <jeff_annospamKILLspamacam-usa.com> wrote:
> I have a pressure transducer design that uses a 4-20 mA current loop.  The
> current output converter (XTR117) that I am using needs to dissipate 0.8W
> (when supplied with high loop voltage (40V is the high end of the 4-20
> industry standard current loop)).
>
> Since the sensor is in a (fully enclosed) circular tube I am very worried
> about heat.  The XTR117 datasheet states that the heat produced by the
> transistor (necessary for the design) will hurt the performance of the
> sensor.  They suggest mounting it so the heat conducts to the housing.

If possible I will attack the issue from a different point. Why 40V? Can
you reduce it to 20V through an SMPS. In that case, you can still
ensure a good enough load resistance range. In this case, the heat
dissipation will go down significantly,.

We do not do transmitters. But for the PLC 4-20mA analog output
modules (up to 12 channels with load up to 750 Ohms)  we have
great heat dissipation problems as well. Then we need to regulate
the compliance voltage based on the load in order to mitigate the problem.

Ref:
http://www.google.com/patents?id=0q9_AAAAEBAJ

Xiaofan

2009\03\20@090323 by Jeff Anno

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Thanks everyone for your replies so far. Dr. Skip asked for more exacting
pics to better clarify.  I posted them on the link below.

http://home.fuse.net/theannos/heat.html

Thanks again,
Jeff



<Snip>
>
> Other ideas, but would need to see more exacting pics and restrictions
> (like
> the tube needs to be watertight, etc.)... ;)
>
> -Skip
>

</Snip>


2009\03\20@091212 by Jeff Anno

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>
> You can probably have a sort of half-moon heatsink
> precision machined from aluminum, and use heat conducting glue to fix it
> to
> the tube, and heatsink paste where it comes in contact with the transistor
> (if board mechanics allow).
>

This is the direction I am leaning in but I am still trying to find a more
economical solution.  I was thinking that there was an expandable heat
sinking foam that I could (precisely) squirt in.  I googled it but didn't
find any products that I could just buy and try.

> Can you make holes in the tube?

Unfortunately no.  The electrical connector gets welded to the top so the
unit is watertight.  This can be used in harsh environments (IP-69K rated).


> OTOH, maybe you just need to run some tests to see if this is a real
> problem
> that you need to solve.

Good point!



2009\03\20@091401 by Jeff Anno

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> Jeff Anno wrote:
> > The tube is made out of Stainless Steel and is 0.800" internal diameter.
>
> Wait a minute.. you're putting your electronics in a steel tube... Are you
> making an IED?
>

I'm actually trying to make sure this sensor doesn't have heat or blow up...
:)


2009\03\20@095549 by Alan B. Pearce

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>> Can you make holes in the tube?
>
>Unfortunately no.  The electrical connector gets welded to the top so the
>unit is watertight.  This can be used in harsh environments (IP-69K rated).

So it is a non-serviceable unit - fill it with thermally conducting epoxy.

2009\03\20@101618 by Michael Rigby-Jones

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> -----Original Message-----
> From: .....piclist-bouncesKILLspamspam.....mit.edu [EraseMEpiclist-bouncesspam_OUTspamTakeThisOuTmit.edu] On
Behalf
> Of Jeff Anno
> Sent: 20 March 2009 13:03
> To: 'Microcontroller discussion list - Public.'
> Subject: RE: [EE] Heat Sinking
>
> Thanks everyone for your replies so far. Dr. Skip asked for more
exacting
> pics to better clarify.  I posted them on the link below.
>
> http://home.fuse.net/theannos/heat.html
>

Is the end of the tube shown in the picture the end that has the
connector welded to it?

Mike

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2009\03\20@103859 by Jeff Anno

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> Is the end of the tube shown in the picture the end that has the
> connector welded to it?

Yes, but the transistor (as it is right now) is actually pretty far inside
the tube, new boards are in process of being laid out.  The transistor shown
in the pictures is actually not connected it was just to illustrate the size
& how it will make contact to the curved surface.

I can put the transistor wherever it needs to be if you have an idea that
requires it to be in a specific location.

As an additional FYI... The connector has wires coming down through the
center that get soldered to the output board (not leaving much room for a
transistor to make contact).  Also, I make a case ground connection (from
each circuit board to the case) through stiff beryllium-copper springs
(stampings) that make contact to the bottom-outer edge of the connector.
These springs are compressed when the connector is mounted to the sensor.


This is confusing, I know, so here is an illustration...

http://home.fuse.net/theannos/illustration.jpg



2009\03\20@105140 by Mark Rages

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On Fri, Mar 20, 2009 at 9:38 AM, Jeff Anno <jeff_annospamspam_OUTacam-usa.com> wrote:
{Quote hidden}

Could you get a wide copper spring to attach to the transistor and
press against the case?  Copper is a good conductor of heat, and
you'll need a firm connection to the wall, which I haven't seen
addressed by the other ideas.

Regards,
Mark
markrages@gmail
--
Mark Rages, Engineer
Midwest Telecine LLC
@spam@markragesKILLspamspammidwesttelecine.com

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