Post by thread:[EE] Heat Sinking

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

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?

-----Original Message----- From: spam_OUTpiclist-bouncesTakeThisOuTmit.edu [.....piclist-bouncesKILLspam@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.

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?

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

Jeff Anno wrote: {Quote hidden}> 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 > > > > 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 ;->

On Fri, Mar 20, 2009 at 4:14 AM, Jeff Anno <jeff_annoKILLspamacam-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

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>

> > 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!

> -----Original Message----- > From: .....piclist-bouncesKILLspam.....mit.edu [EraseMEpiclist-bouncesspam_OUTTakeThisOuTmit.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 ======================================================================= This e-mail is intended for the person it is addressed to only. The information contained in it may be confidential and/or protected by law. If you are not the intended recipient of this message, you must not make any use of this information, or copy or show it to any person. Please contact us immediately to tell us that you have received this e-mail, and return the original to us. Any use, forwarding, printing or copying of this message is strictly prohibited. No part of this message can be considered a request for goods or services. =======================================================================

> 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

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