Post by thread:[PICLIST] [EE] TO-220 heatsinking

Hi list, I'm working on a home project (a drum machine based around a PIC!) that uses a couple of 15V regulators in TO220 packages - a 7815 and 7915. They're both providing about 200mA, and I'm expecting the total voltage drop across them to be about 5V (15-0-15vrms transformer...), hence they'll both be dissipating roughly 1W. According to the National Semi. datasheet and having done some rough calculations myself I know that the device temperature will be well within spec even if the device is used in free-air, i.e. without a heatsink. However, I'm wondering if I should go back and redesign my (rather dense) PCB before getting it printed, to allow space for a heatsink anyway. My question is, can anyone suggest a reason why I *should* consider a heatsink, especially if the device is switched on for very long periods of time, i.e. days? Should I consider a much higher Ta in my calculations as the air inside the enclosure warms up? There will be lots of space above the regulator (within a 2u 19" rack). Thanks in advance for any advice offered. Regards, Trev Trevor Page Pace Micro Technology PLC 01274 538379 The E-mail and any attachments hereto are strictly confidential and intended solely for the addressee. If you are not the intended addressee please notify the sender by return and delete the message. You must not disclose, forward or copy this E-mail or attachments to any third party without the prior consent of the sender. -- http://www.piclist.com#nomail Going offline? Don't AutoReply us! email spam_OUTlistservTakeThisOuTmitvma.mit.edu with SET PICList DIGEST in the body

At 12:19 PM 2/14/02 +0000, you wrote: >Hi list, > >I'm working on a home project (a drum machine based around a PIC!) that uses >a couple of 15V regulators in TO220 packages - a 7815 and 7915. They're both >providing about 200mA, and I'm expecting the total voltage drop across them >to be about 5V (15-0-15vrms transformer...), hence they'll both be >dissipating roughly 1W. According to the National Semi. datasheet and having >done some rough calculations myself I know that the device temperature will >be well within spec even if the device is used in free-air, i.e. without a >heatsink. However, I'm wondering if I should go back and redesign my (rather >dense) PCB before getting it printed, to allow space for a heatsink anyway. My rule of thumb is not to go over 600mW with an un-heatsinked TO-220. But I am rather conservative and like to make stuff that never quits. Without doing any calculations, and not knowing the details of your enclosure, my gut feel is that you'd probably get away with this for one piece, but it isn't great design. >My question is, can anyone suggest a reason why I *should* consider a >heatsink, especially if the device is switched on for very long periods of >time, i.e. days? Should I consider a much higher Ta in my calculations as >the air inside the enclosure warms up? There will be lots of space above the >regulator (within a 2u 19" rack). Consider temperature difference from inside to outside your enclosure, consider what happens if it is used in Barstow or Phoenix in mid-summer, (or whatever the equivalent scenario is where you are), de-rate WELL from Tj(max) for reliability, and also consider what happens if your line voltage is 10% or more high. Also, at light loads compared to what the transformer is rated for, it may put out significantly more voltage than the spec'd voltage at full-load*. On for days may actually be better than heating up and cooling down, the thermal cycling can eventually cause failures in power semiconductors. * If you had 10% high line, and 10% more voltage than at full load, your dissipation in the regulators would almost be *doubled*. Naturally this will also increase the temperature inside your enclosure eventually. >Thanks in advance for any advice offered. Can you button them down to the case, maybe using an overmold TO-220F case if it needs to be isolated? Assuming a metal case, of course. ;-) Best regards, Spehro Pefhany --"it's the network..." "The Journey is the reward" .....speffKILLspam@spam@interlog.com Info for manufacturers: http://www.trexon.com Embedded software/hardware/analog Info for designers: http://www.speff.com 9/11 United we Stand -- http://www.piclist.com#nomail Going offline? Don't AutoReply us! email listservKILLspammitvma.mit.edu with SET PICList DIGEST in the body

>My question is, can anyone suggest a reason why I *should* consider a >heatsink, especially if the device is switched on for very long periods of >time, i.e. days? Should I consider a much higher Ta in my calculations as >the air inside the enclosure warms up? There will be lots of space above the >regulator (within a 2u 19" rack). I would recommend a heatsink anyway, even just a flat sheet of plain aluminium. It does not take much to halve the temperature rise, but it may well save you chasing your tail trying to find out why your circuit goes crazy after a coupe of hours under spot lights, as the internal temperature cutout of the regulator takes effect. Also remember that the tab of 7900 series regulators is NOT at earth potential, so if they are on the same piece of metal this one needs insulating. -- http://www.piclist.com#nomail Going offline? Don't AutoReply us! email .....listservKILLspam.....mitvma.mit.edu with SET PICList DIGEST in the body

Are you giving the TO220s any mechanical support? I don't like to support them only by their leads. If I don't need a real heat sink for thermal reasons, I usually solder (or bolt) them to a big square pad on the PCB. This makes them mechanically very secure, and probably doubles the "free air" power rating. Putting a pad on the board gives metal to metal contact to the TO220 case and I think the pad should be well thermally bonded to the fibergalss. Fiberglass isn't much of a heat conductor. But it is much better than air. Sherpa Doug > {Original Message removed}

Hi all, I'd just like to say many thanks to those who have offered advice on the heatsinking issue. Some very interesting points have been made, and no doubt I shall now redesign the board to make room for some heatsinks. The point about the extra mechanical support they offer is a good one too (it gets a right pain when the TO220 packages start to bend around and fall off after you've handled the board a few times...) So for future heatsink calculations, I suppose the best approach is to actually make some temp measurements inside the enclosure under extreme conditions e.g. under a spotlight or something, and use that as your Ta? Hey, this thermometer I've had on my 15yr old Casio watch may come in useful after all... Trev Trev > {Original Message removed}

----- Original Message ----- From: "Trevor Page" <EraseMEtrevor.pagespam_OUTTakeThisOuTPACE.CO.UK> To: <PICLISTspam_OUTMITVMA.MIT.EDU> Subject: Re: [EE] TO-220 heatsinking: THANKS! Only just catching up with email before going to bed. In all my designs where a heatsink is not needed (in theory) I place a copper pour as big as possible on the top and the bottom of the board, and leave enough room for a TV4 heatsink they only cost ~au$0.50 in qty's of 1000 and do for all the jobs I have needed where a heatsink is more a precaution than a requisite. Regards, Kat. ____________________________________________________________________________ /"\ ASCII Ribbon Campaign | K.A.Q. Electronics \ / - NO HTML/RTF in e-mail | Software and Electronic Engineering X - NO Word docs in e-mail | Perth Western Australia / \ | Ph +61 419 923 731 ____________________________________________________________________________ -- http://www.piclist.com#nomail Going offline? Don't AutoReply us! email @spam@listservKILLspammitvma.mit.edu with SET PICList DIGEST in the body

I have a natty little excel spreadsheet for heatsink calcs, If you wish, I will email you a copy off list. ******************************************************************** This email and any attachments are confidential to the intended recipient and may also be privileged. If you are not the intended recipient please delete it from your system and notify the sender. You should not copy it or use it for any purpose nor disclose or distribute its contents to any other person. ******************************************************************** -- http://www.piclist.com hint: To leave the PICList KILLspampiclist-unsubscribe-requestKILLspammitvma.mit.edu

Douglas Butler wrote, in part: >[...] >Fiberglass isn't much of a heat >conductor. But it is much better than air. Well, it is true that if you only consider true *conduction*, air is very poor at transporting heat (this is why a down sleeping-bag is so warm: those little feathers keep the air from moving, so it can only conduct heat away). But if there is room for the air to move, then *convection* can be a very efficient heat transport mode. This is, after all, how a conventional heat sink ultimately sheds its heat in most cases: it presents a large surface area to the air, which convects the heat away. At least, that's what I think. I could be wrong, I'm only a humble physicist and not a real electrical engineer. Michael V Thank you for reading my little posting. _________________________________________________________________ Send and receive Hotmail on your mobile device: http://mobile.msn.com -- http://www.piclist.com hint: To leave the PICList RemoveMEpiclist-unsubscribe-requestTakeThisOuTmitvma.mit.edu

At 07:43 AM 2/15/02 -0800, you wrote: >Well, it is true that if you only consider true *conduction*, >air is very poor at transporting heat (this is why a down >sleeping-bag is so warm: those little feathers keep the air >from moving, so it can only conduct heat away). But if there >is room for the air to move, then *convection* can be a very >efficient heat transport mode. This is, after all, how a >conventional heat sink ultimately sheds its heat in most cases: >it presents a large surface area to the air, which convects >the heat away. So the fiberglass conducts the heat to a wider area, and natural convection (and, to some extent, radiation) reduce the temperature by transferring the heat to the air, the air transports the heat to the enclosure walls, where it is conducted out and transferred to the ambient air, mostly by convection. I know well that PC board material has significant thermal conductivity- sometimes I have slots routed into the board to thermally isolate patches of the board and make them isothermal to themselves (eliminate gradients across the material)- same concept as cutting a ground plane, if you think about it. Best regards, Spehro Pefhany --"it's the network..." "The Journey is the reward" spamBeGonespeffspamBeGoneinterlog.com Info for manufacturers: http://www.trexon.com Embedded software/hardware/analog Info for designers: http://www.speff.com 9/11 United we Stand -- http://www.piclist.com hint: To leave the PICList TakeThisOuTpiclist-unsubscribe-requestEraseMEspam_OUTmitvma.mit.edu

>You are very right. Even a small fan does wonders for cooling. Most of >my stuff is in small sealed boxes so there is little or no air movement. > I love it when the customer draws a vacuum on the housing to "prevent >moisture problems" and the electronics cook to death in the resulting >Dewar flask! > >Sherpa Doug Why vacuum ? I understand that your customers have usually to do with wetness, vacuum is not a good way to seal those afaik. The smallest leak fills the box with water (completely) even if water needs to enter as vapor because the hole is so tiny. Don't you tell them to fill it with liquid ? (oil or special) ? Peter -- http://www.piclist.com hint: PICList Posts must start with ONE topic: [PIC]:,[SX]:,[AVR]: ->uP ONLY! [EE]:,[OT]: ->Other [BUY]:,[AD]: ->Ads

I'm in the habit of doing a copper pour on both sides of the board after I have all my traces (except ground) routed. The copper pour handles ground. I then screw the regulator down (using a SilPad or similar heat sink pad and a nylon screw if the tab is not grounded (like on a 7900 series regulator)). This is a cheap heatsink! Harold FCC Rules Online at http://hallikainen.com/FccRules Lighting control for theatre and television at http://www.dovesystems.com ________________________________________________________________ GET INTERNET ACCESS FROM JUNO! Juno offers FREE or PREMIUM Internet access for less! Join Juno today! For your FREE software, visit: dl.http://www.juno.com/get/web/. -- http://www.piclist.com#nomail Going offline? Don't AutoReply us! email RemoveMElistservTakeThisOuTmitvma.mit.edu with SET PICList DIGEST in the body

When you work on a boat the humidity tends to be high. If you seal a box with humid air in a warm lab, and then put it in the cold ocean you get condensation. Drawing a vacuum removes the humid air and prevents the condensation. Also trying to draw a vacuum tells you if you did something stupid like leaving a drain plug out. But vacuum introduces problems of it own. Filling the box with liquid means that if the sides of the box bow a little bit, the outside pressure is transmitted to every component in the box. Semiconductors in metal cases or with EPROM windows can be damaged by high pressure. Electrolytic caps often fail. Some resistors are hollow and have been known to implode. Etc... The best thing is to draw the vacuum, then refill the box with nitrogen (cheap & nearly inert). We used to refill with Freon. Sherpa Doug > {Original Message removed}

On Fri, 15 Feb 2002 10:51:39 -0500 Douglas Butler <dbutlerEraseME.....IMETRIX.COM> writes: > You are very right. Even a small fan does wonders for cooling. Most > of > my stuff is in small sealed boxes so there is little or no air > movement. > I love it when the customer draws a vacuum on the housing to > "prevent > moisture problems" and the electronics cook to death in the > resulting > Dewar flask! > > Sherpa Doug When putting stuff in a sealed box (I do this quite a bit), heatsink to the box! (doesn't work very well with a plastic box...) Harold FCC Rules Online at http://hallikainen.com/FccRules Lighting control for theatre and television at http://www.dovesystems.com ________________________________________________________________ GET INTERNET ACCESS FROM JUNO! Juno offers FREE or PREMIUM Internet access for less! Join Juno today! For your FREE software, visit: dl.http://www.juno.com/get/web/. -- http://www.piclist.com#nomail Going offline? Don't AutoReply us! email EraseMElistservmitvma.mit.edu with SET PICList DIGEST in the body

> When putting stuff in a sealed box (I do this quite a bit), > heatsink to > the box! (doesn't work very well with a plastic box...) But there is a marked improvement in having good thermal contact between the internal heatsink and the plastic box. Rth of Junction -> heatsink -> plastic -> air is less than Rth of Junction -> heatsink -> still air -> plastic -> air Steve. ====================================================== Steve Baldwin Electronic Product Design TLA Microsystems Ltd Microcontroller Specialists PO Box 15-680, New Lynn http://www.tla.co.nz Auckland, New Zealand ph +64 9 820-2221 email: RemoveMEstevebEraseMEEraseMEtla.co.nz fax +64 9 820-1929 ====================================================== -- http://www.piclist.com#nomail Going offline? Don't AutoReply us! email RemoveMElistservspam_OUTKILLspammitvma.mit.edu with SET PICList DIGEST in the body

----- Original Message ----- From: Steve Baldwin <RemoveMEsteveTakeThisOuTspamTLA.CO.NZ> To: <EraseMEPICLISTspamspamBeGoneMITVMA.MIT.EDU> Sent: Tuesday, February 19, 2002 3:00 PM Subject: Re: [EE] TO-220 heatsinking {Quote hidden}> > When putting stuff in a sealed box (I do this quite a bit), > > heatsink to > > the box! (doesn't work very well with a plastic box...) > > But there is a marked improvement in having good thermal contact > between the internal heatsink and the plastic box. > Rth of > Junction -> heatsink -> plastic -> air > is less than Rth of > Junction -> heatsink -> still air -> plastic -> air > or, like in a proto of mine once: Junction -> Heatsink -> Plastic --melting (sublimating?) ->air becomes: Junction -> Heatsink -> hole ->air :') -- http://www.piclist.com#nomail Going offline? Don't AutoReply us! email RemoveMElistservKILLspammitvma.mit.edu with SET PICList DIGEST in the body