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'[PICLIST] [EE] TO-220 heatsinking'
2002\02\14@072047 by Trevor Page

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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

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2002\02\14@074421 by Spehro Pefhany

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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"
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2002\02\14@080131 by Alan B. Pearce

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>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.

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2002\02\14@093839 by Douglas Butler

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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}

'[PICLIST] [EE] TO-220 heatsinking: THANKS!'
2002\02\14@103345 by Trevor Page

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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}

2002\02\14@104436 by Kathy Quinlan

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----- Original Message -----
From: "Trevor Page" <EraseMEtrevor.pagespam_OUTspamTakeThisOuTPACE.CO.UK>
To: <PICLISTspamspam_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.

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2002\02\15@031211 by Mike Blakey

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I have a natty little excel spreadsheet for heatsink calcs, If you wish, I will
email you a copy off list.



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2002\02\15@104556 by Michael Vinson

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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.


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2002\02\15@105926 by Douglas Butler

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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

> {Original Message removed}

2002\02\15@111445 by Spehro Pefhany

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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"
spamBeGonespeffspamBeGonespaminterlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com
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2002\02\16@030938 by Peter L. Peres

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>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

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2002\02\19@133748 by Harold M Hallikainen

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       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


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2002\02\19@143011 by Douglas Butler

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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}

2002\02\19@151338 by Harold M Hallikainen

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On Fri, 15 Feb 2002 10:51:39 -0500 Douglas Butler <dbutlerEraseMEspam.....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


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2002\02\19@160453 by steve

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>         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.


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2002\02\19@203539 by Dal Wheeler

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----- Original Message -----
From: Steve Baldwin <RemoveMEsteveTakeThisOuTspamspamTLA.CO.NZ>
To: <EraseMEPICLISTspamspamspamBeGoneMITVMA.MIT.EDU>
Sent: Tuesday, February 19, 2002 3:00 PM
Subject: Re: [EE] TO-220 heatsinking


{Quote hidden}

or, like in a proto of mine once:
Junction -> Heatsink -> Plastic --melting (sublimating?) ->air
 becomes:
Junction -> Heatsink -> hole ->air

:')

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