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-----
> From: Douglas Butler [SMTP:dbutler@IMETRIX.COM]
> Sent: 14 February 2002 14:29
> To:   PICLIST@MITVMA.MIT.EDU
> Subject:      Re: [EE] TO-220 heatsinking
>
> 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-----
> > From: Trevor Page [mailto:trevor.page@PACE.CO.UK]
> > Sent: Thursday, February 14, 2002 7:19 AM
> > To: PICLIST@MITVMA.MIT.EDU
> > Subject: [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
> >
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