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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