Re: How to interpret this package drawing?

On Sun, 06 May 2007 07:00:33 -0500, the renowned John Fields
<jfields@xxxxxxxxxxxxxxxxxxxxx> wrote:

On Sat, 05 May 2007 12:56:46 -0700, John Larkin
<jjlarkin@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx> wrote:

On Sat, 05 May 2007 14:46:41 -0500, John Fields
<jfields@xxxxxxxxxxxxxxxxxxxxx> wrote:

On 5 May 2007 12:18:49 -0700, Michael <nleahcim@xxxxxxxxx> wrote:

Hi - I'm trying to draw out a TO-263-5 from this data***:
http://www.irf.com/product-info/datasheets/data/iru1150.pdf

Thing is - it doesn't actually give dimensions - just minimum and
maximum dimensions. It doesn't even specify the actual width!! What am
I supposed to do here? Take the average of each value?

---
Yup, and set your tolerances so they fall within the minimums and
maximums. :-)

What's stunning is that the nominal lead pitch is not specified.

---
Yeah, it is, but I was wrong about "average"

For the TO-263 package the pitch is the sum of half of the
difference between the max and the min and the min of "G". IOW:

1.829 - 1.575
pitch = --------------- + 1.575 = 1.702mm
2

One nice thing about the GD&T (eg. ASME Y14.5M or ISO) method of
tolerancing (as on the other IRF drawing I linked to) compared to
coordinate tolerancing is that there is no build-up of tolerances. The
theoretically exact "basic dimension" is given and the position of
each pin is specified to lie within a certain tolerance zone. To get
the same effect with coordinate tolerancing you'd either have to put a
stupidly tight tolerance on the pitch, dimension every pin, or add
some kind of note to the drawing.


Best regards,
Spehro Pefhany
--
"it's the network..." "The Journey is the reward"
speff@xxxxxxxxxxxx Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog Info for designers: http://www.speff.com
.

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