Hello all,

I'm a newbie to this forum.  I'm in the IPUG (International PCAD Users Group).
Ray Humphrey of Dynacad suggested you all might be able to help me.  I have a
question for you about how to mount power resistors on a PCB.

I am concerned about the heat being dissipated by a thru-hole mounted power
resistor on a power supply PC board where ambient can be as high as 85C.  The
part I am concerned about is a 1W 56K wirewound ceramic molded resistor.  It's
dissipating .66W worst case.  In past designs, we have placed it up in the air
~0.2" (it's laying horizontally on the PCB) as we noticed slight charring of the
G10 epoxy board - that cures the problem.

I got to thinking, I wonder if there is a standard on this or some guidlines to
follow?

So I looked in my design books but have been unable to find a guide or rule of
thumb for how high off the board a heat generating part should be.

I looked in power resistor catalogs.  The RCD catalog did show a 5W surface
mount wirewound resistor with .05" spacing under it.  I was surprised that the
air gap was so small.

I looked at some commercial power supplies and found that typically the
resistors are mounted end-on which improves the packing density.  We don't like
that kind of mounting.  We also see that some vendors place epoxy power
rectifiers (e.g. EGP50G) as much as .5" away from the PCB.  There seems to be a
variety of solutions.  Perhaps it is whatever works?

I look forward to your answers and suggestions.

Bob Landman
H&L Instruments L.L.C.
Electro-Optics for Industry & Science
34 Post Road, PO Box 580
North Hampton, NH 03862
tel(603)964-1818  fax(603)964-8881

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