Taking up space in a project box

[Jeremy D. Grotte]

I'm building a small controller for a project. I've also got a supply of
cases that fit this project very well. It's about the size of a keyfob type
thing, one small 5way tact switch and an RGB LED. Problem is the case is
too thick. The board fits nicely side to side, but sits too far in the case
to be able to move the tact switch around.
I've tried using some foam to take up the extra space, and it keep the board
in there nice and snug and it works great.
But I've also forgot to mention that the controller also has 3 MOSFETs on
the back of the board each dissipating about 1 watt (worst case). The
MOSFETs end up melting a spot in the foam. The MOSFETs get hot, not
terribly hot, but I wouldn't want to hold one in my hand for more than a few
seconds (i.e. spit doesn't sizzle on them!).
Any swell ideas on what I could use to take up the extra space?
I saw the thread on potting cases, but I don't think that's 100% appropriate
for me. I thought about globbing some hot glue on the top of the board and
holding it to the top of the case instead of spacing it up from the bottom.
No good there either as the glue would eventually seperate and leave me with
a push button flopping around.
TIA...
JDG

 

[Barry Lennox]

On Sun, 27 Jul 2003 02:15:13 -0600, "Jeremy D. Grotte"


snip

Any swell ideas on what I could use to take up the extra space?
I saw the thread on potting cases, but I don't think that's 100% appropriate
for me. I thought about globbing some hot glue on the top of the board and
holding it to the top of the case instead of spacing it up from the bottom.
No good there either as the glue would eventually seperate and leave me with
a push button flopping around

What about a piece of blank PCB material, then the foam pad? The PCB
should insulate and/or spread the hotspots so that the foam is
untouched.

Barry Lennox

 

[Tim Shoppa]

But I've also forgot to mention that the controller also has 3 MOSFETs on
the back of the board each dissipating about 1 watt (worst case). The
MOSFETs end up melting a spot in the foam. The MOSFETs get hot, not
terribly hot, but I wouldn't want to hold one in my hand for more than a few
seconds (i.e. spit doesn't sizzle on them!).

What package are the MOSFET's?

If they are TO-220 (or anything with a screw hole) you might bolt them
to a copper block that fills the extra space on the back. Not only
will it serve as a spacer, it will also be a heat sink. (Well, maybe
more of a "cold reservoir", your case doesn't sound like it has a lot
of heat transfer ability...)

You will need some sort of electrical insulator if the MOSFET cases are at
different potentials than each other.

Tim.

 

[Jeremy D. Grotte]

The MOSFET's are SOT-223 (SOT-23 with an extra pin on the 2 pin side). The
case is an SCR4G from Digikey. Plain ol' case, supposed to screw it
together, I dremel off the screw holes and use a dab of glue to hold it
together. No, the case doesn't have any heat transfer ability. I had
planned on drilling a bunch of small holes in it on the top and bottom of
the ends to hopefully get some convection going on to help out. So, far the
worst case scenario (max input voltage, max duty cycle on the mosfets,
practically sealed case, hot room, etc) has proven that the PCB and the
MOSFETs can handle it, although I can feel the mosfet's themselves thru the
case, not too much, just enough.