Heat Sinks for WB was RE: WBO2 testing

[bcroe at juno.com]

That heat sink might need to dissipate up to 5 watts
in normal operation.  However, that can be much 
higher during warmup.  Thats OK for half a minute,
but if you overload it with a huge lamp it could be
tripple power continuously.  

The T906 lamp mentioned by Richard sounds like a 
perfect test load.  It is large enough to test the current
ramp up, but small enough to easily go to voltage
(about 10.4V) regulation.  Since it is about 1/2 the load 
of the sensor heater, running a T906 for a while should
give you an idea if your heat sink is too small without 
immediate overheating.

Once the circuit is working well, I would expect a 3 X 5
sheet of aluminum to do the job; the cover of a project
box.  But very thin (.025") metal will localize the heat; I 
would replace a thin cover with at least a .062" piece, 
1/8" is pretty good.  A few low profile fins can help too.
Remember the regulator is electrically hot, so you must
use mica washers & grease (or your favorite version) to
insulate the regulator.  This keeps exposed heat sinks 
from blowing fuses & circuits.

Everybody is using a 5 amp fuse, right?  3 would be OK.

Another approach (I used) was about a 2" X 3" heavy 
finned heat sink mounted INSIDE, but with plenty of
air slots (see my pictures).  The regulator then can be 
mounted without insulation, it runs cooler.  

The resistor will not need much, except a bit of air flow.
A ceramic resistor will tolerate pretty high temperatures, 
but could damage parts very close to it.

Running a fan works, but is a lot of complication over a
hunk of metal.

Bruce Roe

On Tue, 18 Dec 2001 20:52:58 -0500 Stephen Andersen
<SAndersen at advan-tek.com> writes:
> It is starting to appear that I have significantly 
> underestimated the amount of heat sink required for 
> the WBo2.  I bought some small heat sinks from RS today
> that are finned anodized aluminum guys about 1.5" x .75"
> x .75" or so.  I guess one of these isn't enough huh?
> 
> If that is the case, I am going to have to completely 
> rethink the packaging of the WBo2 and LCD/RS232 display 
> boards.  I definitely have not allowed for enough room in the
> box with larger heat sink(s).
> 
> Any guidance from those that have completed their WB's and 
> have them functioning?
> 
> Can someone point me to a properly sized Digikey heat sink?
> 
> How do you attach the heat sink to the power transistor (I think
> that is what it is, right?) and the big resistor?
> 
> Thanks,
> Steve

> > -----Original Message-----

> > Behalf Of bcroe at juno.com
> > Sent: Tuesday, December 18, 2001 1:35 AM
> > Subject: WBO2 testing
> > 
> > 
> > Measure the voltage across the lamp, and across the
> > 1 ohm resistor.  If the lamp has less than 10 volts (and
> > the LED is out), you are probably running in the curent
> > limiting warmup mode.   The resistor will measure 
> > about 1.25 volt in that case.  
> > 
> > You probably are using too large a lamp.  With a small
> > voltage drop across the lamp and maximum current, you
> > have a large voltage drop (at max current) across the 
> > LT1086.  This heating it up, apparently to the point of
> > thermal shutdown which dims the light.

----- End of forwarded message from owner-diy_efi at diy-efi.org -----
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