On 6/30/13 12:35 PM, Bob Stewart wrote:
"I believe the original problem was that the raw unregulated voltage may
be marginally too high for a conventional three-terminal to take safely"
Hi Ed,
Not really. The voltage is in line with the product specs for a 7812 (35V
max), as is the current I had projected (a bit over 1A). However, the voltage
difference (20-12) times the current (assumed to be 1A) is not within the
dissipation ability of a small free-air heat sink. When I hooked things up for
a test, the 7812 immediately went into overheat and started reducing the output
voltage to compensate. It would be OK (according to the datasheet) if I bolted
it to the chassis, but at my original post, I had not worked out the actual
current load through the device. And it turned out that the datasheet I was
using for the OCXO had overstated the current draw by some 30%. After working
out what my actual needs were, I compared that to what HP was heat-wasting in
the 37203A PSU, and realized that my needs were smaller. The collective
quickly convinced me that a switching/bucking device would be too noisy, so
I've decided to use a 78S12CT,
which is a TO-3 cased 12V regulator, to pull down the 20V I have available to
me.
So, essentially, I didn't know what I was doing, as I have never done this
before. After understanding how to do this, I decided to get a TO-3 cased
device, which will fit the heat sink available to me in the 37203A case I'm
putting it in. For me, this has all been a good discussion.
The one thing that is missing is how to quantify the heat sink needs for a
linear regulator. Any thoughts? IOW, is there some way to project how many
square inches of heat sink needed for X watts to dissipate?
This is a tricky area. It depends (a lot) on the airflow over the
surface. Check out the applications notes from heat sink manufacturers
like Aavid Thermalloy (http://www.aavid.com) or Wakefield
(http://www.wakefield-vette.com). You can probably find something that
replicates your "plain flat surface" to work with.
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