Expo conv. heater (tomg et al.)

[Magnus Danielson]

>>>>> "SV" == Steve Varner <varner at k-online.com> writes:

 SV> I haven't built a discrete VCO yet (just CEM chips in a kit VCO). I want
 SV> to build a 3046/thermostat circuit, but I have a few questions.

 SV> 1. Do you really need a separate power source for the heater part?
 SV> That's a real pain in the rear. You mentioned regulators and I've seen
 SV> separate supplies mentioned on the web... so that's why I want to know.

 SV> 2. Barry Klein suggested putting the 3046 under a styrofoam cover to
 SV> isolate the chip from heat loss or gain - sort of a chip calorimeter.
 SV> Has anyone done this?

Well, I guess someone has, but it makes perfect sence. This is BTW how
you improve crystal oscillators stability. There are even those that
use double ovens, and oven within a oven. When you reduces the static
heat transfer from the chip (by increasing the surrounding temperature
to be near equal) the regulator have less of a figth to do.

Also, one has to put some care into the design, so that you don't get
a oscillating heating pattern, we are talking well-damped to
over-damped here. But then, you migth miss the fun of another source
of incertainty ;)

 SV> 3. How do you know where to set the temperature trim on the thermostat?
 SV> I mean, how do you know where to set the temperature and how do you
 SV> measure a silicon chip's temperature accurately anyway? Not having the
 SV> temp too high or low was discussed, so this is important.

Well, when you put styrofoam over it, the body will not chill of that
much, so you get about the same temperature at the capsule surface
that you have at the silicon once you have stabilized in temperature.
If you look at the datasheets you can find comments on the heat
conduction of the capsule, using this and some other knowledge you can
get pretty close to the heat of the actual chip. You can also use a
transistor as temperature sensor, since there is this nice little
relation between temperature, voltage and current. You could hook up a
diff pair to act as a temperature sensor with compensation for the Rbe
resistance if you want. This becomes pretty accurate actually.

 SV> PS, if you folks lived in San Diego instead of 2 degrees from the Arctic
 SV> Circle you might not worry so much about temperature   ;-)

Well, the cross-country and downhill skiing isn't the same in San
Diego as we cool guys require. This morning we had a freshly warm
breeze of -17 C (that is 1 F) here in Stockholm, but up north they
have a bit chilly with -42 C (-43 F) and so...

Cheers,
Magnus