Tiny ovens

[Jim Shearer]

In message <970430104338.ZM8378 at devh8.ansys.com>, Steve Jones
<sj2393 at ansys.com> writes
------------------ SNIP ----------------
>Yes.  Well you know, this occurred to me after I posted
>my post.  It should be possible to get the heat from one
>(or two, as you suggest) of the remaining transistors and
>the thermal contact would be excellent.
>
>But I'm a little unclear on one thing.  In what sense
>is one of these a regulator?  Is there still an external
>sensor of some kind?  Could you elaborate just a *little*
>more 8^).

Hi Steve,
         I've crudely represented the temp oven scheme, used by several
designers, below.
  The base-emitter junction of TR1 (part of CA3046) acts as a temp
sensor, buffered by IC1, driving the discrete external TR3. This in turn
sinks current for TR2 (also part of CA3046) acting as a heater. 

                                                  +12v___
             +12v__                                    |
                 |                           Heater   /
                |-|                                 |/
                | |                           0v----| TR2
                | |                                 |\
                |_|                                   \
                 |                                     |
            _____|___________                         |-|
            |               |                         |_|
            |      /- NC    |   |\                     |
            |    |/         |   | \ IC1               /
            |----|  TR1     |---|+ \     _____      |/
                 |\             |   |-|--|___|--|---| TR3
     Temp Sensor   \        |---|- /  |         |   |\ (External)
                    |       |   | /   |        |-|    \
                    |       |   |/    |        |_|     |
                 0v---      |         |    -12v_|______|
    +12v___                 |----| |--|
         |                  |                         Note: All TR's NPN
        |-| Temp Trim       |
        | |<-----------------
        |_|
         |
      0v----
  As the current through TR1 rises with increasing temperature, the
voltage, into IC1+ input, falls. Hence the current into TR3 also falls,
which reduces the current through TR2/TR3, so reducing the temperature
generated by TR2.
(I hope I've gotten the description right:) and this helps describes the
general scheme for you).
  PS. You may have noticed that only 4 TR's of the CA3046's 5 TR's are
used (The two above, and two as a differential pair). The 5th having
it's collector connected to both 0v & -12v via two resistors to prevent
stray current flows.
-- 
Jim Shearer