[volt-nuts] Voltage Reference Mini-Oven

[WarrenS]

It is well known that by changing the nominal operating current of a 1N825 type reference diode, that its voltage turn over temperature point can be moved.
Usually the turn over temperature can be set to the nominal operating temperature,  there by giving a Zero TC reference voltage over a limited temperature range.
For the parts I'm testing, the voltage changes less than 0.1 PPM with a 5 deg F temperature variation, and 1PPM over a 20 deg F range when the zero TC current is set for room temperature.
Not bad, but being an extreme Volt Nut, I always want to do better.

Two ways to make it better, Add an X^2 (second order) temperature compensator or put the reference in an oven.
 
What I did was make a very simple Mini-Oven using an 1N825, a glass thermister & a 1/4 resistor,  
all heat shrink together  in a small package about the size of a 1 watt resistor.
Now by Appling about 1/8 W nominal power to the resistor from a simple temperature controller, the thermistor can be held at a constant Temperature.
This allows setting it up so that normal temperature changes have true ZERO effect on the zero TCed zener voltage.
Next thing I want to make is a simple times 8/5 digital gain amp so that the gain resistors will have no effect when amplifying the 6.25x volt zener  up to 10.000000 Volts.
(or times 10/7 for use with other types of voltage references)

ws