Hi,
And now, for something completely different... (Monty Python)
Lets call it the:
Python ReferenceThis topic is about a small oven, it will house a voltage reference with two LT1236 IC's.
This wil not be a Ultra reference! just one to see what is posible.
The case comes from the dump store, it contained a beautiful GHz oscillator, which I demolished...
OK first picture, inside the box, removes all components.
![](http://www.bramcam.nl/NA/NA-Dual-LT1236-Oven/NA-Dual-LT1236-Oven-03.png)
Bottom of the box.
![](http://www.bramcam.nl/NA/NA-Dual-LT1236-Oven/NA-Dual-LT1236-Oven-04.png)
Made a small piece of thin circuit board, the reference will be build "Manhatthan Style".
![](http://www.bramcam.nl/NA/NA-Dual-LT1236-Oven/NA-Dual-LT1236-Oven-05.png)
The holes for the NTC resistors are drilled and the NTC are glued with WLK30.
![](http://www.bramcam.nl/NA/NA-Dual-LT1236-Oven/NA-Dual-LT1236-Oven-06.png)
Work in progress, unfortunately it never stays so neat...
![](http://www.bramcam.nl/NA/NA-Dual-LT1236-Oven/NA-Dual-LT1236-Oven-08.png)
uA723 on its back...
![](http://www.bramcam.nl/NA/NA-Dual-LT1236-Oven/NA-Dual-LT1236-Oven-09.png)
Firts stability test, bad, bad, bad....
This is the oven current on the display of the DMM.
![](http://www.bramcam.nl/NA/NA-Dual-LT1236-Oven/NA-Dual-LT1236-Oven-10.png)
It is working now, the large capacitor, it will end up with in a different place, just for testing on this spot.
![](http://www.bramcam.nl/NA/NA-Dual-LT1236-Oven/NA-Dual-LT1236-Oven-12.png)
NTC to measure the temperature in the box (5K connected to a 34461A)
![](http://www.bramcam.nl/NA/NA-Dual-LT1236-Oven/NA-Dual-LT1236-Oven-11.png)
Closed box.
![](http://www.bramcam.nl/NA/NA-Dual-LT1236-Oven/NA-Dual-LT1236-Oven-13.png)
Not made for this oven, but nice to have, a polystyrene box.
![](http://www.bramcam.nl/NA/NA-Dual-LT1236-Oven/NA-Dual-LT1236-Oven-17.png)
This is the current through the oven, there is no oscillation visible!
![](http://www.bramcam.nl/NA/NA-Dual-LT1236-Oven/NA-Dual-LT1236-Oven-15.png)
The power used, by the oven, LAB Temp ~22C
![](http://www.bramcam.nl/NA/NA-Dual-LT1236-Oven/NA-Dual-LT1236-Oven-16.png)
The temperature shift after power on.
![](http://www.bramcam.nl/NA/NA-Dual-LT1236-Oven/NA-Dual-LT1236-Oven-14.png)
It was difficult, to get the oven work properly...
I dit my best, to get sensor and the powersource (2xTIP3 close together to minimise the lagise the lag.
I used all kinds of compensation, nothing works...
This is how i started, the first schematic, do NOT build this one.
![](http://www.bramcam.nl/NA/NA-Dual-LT1236-Oven/NA-Dual-LT1236-Oven-01.png)
To keep it short, there was to much gain in the circuit.
I replaced the FET's for BJT's TIP31.
Repaced the resistor at the output of the uA723, made the 3.3 Ohm resistors 4.7 Ohm
Still to much gain, better behavior...
I had to lose some gain, but how?
Looking at the internals of the uA723, the only option was pin13, the AC compensation pin.
This is a Pull Up current source, and has a high impedance, a good place to ad a parallel resistor to power supply.
This lowers the gain that is available, BINGO!
After a couple of resistors to trie, the oven control work well.
Except for the power supply, there are no capacitors which determine the stability, they are not necessary.
![](http://www.bramcam.nl/NA/NA-Dual-LT1236-Oven/NA-Dual-LT1236-Oven-02.png)
This is the working circuit, the magic is in R6, a 330K resistor, controling the gain.
I have chosen the resistance value of R6, that the ovencontroller regulated, without overshoot.
A few points ....Ovens must as voltage references, aging a little.
Ovens for voltage references, do not need to have a stability such as for Reference oscillators 0.05C is enough.
For a simple oven, you wil get between 25 and max 200 times reduction of the temperature change, 200x reduction, is already art
There is much to tell about how your ovens build, but I have no more time today.
Nextime!
Kind regarts,
Blackdog