Hi,
On this forum, several oven projects are discussed for heating voltage references.
This is one of my controllers equipped with an LM723 and three Power Mosfets.
These are the requirements for a proper controld oven:Good coupling between the sensor and the heat source.
Good distribution of the heat source over the housing to be heated.
A stable controler.
Good isolation of the heathed unit, best is to use a doubble isolation, two layers of polystyrene with a few millimetres of space around it.
Treat the oven control as if it were your best voltage reference, use good resistors and other components.
If you want to be cheap, you will get a bad oven!
I use "old style" Power MOSfets here the IRFP2410 with a relativ high Rds on resistens, do NOT use modern low Rdson MOSfets or Logic MOSFets.
It wil be troublesome to get is stable with modern MOSfets.
The old style MOSfets are cheap and i chose these also for the large housing, the TO247.
It is posible, if you want to connect the drains to the raw power supply en connect the uA723 to a 78L12 regulator, but i wil not do this.
For maximum stability use a 1,5A regulator like a 7812 or LM317, maximum current will only by drawn for say a few minutes.
At a local flea market I bought three of these boxes for 10€, 50x90x33mm and the bottom is 7.5mm thick, probably used for HF projects.
Here you can see how the IRFP240 MOSfets is mounted isolated on the housing.
There is also an LM35 in TO220 version mounted, which was for an extra test but not easy to use because of the high noise value, it wil be removed.
I don't want to have a galvanic connection between the over circuit and the voltage reference for this kind of project, so the MOSfets are mounted on aluminium oxide plates.
To make the distribution of heat as good as possible, I applied a small selection to a handful of the IRFP240 MOSfets.
I tested at 12V Vds, at 50ma, selected 100mA 200mA drain current and the MOSfets that were within a few percent equal.
I used some stripboard for ease connecting te components around the MOSfets.
Yellow circle is de 5K Thermistor connected to one of my 34461A DMM's i use to measure the oven temperature.
The blue circle is the
10K thermistor for the oven controler and it is directly under the IRFP240 for a tight coupling with the heat source.
The oven controler, to the right of the uA723 you can see the 100uF capacitor with the 100K resistor, which is used to keep the loop stable.
The values of these components depend on how the big housing is, and how well the Thermistor is connected to the heat source, if you try this controler start with these values
and if its stable, make the capacitor smaler until the heather current starts oscilating, make the capacitor now 2x as big, this is a rule of thumb :-)
Red is +12V for the uA723, Blue is de Gate driver, Green is the current sense and 2x white is the 10K Thermistor, and black is ground.
This is the first version, with a different loop controle, on the spot of the trimmer resistor there is now the 100uF capacitor.
Look how i use the space in the IC socket to place the 1uF decoupling capacitor and the connection between pin-3 and pin7.
Room enough for some nice voltage references :-)
This is the temperature drift after the oven has been on for about 15 minutes.
The power used is the oven is stable, arond 1,5-Watt's if the LAB temperature is 23C, the Max. power on startup conditions is at 12V 16-Watt's.
You can play with the source resistors to make it lower or higher is necessary for your oven housing.
My housing needs aboud 0.1-Watts per C.
Things to think about...The drift you see on the picture is from about 1C of dift in my LAB.
It wil, just like a voltage reference needs minimaal 1000 hour to get stable.
Keep is as air tight as you can make it. (mine is not at the moment of these measurements)
Think about heath leakage of the wiring.
Use a stable powersupply for the oven controler
Shoot at it!
Kind regards,
Bram