Hi
It is good to see people are building ovens!
I want to share some points that are important for a good oven.
1e
Use copper (difficult to find boxes) or aluminium, but "real" aluminium!
A round aluminium tube with a 4mm wall is good, but maybe for most of you to difficult to make a oven out of it.
Why, you need good resistance wire and expensive 3M glas tape (#69)
If you are carefull you can use resistance wire and Kapton tape.
The easiest way is to use square aluminium pipe with a wall thickness of 4mm with the four MOSFets mounted directly on the pipe as I have shown in this topic.
You need the minimum 4mm wall thickness because you have to drill the four thermistors behind the MOSFets and you need the at least the 4mm for the good transportation of the heat.
The blue glow at the top is the conformal coating I used.
The heat conduction of the Hammond and Bimboxes are not good and also the wall thickness of almost all these boxes is too thin.
The large temperature differences in the walls of the housing, will create thermic in the oven and you do not want this.
The second problem is that it wil be difficult to get the loop stable.
A good oven has no current and/or temperature oscilation!
2e
If you still want to use Hammond or Bimboxes, then take enough heat sources.
My preference is for the older type of MOSFets such as the IRF140, IRF240, then use 4 or 6 pieces and they are cheap.
Use just like i dit 4 thermistors directly under the MOSFets.
Do this as well as possible, as I have done by installing 4 thermistors directly underneath the MOSFets chips.
You have then mounted the MOSFet directly on the housing on the outside without insulation with thermal paste
and then glue the thermistor on the inside directly underneath the place where the chip is in the MOSFet.
You can use 10 seconds glue, but better is to use a blob of thermal glue over the thermistor, like this.
This is another oven setup for my PT1000 measuring amplifier, here I use Kaptop tape and resistance wire for heating the oven.
But it's about the sensor, which is buried under a layer of thermal glue,
Don't forget to also glue a part of the wiring, this here is an Analog Devices TMP37 sensor.
3e
Each part is carefully defined in this uA723 scheme, thinking you know better if you don't feel like following my directions will almost certainly result in a poorly functioning oven.
It is little like this:
You can make an adjustable power supply with an LM317, but is it not a LAB power supply because you can control it from 1.25V to 30V?
No, it has high internal resistance, high noise, no current control, and so on and so forth.
( I realy like the LM317 and is brother and sisters, but it will never be a good LAB power supply)
4e
For a good oven this is necessary:Good heat conductive material diamond is best
The worse the heat conductive properties are of the oven housing, the more heat sources you need for an even distribution of heat to prevent thermic in the oven.
Tight coupling between the heat sources and the measuring sensor for a stable running controler.
Good thermal isolation of the oven, there are more ways to do this, some examples.
Here i use "air" as a isolation material for the inner oven, only two nylon screws keep the oven in the middle of the outer oven.
Build in PWM controler, i will never do that again, that is asking for EMC trouble.
A different oven, and now the oven is hold inside the polystyreen bij small strips of polystyreen,
so there are no large surfaces of the insulating material that makes contact with the oven.
And a different setup of how to isolate a oven.
This time there is a layer of a product called "Blue Deck" around the oven.
http://www.en.bluedec.nlMake your oven airtight as possible, most Hammond and Bimboxes are not airtight and if they are, the user start to dril holes in it
Think of the heat leak through the wiring.
Build the controler like you are building a high quality voltage reference, so use good bridge resistors en a high quality thermistor.
If you follow these direction, your oven wil stay probably within 0.05C between 20C and 30C ambient temperature over a lang period of time.
But..
A Chain is As Strong As Its Weakest LinkDo you want a better controler than this uA723, than look for "Jim Williams" and "oven" look for document from its MIT time.
Than you wil see i do not talk BS about ovens
The high quality controlers from Jim are unnecessary for modern voltage references or measuring amplifiers.
Do you want to go a little step higher in controler performance, than use a TI INA125 IC.
You can have than a gain that is simple to trim, so you can those between high gain and large compensation capacitor or lower gain and a smal capacitor.
You will have to decide for yourself what the "sweet spot" is for your oven.
Happy building, also if you do it your way!
Kind regards
Bram