Small Oven Controler For Voltage Reference

[Kleinstein]

With the rather low frequencies relevant for the thermal setup there is not big difference between having the compensation from the output or the compensation pin (trans-conductance output)  of the LM723.

For the regulation it might be even better to take the slow compensation even further down, from the resistor to ground used to limit the current. this way the nonlinear FET curve and possible windup would have less effect.

[blackdog]

Hi,

First this, all the remarks about the compensation pin of the uA723: "bin there done that" 

Next the schematic of the controler which is built in the thick HF housing with the yellow color.
Because the oven itself has a different thermal mass, the compensation is also different from the oven built of a piece of rectangular aluminum pipe which I also showed here.
Also look at pin-13, it has now a 220pF HF compensation capacitor, it was sometimes oscilating and 100pF was enough, but i doubbled the value to be save..
This oven works wel because of the verry thick walls of the housing, and the small distance between the powersource and the measurements thermistor.


And now we come to what cellularmitosis is trying to build :-)
"You will fail my young panduit" (Yoda) 

The box you ordered is not made of aluminum but of clutter and not of good quality aluminum with a wall thickness of at least 4mm.
And how do you attach the Thermistor to your Hammond box?
There is a error in your schematic, the resistor in de bridge connected to the thermistor is not 10K but about half of this (5K) for about 42C oven temperature.

A oven schematic cannot be seen separately from the oven housing, they belong together.
That is also the reason why I am showing the differences here and also explaining them.

Look at the controler and his compensation voor de square aluminium tube version totaly different.
This schematic and this oven is the best i made with a uA723 controler, and is more than enough to keep voltage references on temperature.


I can only tell you guys to follow my directions to get something usable 
Building a good oven is more complex than building a good Power Supply and everybody think they can build a power supply...

For the uA723 oven controler.
Best is the square tube version with the thermistors drilled in the walls just behind the chip of the MOSFet, you can see it in my pictures of this oven.
Use the components in the schematic, different MOSFets or Thermistors, you are on your own 
Think about the wiring, it is transporting two directions the heat!
Best is to use two layers insulation material, make shure the oven comes only a little in contact with the insulation material.
Second layer also some air arond it and than the second layer insulation material.
Air is a far better isolator than the Polysyreen i normaly use, so that is why less contact between the oven and the insulation material is important.

If you do your best and follow these directives, you can have a oven with a "gain" of 200x that is the temperature in de oven wil change only 1/200C per C.

Can it be better, yes but not much with this controler, than another controler is necessary with direct DC feedback from the power section to get a far higher gain.
But also the isolation, wiring encetera, encetera will need far more attention.

Is this all necessary for a voltage reference or measuring amplifier where i also using is for, that is for you to decide.
My decision is that i do not want to worry about the oven, if there is a good voltage reference in it.
I then choose for a bigger confidence and build it as good as possible with the indicated components.

Designing and building ovens is fun, you can learn a lot from them, so have fun!

Kind regards,
Bram

[cellularmitosis]

Thanks for the feedback, Bram.  Actually, I am building a 25C oven, which is why I used the 10k resistor.

[blackdog]

Hi cellularmitosis,

Do you cool te envirement down to 10C then?
Less then 10C between outside en the set oventemperature wil be extra difficult.

Kind regards,
Bram

[cellularmitosis]

At least for the microcontroller-based PID oven, 3.5C of headroom wasn't a problem.

[blackdog]

Hi cellularmitosis,

I had also good results with a Arduino PID controler.
But... if you have already good results why build this uA723 design?

The same rectangular aluminium oven i want to test with a Arduino PID controler.
The uA723 has not a high openloop gain, but with good design you can work at 5C difference.
But there are so many thing that play a role in your oven stablity, think about the wiring and isolation i talkt about. 

The fact that your oven is set to 25C does not mean that it will continue to do so at varying ambient temperatures, say from 20 to 30 Celsius.
I assume that with a Peltier element you are going to make a colder environment and then put the oven on 25C?
Be aware that you then have two loops that influence each other.

Kind regards,
Bram

[cellularmitosis]

Hmm, perhaps 25C regulation in a 22C environment is not an attainable goal for the LM723.

Currently, I seem to be running into some sort of problem where the current limiting circuit is loading down the inverting input.  I'm not sure what is causing this, but if I disconnect pin 3, the inverting input returns to where it should be.

I just realized I don't have a base resistor on the current limiting transistor.  At one point I managed to have the circuit draw about an amp, while using a 1R current shunt (which should not allow 1 amp to pass).  Perhaps I have damaged my LM723 somehow, and the current limiting circuit is no longer behaving correctly.

Edit: Forgot to respond to this question: The reason I wanted to try the LM723, even though I already had some results with a digital approach, was because I wanted to see if a pure analog approach could work as well, which would be a simpler circuit for hobbyists to construct and possibly also keep any digital noise further away from the circuit.

[kj7e]

Not trying to hi-jack this thread, here is another way to build a very simple and effective analog (low noise) oven controller, adjustable from ~30-40C.

More details in this post;
https://www.eevblog.com/forum/metrology/kx-reference/msg1241349/#msg1241349

[blackdog]

Hi cellularmitosis,

I forgot to mention that most Peltier controler can ad and substract heath, that is a big difference with a lineair controler...

kj7e
Can you show us how you used this controler, some pictures about the mecanics would be nice, please... 
That is then wat kind of box were are the termistors mounted, what kind of wiring dit you use...
Where dit you measure the temperature?

Kind regards,
Bram


 

[kj7e]

Hi Bram, see the link in the post above and here for the oven enclosure details and performance results;
https://www.eevblog.com/forum/metrology/kx-reference/msg1236890/#msg1236890

If I were to build it again, I would add more heating elements for a more uniform thermal distribution and to reduced thermal gradient.

To copy from my post here;
https://www.eevblog.com/forum/metrology/kx-reference/msg1264331/#msg1264331

"Also mounted the heater element pass transistor to outside front of the oven for added efficiently and this also helps heat a third side.  I had some extra 10K NTC sensors laying around so I started experimenting again today and may have made vast improvement.  Currently, the heater resistors are on the ends | ----- | of the oven, one sensor was mounted just offset from the left resistor.  This gave great and stable feedback and the area near the resistor stayed at a very consistent temp.  Where I found some variation is with the internal air temp due to the top, bottom, front and back of the oven not being heated and only relying on the conductive heat from the left and right sides.  Better insulation would help, but there is no more room for more insulation in the box.   So my idea was to place the oven body temp sensor center bottom of the oven,  this surface, while insulated, is in contact with the bottom cover of the box.  I figured this area would be more representative of the internal air temp and the most effected by ambient change.  So far I'm able to hold the internal air temp well within 0.1 dec C from the same 15-30 dec ambient.  The downside is a bit more initial oscillation and time to stabilize, but it seems to be a good trade off.  Ill post of some temp plots and sensor locations after more testing."

Later, I added a second 10K NTC to measure the internal air temp of the oven in series with the oven body temp sensor.   Its the internal air temp I care about.  This creates 50/50 average between the two sensors, the body temp sensor is needed for stability but the air temp sensor helps regulate the internal air temp.

In the photo below, you can see the air temp sensor bent over between the two boards, the oven body temp sensor is mounted to the bottom of the oven below the green board on the left side.

[cellularmitosis]

Thanks Damon.  Was the 3055 also mounted to the aluminum case?

[kj7e]

Thanks Damon.  Was the 3055 also mounted to the aluminum case?

Yes, you can see the TO-220 mounting screw on the left side above the KX board.  In hindsight, I would have placed this more away from the KX board.

[cellularmitosis]

I had been using the LM723 datasheet, but today I happened to take a look at the uA723 datasheet and found its schematic is a bit more approachable.

[cellularmitosis]

Currently, I seem to be running into some sort of problem where the current limiting circuit is loading down the inverting input.  I'm not sure what is causing this, but if I disconnect pin 3, the inverting input returns to where it should be.

Oopsie, I accidentally drew pin 1 as the base of the current limiting transistor.  This should be pin 2.  I effectively had no current limit at all, which explains the strange behavior I've been seeing.   

[blackdog]

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.nl




Make 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 Link

Do 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

[cellularmitosis]

Beautiful!  What did you use to cut the polystyrene?  A hot wire setup?

[blackdog]

Hi,

One of my IT clients is an architectural firm, if I need some insulation for an oven, I ask if I can use their professional Hot Wire machine. 

But... Do you want to see more oven photos?   

Lets start then!

This one is designed to calibrate TO92 temperature sensors, the temperature is set between 40 en 45C and is about 5 years old.
Something went bad, the hole u see has a broken dril in it 
This prevents me from inserting the sensor to be tested deep enough into the hole.
In the left hole is one of the bridge resistors and the second with the yellow wires, is the thermistor.
At the top the loop compensation parts , 47K5 and 220uF old style via pin-13 of the uA723.



I connected this afthernoon the oven to a power supply hanging on the wires in a small polystyreen box for some isolation.
The variation is a few mili-C for 1.5 hours.



This is a design from a few weeks ago, the first test version made according to the latest uA723 schematic.
I glue a very thin piece of double-sided pcb material to the BPR10 resistor with thermic glue.
And build carefuly the electronics on this board, the BD137 is a total plastic version.


This is a close-up, just above the "big bridge resistors" (0.1%) you can see the thermistor glued to the BPR-10.
There is a hole in the thin board so that the termistor is touching the resistor layer of the BPR-10.
This was not good enough for me, the yellow and green wire is for the loop compensation and i do not want them in the circuit!
The green wires are a 5K smd thermistor to measure the oven temperature with one of my 34461A DMM's



Another oven, this one is also fitted with Blue-Deck insulation.



And of course I also tested with Hammond or Bimbox boxes.



Sorry for the bad picture, also 4 a 5 years old, to many IC's used, also a uA723 controler would be better fore this box.



This is the latest, about a week old a test setup to find out if a can use this oven for the traveling 10V reference i'am designing.
Also a BPR-10 resistor but 10 Ohm so the current can be high enough for a 3.7V Li-Ion battery.
What is different in this version?
The BD137-16 is closer to the middle of the BPR-10 resistor and the thermistor is almost in the middle under the blue blob of thermal glue.
Again I made a small hole in the copper so that the thermistor makes a good contackt with the BPR-10 resistance.
Is it now perfect, no, but almost, the current while heating has one wave (about 1,5 second) and then its stabel, everithing is now so tight together i do not need loop compensation components.
The temperature has a little overshoot of 0.3C and then its stable.
Are there any other problems? YES! isolated this oven uses with normal temperatures about 140-mWatt.
The uA723 itself uses 45-mWatt, and with these small ovens, the energy that leaks away via the wiring becomes proportionally greater.
Next week I will find out how I will do the mechanical construction and where I can apply very thin wire to limit the heat leakage,
use 0.05 and 0.1 mm wire to connect some parts of the oven.

On the other side i glue 2x a selected LT1021CH-5V on the BPR-10 resistor, this wil use some extra energy, but also at energy of the LT1021 IC's itself.
This is about 20-mWatt for both IC's and I hope that the additional thermal mass added by the LT1021 ICs, will somewhat increase the total consumption,
so the uA723 can do better its work.
Otherwise I will have to raise the oven temperature from 40 to 45C in order to be able to operate around 30C ambient temperature.



And yes, i have even more test ovens 

Kind regards,
Bram

[cellularmitosis]

Thanks Bram, lots of great inspiration in this thread!   

[kj7e]

My next Vref oven I would build in a double enclosure.  First a steel enclosure for EMI reasons to house the oven controller, per-regulator and a smaller aluminum oven for the Vref and buffer.  I would use a smaller oven and work on heating it on as many sides as I could to minimize thermal gradients.  I would even consider a double oven where the outer does most of the heating, then the inner small step and finer control, this would make it easier to lessen thermal gradients.  However, the KX LTZ1000 board is not very temp sensitive and I would not see any benefit to holding the temp any more stable than what I have now.

[blackdog]

Hi cellularmitosis,

Thanks 

What!!! you need more?

Let me show some big falings...
This is a quad 1N827A setup in a oven, and it is not good, to drifty.
Why, I stopped searching for the problem, it took too much time to find out...
but probably too much heat was leaking through the thick wiring.



The schematic of this oven.



The reference zeners 1N827A.



And how the reverence/oven is mounted in the polystyreen box, a lot of air around it.



The next reference oven is the one of which I am most proud and also the one of which I am most ashamed. 
Just soldered open for you to show how this oven is built up.



Here began one of the many mistakes I made at this reference oven.
At the spot of the white wire was a thick piece of copper wire, this gave when heating this oven a lot of mechanical stress on the assembly.
I have cut away the thick wire here and replaced it with the thin wire,
That made it a lot better, but still not good!
The thick wire was connected to a connection of the APEX reference which is glued to the IRF540 heating transistor.



And this is the schematic.


Side view.



And more



And here you can see that even at the bottom, parts are mounted..
I'am proud of how i could fit al the components in this uMetal box, proof that I can build in three dimensions. 
But a BAD reference!



And the last one, a piece of square aluminium tubing with a e-bay temperature controler set to 0.1C difference.
Garbage man, you can also remove this!



Bey!,
Bram

[cellularmitosis]

My next Vref oven I would build in a double enclosure.  First a steel enclosure for EMI reasons to house the oven controller, per-regulator and a smaller aluminum oven for the Vref and buffer.  I would use a smaller oven and work on heating it on as many sides as I could to minimize thermal gradients.  I would even consider a double oven where the outer does most of the heating, then the inner small step and finer control, this would make it easier to lessen thermal gradients.  However, the KX LTZ1000 board is not very temp sensitive and I would not see any benefit to holding the temp any more stable than what I have now.

It is somewhat ironic that it is only the less expensive references which really need the more expensive oven

However, a better oven could be beneficial for a Vref which also has scaled output, and also for resistance references.

(I've also been thinking it might be neat to have a combined LTZ + resistance reference.  One of the 70k resistors could be replaced with a 100k, a 13k R4 could be split into a 10k + 3k, R5 would be 1k, a 120R R1 could be split into 100R + 20R.  With kelvin connections for each, you could power off the LTZ and use the device as a 100k,10k,1k,100R resistance standard.  Or just 10k if you only need artifact calibration).

[cellularmitosis]

Thanks Bram for also posting some failures -- good lessons for the rest of us!

This afternoon I took a second look at the strange behavior I'm seeing from my LM723.

I took a step back from the oven and breadboarded up a test circuit (using a fixed resistor rather than a thermistor, and using a "dummy" 2N3904 heater transistor).

I used a 10k/10k divider for the non-inverting input.

I used a 4.7k in place of the thermistor (above the inverting input), which simulates a thermistor which is heated up a bit.

We would expect 3.5V at the non-inverting input (7V / 2), and about 4.76V at the inverting input (7V * 10k / 14.7k).  With the inverting input higher than the non-inverting input, we would expect the output of the internal "op amp" to go low, which should shut off the output, and the 2N3904 should not conduct.

I breadboarded this up and probed various points of the circuit, and the results are shown in test1.png (attached).

We see that the inverting input is sitting at 3.28V, which is about 1.48V lower than the expected 4.76V.  This means 0.79mA flows through R1, and 0.33mA through R2, indicating the inverting input is "stealing" 0.46mA.  Further, we see 1.46V at the output, when we expect the output to be "off".

I then tried disconnecting the base resistor of the current limiting transistor and again probed the circuit, the results of which are in test2.png (attached).

We find the inverting input is now at 0.93V, indicating the inverting input is now "stealing" about 1.2mA (1.28mA - 0.09mA), and we also find 0.74V sitting on the base of the current limiting transistor.

So, the "op amp" in the LM723 doesn't seem to behave like a pure op amp.  I'm guessing the issue is that the current-limiting functionality is connected to the inverting leg of the circuit, and there is a strange interaction there.  It seems that using the thermistor in the upper half of the inverting divider isn't workable.

Perhaps we can instead use the thermistor in the lower half of the non-inverting divider.

Any feedback or insight is welcome -- this is new territory for me!

[cellularmitosis]

I switched to a discrete opamp-based oven circuit -- it is looking good! 

More details in the morning

[blackdog]

Hi cellularmitosis,

Yes! teasing time.

I shall push you in the wright direction...
If i see your schematic it is different from my uA723 controler, you have to pay for that! 
And then I mean that you will learn what it can cost to customize a schematic, you will pay in time...

If you use a simple setup with a bjt what will the voltage be on the outputpin of the uA723 and if a look at your last drawings is see 0.11 and 1.46V.
And now learn  look at the datasheet think about pin-9 of the uA723, that will be your problem solver, tell us why.

I hope you are not going to dream about this circuit, have a good night's rest.

Kind regards,
Bram

[cellularmitosis]

ahhhh, now this is all making sense   I ran into a similar regulation problem with my OP07, and quickly realized it was because the output couldn't swing below roughly 1.4V, so I added a reverse-biased zener on the output to solve the problem.

And now I finally realize why there is a strange zener hanging off of the LM723 output, exposed as pin 9...

Thanks for the nudge, Bram 

Edit: hmm, your 4.7k resistor is biasing the output voltage up, which is why you don't have this problem when using a mosfet, is that correct?