Author Topic: Small Oven Controler For Voltage Reference  (Read 12726 times)

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Offline blackdog

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Re: Small Oven Controler For Voltage Reference
« Reply #25 on: April 28, 2018, 03:47:10 pm »
Hi,

Are you all still there?  :-DD

This afternoon I measured the four thermistors, the oven was on the LAB table for an hour without having been touched.
That's why I was almost certain that the temperature of the oven would be even.
The value I measured of the four thermistors were these: 2x 115K1 and 2x 115K4 and this is very good, nothing to complain about.

Selecting the MOSfets
In this way I have selected four out of eight MOSfets,
I used the two remaining thick aluminium cabinets which I had, as a heathsink.
There are many holes in it, so it was easy to mount four per box quickly.



And this is the schematic used to measure Ugs at 50 and 200mA drain current.
First i used the Bryman BM869s hand DMM for measuring the drain curren, but it's a drama to read that when you vary the Gate voltage.
This meter is so slowwwwww when measuring current, so i switcht to my Fluke 287, no problemo!



Now the MOSfets are mounted and the Drain in solderd to a soldering lip, why? the MOSfet is in fact mounted without insulation...
Do not fool yourself, I measured all four MOSfets, two measured less than 0.05Ohm to the oven body, but the other two had a resistance of more than four Ohms.
You can be sure that if the resistance is high, you have used too much heathsink compound.  ;D
On some pictures you see dust particles, i will clean this later.



I also pay attention to the way of assembly, like here, the screws are just long enough, no coat hooks in my oven!



The next step will be to assemble the electronic parts, I'll keep you informed!

Kind regards,
Bram
“Two things are infinite, the universe and human stupidity, and I am not yet completely sure about the universe.”
 

Online David Hess

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Re: Small Oven Controler For Voltage Reference
« Reply #26 on: April 28, 2018, 09:36:23 pm »
And this is the schematic used to measure Ugs at 50 and 200mA drain current.
First i used the Bryman BM869s hand DMM for measuring the drain curren, but it's a drama to read that when you vary the Gate voltage.
This meter is so slowwwwww when measuring current, so i switcht to my Fluke 287, no problemo!

This measurement is a lot easier to make with the gate tied to the drain and a power resistor, say 10 ohms, in series with the source.  Adjust the power supply for 50 or 200 milliamps (or measure 0.5 or 2 volts across the 10 ohm source resistor) and then read Vgs.

In a production environment, I would use an operational amplifier to drive the gate and force the drain current to whatever is required.
« Last Edit: April 28, 2018, 09:37:58 pm by David Hess »
 

Offline blackdog

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Re: Small Oven Controler For Voltage Reference
« Reply #27 on: April 28, 2018, 09:55:48 pm »
Hi David,  :)

The reason I did it in this way is that I wanted to stay as close as possible to the real-life situation.
The drain voltage also determines the current through the MOSfet, and at around 3.5V (~200mA) it is not de drain voltage de oven will work at.
The MOSfet is used at the point where it is just starting to conduct, so I wanted to be a little sure that I would measure the good value.

Yes, with a opamp it all goes a little bit better, but measuring the 8 Mosfets didn't take more than 15 minutes together with the mounting on the boxes.
The potmeter with the 9V battery, I had already prepared, because I had it already used it for some other projects.

Kind regards,
Bram
“Two things are infinite, the universe and human stupidity, and I am not yet completely sure about the universe.”
 

Offline blackdog

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Re: Small Oven Controler For Voltage Reference
« Reply #28 on: April 29, 2018, 09:35:18 pm »
Hi

Today I wanted to show you some of my oven developments in a new topic, maybe you could use that for inspiration.  :)
But, ^@#%$%^#$%@#%#$

I was soldering the last parts to the oven that I have been working on in the last few days and was doing the first tests.
What I measured and saw did not feel good, there was "mental" behavior of this oven.
I have quite some experience in building this kind of oven, and I started by watching how the current drops as the oven warms up.

If the oven is close to the final temperature, the oven should start using less power quickly and should not become completely powerless.
If the current becomes almost "0" then the loop compensation is not good, that is one problem.
But with this oven I also connected a scoop channel to the output of the uA723. w.t...
A lot of "random" noise, lookt like the infamous popcorn noise, forgot to make a scoop picture of this.

But first a picture when the oven "was" ready.
I didn't have any more 0.1% resistors of the good value, so here are 1% resistors used this week the good resistors come back in here.


Without looking at my test setup, the noisy oven had stopped working.
At the bottom you can see that there are always bursts of current and at the end the maximum current and then nothing anymore....
The oven stopt working, i search myself silly...
Everything at the inputs seemed all right, also the reverence voltage was present with its 7.05V.
The +input of the uA723 also had nicely half the reference voltage as it should have been.
Only the output of the IC it has a voltage of about 1.7V, and then the MOSfets never conduct.


Noise at the output of the uA723 and an output that is far too low in voltage.
It must be a defective uA723, a pity of the neat wiring....
Out you go!



And I've soldered a new one in, "bad language" exactly the same behavior...
Grrr.
I had another hp 34401 on the bench and soldered one side of the thermistors off
and left the other side connected to the circuit and tried to measure the resistance of the thermistors.
Very strange results I got at the resistance measurement.
In the end I cut all four thermistors loose and measured them separately.
When I started measuring the resistance connections to the oven housing, the monkey came out of the sleeve. (This is a Dutch expression)
There was quite a lot of "leak" present to the aluminum of the oven.
This is the first time I have experienced this, when i mount/glue sensors in aluminium.
It's also the first time I've used the glue I've shown for this application.

To make a long story short, I have drilled out all four thermistors and placed four 5K thermistors in series so that I can all use 10K 0.1% resistors at the uA723 inputs.
4x5K makes 20K at 25C and at about 42C the total of the thermistors wil be about 10K.
This time it's not an ebay thermistors but beautiful from Vishay 5K and 0.5% not too expensive and very good.

The current is now nice and stable and the oven works fine, noise is gone.
The wiring is not as neat as it was, but it works fine.
I have to tune the loop just a little more for optimal performance.
The thermistors are put in the holes with termal compound, later i wil blok the hole with some hobby glue.



I can say that this was a learning day...  :-DD

Kind regards,
Bram
« Last Edit: April 29, 2018, 09:42:57 pm by blackdog »
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Offline blackdog

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Re: Small Oven Controler For Voltage Reference
« Reply #29 on: April 30, 2018, 10:57:44 am »
Hi all,

Again some steps in the development of this oven.
I would like to show you what you all encounter when developing a piece of measuring equipment.
That's why I'm doing a lot of postings here now, just a few photos and it has these specifications, says little about the problems you encounter when building a project.

This is the modified schematic with now four thermistors and the adjusted value of the bridge resistors.
Because of this configuration with the four 5K thermistors, the other bridge resistors can all be 10K.
The four thermistors in series also result in a value of 10K at an oven temperature of approximately 42C.
So I'm not interested in a precise oven temperature, around 42C is good to about 35C LAB temperature, if you don't put the oven in a warm place.



The loop compensation is now also set for the adjusted impedance at point 4 of the uA723.
It is possible to further increase the control speed, the loop is now a little over compensated.
But I'm not going to change that until the lids are on the oven and a PCB with the voltage references is mounted.


The upper meter shows the current flowing through the oven, the measuring time is approximately 11.5 hours.
The peak current in the beginning is on purpose, I removed the insulation for a moment to be able to see if the current through the oven behaves properly.
The bottom meter shows the deviation of the other oven  i still measure, it is the second oven I showed in this topic.
The suppression of the LAB temperature for this second oven I estimate about 130x with the current insulation, nothing to complain about and more than enough for voltage references.


This is the current trough the oven coming up to temperature, very controlled it runs to the desired value, no over or undershoot.



I deliberately don't show a temperature chart of this oven yet.
This because it is much more important to get the current through the oven stable, by adjusting the loop compensation.
Later this week i hope to have 2 DMM free so i can show the oven temperature and the oven current on the same time.
I use 2x the Keysight 34461A and the horrible BenchVue software for that  :-DD

Later more...
Bram
“Two things are infinite, the universe and human stupidity, and I am not yet completely sure about the universe.”
 

Offline blackdog

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Re: Small Oven Controler For Voltage Reference
« Reply #30 on: April 30, 2018, 06:59:31 pm »
Hi,

Just a small side step, what could you mount in one of the ovens as a voltage reference...
A selcted 1N829A, LTZ1000A, APEX VRE310AD, AD588JQ, LT6655? no... i am aging 14 pieces of LT1021CH for more than 3 years now.
And i want to use 5 to 8 pieces of them in a oven.

Below is one of the first examples of how the circuit will be constructed.
It uses 5x the LT1021CH in parallel, then a times two amplifier, this will all be in the oven, everything in the right box, will be mounted on the banana sockets (Pomona)
Because its a working reference, you need some protection and i added that on the banana sockets.
I'm not sure how I'm going to do the trimming, the schematic shows a trimpot, but maybe I am going to use an i2c DAC with a very small trimming range, that is optically linked via i2c.
It is still a work in progress...


Shoot @ it!

Kind regards,
Bram

“Two things are infinite, the universe and human stupidity, and I am not yet completely sure about the universe.”
 
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Offline zhtoor

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Re: Small Oven Controler For Voltage Reference
« Reply #31 on: April 30, 2018, 08:10:24 pm »
Hi,

Just a small side step, what could you mount in one of the ovens as a voltage reference...
A selcted 1N829A, LTZ1000A, APEX VRE310AD, AD588JQ, LT6655? no... i am aging 14 pieces of LT1021CH for more than 3 years now.
And i want to use 5 to 8 pieces of them in a oven.

Below is one of the first examples of how the circuit will be constructed.
It uses 5x the LT1021CH in parallel, then a times two amplifier, this will all be in the oven, everything in the right box, will be mounted on the banana sockets (Pomona)
Because its a working reference, you need some protection and i added that on the banana sockets.
I'm not sure how I'm going to do the trimming, the schematic shows a trimpot, but maybe I am going to use an i2c DAC with a very small trimming range, that is optically linked via i2c.
It is still a work in progress...


Shoot @ it!

Kind regards,
Bram

hello Bram,

how about a low pass filter (maybe bootstrapped) before pin 3 of ltc2057?

best regards.

-zia
 

Offline blackdog

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Re: Small Oven Controler For Voltage Reference
« Reply #32 on: April 30, 2018, 08:18:45 pm »
Hi zhtoor,

You mean something like this?



Kind regards,
Bram
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Offline zhtoor

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Re: Small Oven Controler For Voltage Reference
« Reply #33 on: April 30, 2018, 09:09:03 pm »
exactly, only place some suitable low-value (0.01 - 0.1uf) caps in parallel with 330uf ones for taking out fast pulses if any.

best regards.

-zia
 

Offline blackdog

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Re: Small Oven Controler For Voltage Reference
« Reply #34 on: April 30, 2018, 09:29:38 pm »
Hi zhtoor,

Where should those fast pulses comming from?
Have you done some measurements on  good 330uF capacitors? these capacitors have a very low impedance over a wide range of frequencies.
And there's nothing in this circuit that generates fast pulses.
There is also a good film capacitor included (C8) at the +input of the LT2057.

I can tell you from a reliable source, that this circuit works very well, it has been my 10V LAB reference for about four years now. (change it a little about 3 years ago)
After my HP 3458A was calibrated again this january, the reference had measured no more than 3PPM in three years.
The LT1021 used in this design, where at least two years aged, before it was included in this reference, it is in a oven of about 43C.
Later this year it wil be a double oven, but is have at the moment no extra time to spare for this project.

Kind regards,
Bram
“Two things are infinite, the universe and human stupidity, and I am not yet completely sure about the universe.”
 

Offline zhtoor

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Re: Small Oven Controler For Voltage Reference
« Reply #35 on: April 30, 2018, 09:38:28 pm »
sorry i did not see the 100nf film cap.

choppers are bad-boys when it comes to generation of pretty fast charge-injection pulses at the inputs,
according to MisterDiodes, (who has measured these).

start at page 18 of:-

https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=1&ved=0ahUKEwjc8bbJ-uLaAhXEFSwKHaf0DH0QFggkMAA&url=http%3A%2F%2Fe2e.ti.com%2Fcfs-file%2F__key%2Ftelligent-evolution-components-attachments%2F00-14-01-00-00-70-21-03%2FChopper-Noise.pdf&usg=AOvVaw2AKCrO2oJodvZcQSG5mzV5

best regards.

-zia
 

Offline blackdog

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Re: Small Oven Controler For Voltage Reference
« Reply #36 on: May 01, 2018, 05:32:49 am »
Hi zhtoor,  :)

I hadn't seen that document about chopper opamps yet.
The problems with the charge injection do not occur in my setup because of the low impedances and the extra low pass filter that is set up around the opamp.

Or at least, I can't measure them and never noticed them in my LAB reference.
In any case, I will go through the document carefully.

Thanks

Bram
“Two things are infinite, the universe and human stupidity, and I am not yet completely sure about the universe.”
 

Offline chickenHeadKnob

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Re: Small Oven Controler For Voltage Reference
« Reply #37 on: May 01, 2018, 09:42:22 am »
I don't see the point of starting with 5 volt refs and ending up with 10 volt output. LT1021 is available in 10V versions, you could parallel those and have one less source of drift.
 

Offline blackdog

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Re: Small Oven Controler For Voltage Reference
« Reply #38 on: May 01, 2018, 10:02:48 am »
Hi chickenHeadKnob  :)

Maybe you can understand if you know, that I have at least 30 LT1021CH 5V and most of them are now well aged :-)

And there is another point that may be also important to others users, when they want to power the circuit out of a 12V battery.
The LT1021CH-5V is always supplied with a sufficient and clean power from the output of the reference circuit.
This is not posible with a LT1021CH-10V
Q1 J310 makes the circuit act as a LDO.

The best setup with a LT1021 is a 7V version, but you will need stange values resistors.

Do not forget that a LT1021-10V needs at least 14  a 15V power supply to be in the sweet spot.

But be completely free to do it the way you want it to.

Kind regards,
Bram
“Two things are infinite, the universe and human stupidity, and I am not yet completely sure about the universe.”
 

Online David Hess

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Re: Small Oven Controler For Voltage Reference
« Reply #39 on: May 01, 2018, 10:23:28 am »
The problems with the charge injection do not occur in my setup because of the low impedances and the extra low pass filter that is set up around the opamp.

Or at least, I can't measure them and never noticed them in my LAB reference.
In any case, I will go through the document carefully.

Chopper noise is a problem in high impedance applications and where the noise at the input can affect the source.  Usually neither is a problem because chopper wideband noise is high so filtering is used to limit bandwidth anyway.

The big problem I remember from before engineers figured out how to use them effectively was intermodulation producing idle tones and DC offsets.  Preserving the offset voltage drift is not easy either.

 

Offline chickenHeadKnob

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Re: Small Oven Controler For Voltage Reference
« Reply #40 on: May 01, 2018, 10:26:29 am »
Ok, I understand now, thank you. Those are not the engineering choices I would make. I can see 12v battery operation would also then rule out putting two 5v refs in series since you have lots of those. I would have instead gone  to 15 - 18 v BAT. From what I understand of Mr. Diodes posts the laser trimmed onchip resisters in the LT1021 are subject to more aging drift than the best quality wire wounds or Caddoc TaN types and you are stuck with those in either the 5V and 10V chips but not the 7.xx.
 

Offline blackdog

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Re: Small Oven Controler For Voltage Reference
« Reply #41 on: May 01, 2018, 10:35:09 am »
Hi chickenHeadKnob,

My LAB reference with 4x LT1021 dit not drift more than about 3PPM in 3 years, if i believe my calibrathed Agilent 3458A  :-DD

Kind regards,
Bram
« Last Edit: May 01, 2018, 10:57:09 am by blackdog »
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Online Andreas

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Re: Small Oven Controler For Voltage Reference
« Reply #42 on: May 01, 2018, 02:56:33 pm »
the laser trimmed onchip resisters in the LT1021 are subject to more aging drift than the best quality wire wounds or Caddoc TaN types and you are stuck with those in either the 5V and 10V chips but not the 7.xx.

Hello,

that is only part of the truth.
All epoxy packaged resistors will have some kind of humidity sensitivity (seasonal changes)
which you do not have on resistors within a metal can reference.

So the weak point of the cirquit are the resistors R12+R17.
But even those seem to cancel out as Bram has measured.
3 ppm in 3 years is a very good value for a reference.

with best regards

Andreas
 

Offline blackdog

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Re: Small Oven Controler For Voltage Reference
« Reply #43 on: May 01, 2018, 03:08:10 pm »
Hi,

I had the precision resistors deleverd today, so it was time to do some soldering  :)
This are the resistors i mounted today in the bridge circuit of the oven.
These are Vishay PTF56, 10K, 0.1% and 10PPM, i orderd them from Farnell.
Of course you can also use "normal" 10K 1% resistors, i dit this many times, but sorry, i like good/stable performance.  :box:



I regularly test the parts I'm going to use, and I've done that with these resistors as well.
The resistors are measured with one of my two TEK DMM 4050 multimeters on the four-wire mode.


This is a picture that of the first test with the new resistors in the orange circle.
In the two yellow circles you can see that I applied the first layer of plastic glue to close the hole of the sensors.
I also changed the connections of the thermistors a bit, so that the wiring doesn't floats anymore.



The termistor wiring en closing the hole is better visible on this picture.



This is a picture of a KeySight 34461A DMM that measures the current through the oven.
At A I was measuring the voltages of the bridge circuit, this caused interference in the image.
I turned off the oven but let the measurement run,and I thought, let me try not to apply any loop compensation...
I managed to do that at a number of very small ovens, later on more on that in a different topic.
At point "B" is therefore visible that this oven really needs loop compensation, the current went all the way to 0 and back to maximum etc, etc., which of course is not desired.
At point "C" the loop is stable, but also a bit slow, further I would like to use as few electrolytic capacitors as possible in circuits.
So I have replaced this capacitor with a 4.7uF film type and the series resistance I have increased to 1M, TADA!
Beautifully quickly the current drops without any form of aberration, I have already said that I do not like to go to the limit, especially not because the lids still have to be fitted and there also needs to be a PCB in the oven and wiring of the voltage reference.
All of these have a thermal mass and partly determine the loop compensation.



This picture is to show how "quiet" the loop is, the oven currently has two layers of insulation.
The temperature is approximately 41.3C.
This picture shows the variation of the current through the furnace after pressing the meter zero in a measurement that took a little longer than half an hour.
The fact that the current still goes down does not mean that the oven is not at temperature, this is because here in Amsterdam the sun has finally started shining again.
This morning it was really cold for the beginning of the month of May.
But much more important is that in this chart no noise at all is visible, task accomplished.  ;)



This is an update of the schematic, the new components value are now in there.



I will leave the oven on for another day and then glue the four thermistor holes again with plastic and the electronics will be coated with Conformal Coating.
Remarks, I like to hear them!

Kind regards,
Bram

“Two things are infinite, the universe and human stupidity, and I am not yet completely sure about the universe.”
 

Offline Wolfgang

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Re: Small Oven Controler For Voltage Reference
« Reply #44 on: June 30, 2018, 11:53:10 am »
Hi,

just a suggstion: instead of the wobbly small thermistors glued into something you could use a Pt1000 sensor. Its perfectly stable and can be screw-mounted to the surface to be measured.

Regards
   Wolfgang
 

Offline blackdog

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Re: Small Oven Controler For Voltage Reference
« Reply #45 on: June 30, 2018, 02:20:26 pm »
Hi Wolfgang,  :)

The thermistors are there by design...
The uA723 has a low openloop gain and i dit not want to use modern MOSFets for some extra gain.
The whole schematic is now in balance, special the version made with the square aluminium tube and the 4 thermistors.
Almost no loop compenation necessary.

The version made from the old HF cabinet needs much more compensation to become stable due to the large thermal mass and the single Thermistor.

The last few weeks I have been testing several ovens, this takes a lot of time...  :)

Regards,
Bram
“Two things are infinite, the universe and human stupidity, and I am not yet completely sure about the universe.”
 

Offline Wolfgang

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Re: Small Oven Controler For Voltage Reference
« Reply #46 on: July 01, 2018, 07:00:46 pm »
All thermal stuff is never fast when it comes to testing it.
Much success !
 

Offline cellularmitosis

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Re: Small Oven Controler For Voltage Reference
« Reply #47 on: July 03, 2018, 11:21:35 pm »
Thanks for this thread, blackdog.

I'm planning on building an oven based on this design over the July 4th holiday.  I started by drawing up a single-fet version of your design.  I modified the R/C values a bit to see if I can get away with an all-ceramic capacitor design.  I'll have to tweak those values for a Hammond 1590B case.

Edit: marking the schematic to indicate non-working status.
« Last Edit: July 07, 2018, 11:35:36 pm by cellularmitosis »
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Online David Hess

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Re: Small Oven Controler For Voltage Reference
« Reply #48 on: July 04, 2018, 01:00:31 am »
Is frequency compensation between the output and inverting input useful enough to include when the transconductance output is available for frequency compensation to ground?
 

Offline eurofox

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Re: Small Oven Controler For Voltage Reference
« Reply #49 on: July 04, 2018, 05:08:07 am »
I build as well an oven control system based on the idea of kj7e design and reach stability of +/- 0.1°C at 35°C.

It is a simple design easy to tune, use less than 2W to heat a Teco box none isolated, isolation will be done of course when I finalise the LTZ1000 project that will work on battery.
eurofox
 
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