Author Topic: Small Oven Controler For Voltage Reference  (Read 8917 times)

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Offline blackdog

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Small Oven Controler For Voltage Reference
« on: April 22, 2018, 08:24:19 pm »
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
« Last Edit: April 22, 2018, 10:14:17 pm by blackdog »
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Offline zhtoor

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Re: Small Oven Controler For Voltage Reference
« Reply #1 on: April 22, 2018, 08:52:11 pm »
hello Bram,

a couple of humble suggestions:-

1. how about removing the controller from heated zone, why subject the controller to temp. cycling?
2. mounting sensors (thermisters) in middle of 4-sides by drilling small holes in the thick plate and inserting the sensors.
3. mounting the heat-sources (iRFP240's) on the 4-faces instead of the bottom to lower the chance of temp. gradients.
4. using accurate pt-100/1000 sensors (at least 1) like the TE-connectivity ones. NB-PTCO-002/006 (farnell.co.uk)
5. schematics?

best regards.

-zia
 

Offline blackdog

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Re: Small Oven Controler For Voltage Reference
« Reply #2 on: April 22, 2018, 09:19:32 pm »
Hi zhtoor,

This is not a perfect oven  :)
It is more to let other people see what is posible with a old nice box with a thick bottom plate.
4 Mosfets on the side, yes thats is posible, how mutch it would be better...
This wil take a lot of measurements.

The best oven will be a diamond spere placed in a good dewar  :-DD

The reason i used a uA723 is because it has all the electronics you need to make a nice controler (not the best controler!)
I have build many differenty ovens, maybe later today i will show some, also a setup with 4 MOSfets on the side of a aluminium square tube with more sensors.
Sometimes i use good Thermistors and sometimes i use PT1000 sensors, depend on how good i want it to be.

I have also a better controler than the uA723, is uses a instrumentation amplifier, but that one is expensive, it is using a INA125.

The controler wil be more stable in the oven.

The schematic wil be placed later, i forgot it, i'am getting old...

Kind regards,
Bram
 
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Offline blackdog

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Re: Small Oven Controler For Voltage Reference
« Reply #3 on: April 25, 2018, 06:51:37 am »
Hi,  :)

I had promised the zhtoor to show some other ovens.
This is a made of rectangular aluminum pipe, 60x40mm and 80mm long, the wall thickness is 4mm.
I drilled two holes in the aluminium under two of the MOSfets in which a 100K thermistor was glued.
This oven uses 4x IRFP140 MOSfet for heating.
The used control unit, is also equipped with a uA723 IC.

But first now the schematic, it is a little different than the first one i showd.
This is using a dual parallel 100K Thermistor.



This picture is a plot of the stability, a small overshoot and than stable, i think i can tune it a little bit better, but I'm not going to do this until both covers have been fitted.
These lids also have a thermal mass and they also change the loop constant.
I also measured  the oven current and there is no sign of instability there.



On the aluminium i glued a one piece of thin double-sided circuit board to make the connection.
And also the uA723 wil be held temperature stable this way.
On the bottom left, is the blue hole that I drilled for the 100K sensor under the IRFP140.
The blue stuf is WLK30 thermic glue.



Sensor hole, sorry for the bad picture.



This is the PT1000 sensor that is connected to one of my TEKTRONIX DMM 4050 multimeters with the 4 wire technology to read out the temperature.
The sensor is fixed with Kapton tape, the connecting wire is also fixed to the aluminium along its entire length.
This reduces measurement errors by dissipating heat through cabling.



Maybe later, some others ovens i played with.

Kind regards,
Bram
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Offline zhtoor

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Re: Small Oven Controler For Voltage Reference
« Reply #4 on: April 25, 2018, 07:02:49 am »
Hello Bram,

i am most grateful for your insights on the subject.
what would you say the temperature stability of this oven would
be when holding say an un-heated LM399 or an un-heated LTZ1000?
say +- 10mK temperature ripple?

best regards and keep up the good work.

-zia
 

Offline blackdog

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Re: Small Oven Controler For Voltage Reference
« Reply #5 on: April 25, 2018, 07:22:27 am »
Hi Zia,

I think a single oven wil not do the trick.
I have also a nother design, i uses a double oven, the outside oven controled by a Arduino on about 38C, PID controled.
and the inner oven linear controled to about 42C.

I do not have the time to show everithing, too busy with work...

A single oven attenuates the ambient temperature between 50 and 200x, better for a single oven is difficult.
there are so many variables that play a role in keeping the oven at the right temperature.

I like it to play with these techniques, but its also rely time consuming.
It is also difficult to readout 0.001C, the noise of the DMM4050 is high when measurening temperature with a PT1000 and a little drifty.

A good 5K thermistor connected to a KeySight 34461A wil give you a higher noise free resolution, but i'am still not shure how mutch drift this meter has...
I have to do a test with a Vishay 5K resistor "S" series or better to see how the 34461A drift on the temperature range.

Kind regads,
Bram
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Offline blackdog

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Re: Small Oven Controler For Voltage Reference
« Reply #6 on: April 25, 2018, 08:20:11 pm »
Hi,

To make things clearer, if you want to build an oven for an unheated LM399 or a LTZ1000, you will have to make a double oven.
It is difficult to make an oven that is better than the one in an LM399 or LTZ1000.
This is because there is a very good connection between the sensor and the heating element.
But if you make the standard circuit for an LM399 or LTZ1000 and put it in an oven, then you have the best of both worlds.
My opinion is that a single well built oven is a nice addition to a well built LM399 or LTZ circuit.

If you need a low noise reference, you can use an unheated LM399 or LTZ1000 in a double oven.
But it takes quite a bit of "skill" to make up for this.


Here some links to a Dutch Forum where i show some oven developpement, use google translate if you cant read Nederlands.

https://www.circuitsonline.net/forum/view/107384#highlight=oven

https://www.circuitsonline.net/forum/view/114717#highlight=oven

https://www.circuitsonline.net/forum/view/133966#highlight=oven

Happy reading  :-DD

Kind regards,
Bram

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Offline blackdog

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Re: Small Oven Controler For Voltage Reference
« Reply #7 on: April 25, 2018, 10:01:19 pm »
Hi,

During my lunchtime I changed the loop constants of the oven with the 4x IRFP140, now the temperature overshoot has disappeared...
And the current through the oven is still stable, this current value is a good indication if the loop is stable.

If the current through the MOSfets goes to almost 0 mA during warm-up, then you know that the loop constant is not yet good enough.

This is the schematic with the different values for the loop controle, C1 is now 47uF and R6 is 221K.



This picture shows that the overshoot is gone...



But again, the loop controle will be different for your oven, play with C1 and R6 and watch the current trough the MOSfets, Bang-Bang is wrong!  ;)

Kind regards,
Bram
« Last Edit: April 25, 2018, 10:46:41 pm by blackdog »
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Offline zhtoor

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Re: Small Oven Controler For Voltage Reference
« Reply #8 on: April 25, 2018, 11:45:39 pm »
Hi,

To make things clearer, if you want to build an oven for an unheated LM399 or a LTZ1000, you will have to make a double oven.
It is difficult to make an oven that is better than the one in an LM399 or LTZ1000.
This is because there is a very good connection between the sensor and the heating element.
But if you make the standard circuit for an LM399 or LTZ1000 and put it in an oven, then you have the best of both worlds.
My opinion is that a single well built oven is a nice addition to a well built LM399 or LTZ circuit.

If you need a low noise reference, you can use an unheated LM399 or LTZ1000 in a double oven.
But it takes quite a bit of "skill" to make up for this.


Here some links to a Dutch Forum where i show some oven developpement, use google translate if you cant read Nederlands.

https://www.circuitsonline.net/forum/view/107384#highlight=oven

https://www.circuitsonline.net/forum/view/114717#highlight=oven

https://www.circuitsonline.net/forum/view/133966#highlight=oven

Happy reading  :-DD

Kind regards,
Bram

thanks again Bram.

please have a look at using the LM399 heater as a die-temp sensing diode for accurate temperature control of the die here:-

https://www.eevblog.com/forum/metrology/a-low-cost-oshw-voltage-calibration-reference-project/75/

now, if this sensor is used to set the oven temperature which is in thermal contact with the LM399
(possibly nine of them with their temp. sensing diodes being averaged) and also holding the attendant
electronics (opamps / resistors etc.)

ie; using a set of LM399's as temperature sensors mounted strategically in the "oven" to make a
lower temperature double oven @ 25 degC, maybe peltier modules would be required for the outer/inner oven.

best regards.

-zia
 

Offline zhtoor

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Re: Small Oven Controler For Voltage Reference
« Reply #9 on: April 26, 2018, 01:40:47 am »
hello Bram,

how about enlarging the tab hole of :-

https://www.digikey.com/product-detail/en/infineon-technologies/BTS282ZE3230AKSA2/BTS282ZE3230AKSA2-ND/4841056

to ca. 4.75mm to insert an LM399 (without the polysulfone cap) and using this contraption as the "inner" oven.

btw. the part can also be used with other ovens with an in-built temp. sensor diode.

or:

a more complex / cheaper way would be to use the source-drain body diode in iRFP240 as an on-die temp. sensor
but the disadvantage would be a discontinuous control with interspersed temperature sampling periods.

best regards.

-zia
« Last Edit: April 26, 2018, 02:43:45 am by zhtoor »
 

Offline blackdog

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Re: Small Oven Controler For Voltage Reference
« Reply #10 on: April 26, 2018, 04:07:39 am »
Hi zhtoor,  :)

I've tried to use the back diode or a MOSfet, it's too complicated.
The Infinion TEMPFet is not usable, the sensor is a thyristor, not a normal diode.

For the two oven design i shown in this topic, it is posibel to use these transistors from ON Semi: NJL3281D with a internal diode.
Datasheet: https://www.onsemi.com/pub/Collateral/NJL3281D-D.PDF
A nice side effect is the large housing of this transistor, it uses a TO-264.

You can't fit a LM399 in the hole, but you can use thermal glue, it fits 6x LM399  :-DD
Here you can see the NJL3281 transistors on some aluminium tubing.


Maybe for some inspiration?

Kind regards,
Bram

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Offline zhtoor

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Re: Small Oven Controler For Voltage Reference
« Reply #11 on: April 26, 2018, 05:28:34 am »
You can't fit a LM399 in the hole, but you can use thermal glue, it fits 6x LM399  :-DD

now that is GOOD  :-+
one can make a heatsink-like adapter out of solid aluminum which houses 9x LM399's (with thermal grease/glue) and fits the TO-264 package neatly.
or a solid square block with 4x these transistors on 4 sides and a cavity milled in it for housing electronics / refs.

best regards.

-zia
« Last Edit: April 26, 2018, 05:42:58 am by zhtoor »
 

Offline blackdog

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Re: Small Oven Controler For Voltage Reference
« Reply #12 on: April 26, 2018, 06:26:19 am »
Hi,

Just to show that it fits, four LM399 pieces without insulation is possible to glue on this transistor...
These are LM185 zeners in the same TO-46 housing as the LM399.


No, i will not build it  :)

Kind regards,
Bram
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Offline David Hess

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Re: Small Oven Controler For Voltage Reference
« Reply #13 on: April 26, 2018, 08:11:32 am »
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.

To me that looks like a TO-220 LM350T 3 amp variable voltage regulator (3 amp version of an LM317) and not an LM35 (LM35DT?) temperature sensor.

The reason i used a uA723 is because it has all the electronics you need to make a nice controler (not the best controler!)

The 723 is a great choice for this application since it includes a differential input error amplifier, reference, and power output stage if needed.  There are some combination operational amplifier and reference ICs but they are more expensive.
 

Offline blackdog

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Re: Small Oven Controler For Voltage Reference
« Reply #14 on: April 26, 2018, 04:47:07 pm »
Hi David,

It really is an LM35 in TO-220 housing and he has also been connected working.
The other ICs I used for oven projects are the LM10 and the INA 125.

And of course an Arduino Nano I hacked, in order to get a more stable ADC.
I have used a good 1.23V reference IC and better decoupling of the powerlines on the Arduino(PID controler)

Kind regards,
Bram
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Offline David Hess

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Re: Small Oven Controler For Voltage Reference
« Reply #15 on: April 26, 2018, 08:09:50 pm »
It really is an LM35 in TO-220 housing and he has also been connected working.

So it is.  I am sure I have seen that before but then forgotten.  Providing the LM35 temperature sensor in a TO-220 package is a great idea but sure makes for a confusing part number.

Quote
The other ICs I used for oven projects are the LM10 and the INA 125.

The LM10 is great for this sort of thing but sure is expensive compared to a 723 unless you need its low voltage and low power features.
 

Offline blackdog

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Re: Small Oven Controler For Voltage Reference
« Reply #16 on: April 26, 2018, 11:08:15 pm »
Hi,

The nice thing about the LM723 is that it has all, also the max. Current/Power limiter, you dont need no extra active components.
The loopgaing en reference stability is more than enough for making a reasonably good component oven.

But keep in mind that the impedances that the inputs see remain below 10K.
My second design I showed here is just above that for the Thermistor input, if you use a 100K Thermistor under each transistor you meet the requirement of less than 10K of impedance.
Also don't go to low in impedance, otherwise the reference section will be overloaded.

A rule of tump is that at 42C the thermistor is about half its resistence value, easy to remember. (100K thermistor is at 42C about 50K)
And if a component oven is 41.45C or 42.67C, i dont care, but maybe for you guys it is important,
trim one of the bridge resistors to tune the temperature of your liking.

Kind regards,
Bram
« Last Edit: April 26, 2018, 11:14:45 pm by blackdog »
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Re: Small Oven Controler For Voltage Reference
« Reply #17 on: April 27, 2018, 05:45:52 am »
Hi,

I had had had enough of the work today.... and thought, let me look for some relaxation in the electronics  ;D

I have made some adjustments to the loop compensation.
This I did, after some research of the internal construction of the uA723.
The loop compensation connection (13) has a high impedance and when the loop compensation is on that pin, the open loop gain goes down.
This in turn results in poorer control properties, so instead of pin 13, I used the output pin 10 which has a low output impedance.

After this adjustment the capacitor could also become smaller, and C1 is now 10uf and the overshoot is minimal with this value.


I don't think I can get any more performance from this circuit, remarks, shoot at it!

Kind regards,
Bram
« Last Edit: April 27, 2018, 06:30:21 am by blackdog »
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Re: Small Oven Controler For Voltage Reference
« Reply #18 on: April 28, 2018, 07:36:25 am »
Hi,

This afternoon I thought about trying to make one of the ovens even better.
Again this time four MOSfets, but this time directly mounted on the aluminum tube.
I then decided that power supply from the oven electronics is connected with the + to the oven tube.
This is not a problem if two separate power supplies are used.

The oven tube looks smoot, but it is not!  >:(

I had to file with alcohol for a long time to get the sides fairly flat.



One side ready, not perfect, but mutch better than it was.



The aluminium tube looks like this when it comes from the supplier, but i drilled the holes.


Here you can see the four holes for mounting the 100K thermistors.
Make sure that all holes are deburred.



Here I have prepared a piece of 0.5mm of double-sided print material to glue to the oven tube.
The material is fixed with 10 second adhesive.



Ready for the next step, the Flux.



Here the tube is wrapped with capton tape and the copper is sprayed with flux,
the cotton tips ensure that the holes for the thermistors do not fill with flux.
It is almost a peace of art  :-DD



Shiny Flux!



Making poison!
When you read the manual of this glue, you barely dare to use it....



The prepeard 100K thermistors, teflon sleves and kynar wire, it was a lot of work.



The thermistors glued in the oven.



If i have time tomorrow, i will select four of these MOSfets for the oven



Thats all for today!

Kind regards,
Bram
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Offline David Hess

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Re: Small Oven Controler For Voltage Reference
« Reply #19 on: April 28, 2018, 09:20:18 am »
I have made some adjustments to the loop compensation.
This I did, after some research of the internal construction of the uA723.
The loop compensation connection (13) has a high impedance and when the loop compensation is on that pin, the open loop gain goes down.
This in turn results in poorer control properties, so instead of pin 13, I used the output pin 10 which has a low output impedance.

Pin 13 is the transconductance (current) output and is used in the same way that similar external compensation pins on operational amplifiers are used.  Schematics usually show a capacitor back to the inverting input but a capacitor to ground also works.  Unfortunately there is not all that much data for the 723 on different compensation schemes.

For voltage regulators which are relatively fast, I doubt you can get away with *not* using the compensation pin but in your case the external frequency compensation is so heavy that this may not be a problem.
 

Offline zhtoor

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Re: Small Oven Controler For Voltage Reference
« Reply #20 on: April 28, 2018, 11:11:58 am »
Hello Bram,

do keep up the good work, make lots of money and retire to do this full time for our benefit  :-+

have a look at an interesting implementation from the distant past: fluke 335 A/D.

best regards.

-zia
 

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Re: Small Oven Controler For Voltage Reference
« Reply #21 on: April 28, 2018, 09:10:34 pm »
Bram, thanks. Nice thread ! :-+

How do you attach the mosfet? Aluminium Nitride? Send me a PM if you need four.
 

Offline blackdog

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Re: Small Oven Controler For Voltage Reference
« Reply #22 on: April 28, 2018, 09:56:20 pm »
Hi Ramon,

For this oven the MOSfets wil be mounted whitout isolation.
Only some thermal compound will be used.

There will also be a small revision of the schematic.
This is because I now use four thermistors and the impedance of the node of the thermistors is now half of what is was.
This will probably also change the loop constant a little and need some adjustmend.

Furthermore, this week I also calculated some data from the NJL3281D transistors.
The sensitivity with the four diodes in these transistors is about 8x less than with the thermistors,
this will result in poorer control properties with a uA723, this because the open loop gain of this IC is only 30 to 40dB.
Also it is just outside the input common mode of the uA723 without extra precautions.

It is very well possible to use these transitors, but you may need a different IC for proper functioning, but that is outside the scope of this topic as far as I am concerned.

I used these thermistors for the oven:
https://www.banggood.com/10-Pcs-3D-Printer-Thermistor-NTC-100k-100ohm-Match-MK2a-1-Sensor-p-932646.html?rmmds=search&cur_warehouse=CN

In the Netherlands the used MOSfet is cheap.
https://www.eoo-bv.nl/zoeken?controller=search&orderby=position&orderway=desc&search_query=irfp140&submit_search=

Also in Germany
https://www.reichelt.de/IRFP-IRFRC-Transistoren/IRFP-140N/3/index.html?ACTION=3&LA=446&ARTICLE=41674&GROUPID=2893&artnr=IRFP+140N&SEARCH=irfp140&trstct=pos_0

If you are going to use other MOSfets, that's your problem.  :-DD

Kind regards,
Bram
« Last Edit: April 28, 2018, 10:46:23 pm by blackdog »
“Two things are infinite, the universe and human stupidity, and I am not yet completely sure about the universe.”
 

Offline ramon

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Re: Small Oven Controler For Voltage Reference
« Reply #23 on: April 28, 2018, 10:47:54 pm »
I want to simplify you schematic as much as possible.

  - instead of four parallel mosfet I will use just only ONE.
  - No big BOX: Just mosfet attached to aluminium nitride sheet and one temperature sensor on the other side.

It cannot be seen clearly on the picture I send before, but the bigest mosfet I have in the drawer is IFRP460.
I think that a single IRFP460 can be used to substitute 4x IRFP240, if the area to heat/regulate is reduced.

  IFRP140: input capacitance = 1700 pF, Power Disipation = 180 W
  IFRP240: input capacitance = 1300 pF, Power Disipation = 150 W
  IFRP460: input capacitance = 4200 pF, Power Disipation = 280 W

Another are that can be modified (improved?) is the resistor divider.
We can add 'filling' resistors on high side, bottom side (or both) to avoid self-healing.
 

Offline blackdog

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Re: Small Oven Controler For Voltage Reference
« Reply #24 on: April 28, 2018, 11:40:47 pm »
Hi Ramon,  :)

It is not clear to me what you are trying to build.
If you do not use a box, the uA723 also can not be co-heated.
This makes the temperature control worse, but maybe that's not so important for your application.

The capacities of the MOSfet are not important at all in this application.
The steepness, however (admittance) will. (need different values for the loop compensation) if you can get is stable  :)

If you are not going to use a housing, then the leakage of heat will become a problem because of your connecting wires.
Heating the MOSfet only, when the MOSfet is well insulated will not exceed 0.1 a 0.2 Watt.
If you don't pay enough attention to the wiring, the heat loss through the wiring can be as high as 40%...
And there go's your temperature stability...

I do not understand what you mean by: We can add 'filling' resistors on high side, bottom side (or both) to avoid self-healing.

The good results I achieve are due to the fact that I have thought and experimented a lot with this setup.
Applying a well-functioning schematic to an entirely different assembly will never produce the same good results.
But, don't let me stop anyone from doing experiments, because you learn a lot from them.  :D

With some extra isolation, the power the oven is using dropt from 2.1-Watt to 1.5-Watt at 23C LAB temperature.



And this is 9 hours temperature logging, the wide band is due to the noise level of the TEK DMM4050 meter when you measure with a PT1000 sensor.
The top of the chart is +7 mC and the bottom is -6 mC compared to the start of the measurement yesterday evening.
The LAB temperature had dropped by about 2C during the night.
At the moment, after more than 16 hours of operation, the temperature is approximately +0.004C at approximately 23C LAB temperature.


But keep in mind that also the TEK DMM 4050 used drift with the change of LAB temperature!

zhtoor
The ovencontroler you are showing is almost the same as the internals of the uA723, look at the schematic of the uA723, you wil see a differential amp and a darlington.
The uA 723 uses a current source in the gain stage and Fluke uses a 75K resistor (R3)
Both have a Darlington output stage.

Now it's time to select some MOSfets!

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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