Temperature controlled oven for Vref - Page 1
Forum section: electronics / projects

author: MathWizard
author_rank: Super Contributor
timestamp: August 01, 2025, 06:53:28 am
content: I need a more temperature stable Vref from a TL431 circuit, so I'm experimenting with a little oven I made. I used a breathmint, tin coated steel can, it's roughly 22mm x 40mm x 72mm (0.85x1.57x2.83 in). It has an area of about 0.011m^2, 16in^2. I sanded off most all the paint, and soldered a TIP32C onto the outside of both large surfaces. So far I've placed it between 2 peanutbutter jar covers, packed with pink fiberglass insulation, it almost fits closed, and looks like a 2" tall, 5" diameter hamburger, but there's a 1mm gap so it's not airtight.(I'm planning to get better Vref IC's some day, but for now the TL431A is ok)I have a thermocouple taped to the outside of the tin box for testing, and 2, 10k glass encapsulated NTC's in parallel inside the tin, w/ the door almost closed. I used 28AWG wire for everything in/on the tin can.With roomtemp about 24C, it takes about 1.4W to keep it at 41C, and so far I used about 5.3Wmax to heat it up to temp, which might take 2-3minutes. On a 12V rail, I'm using 0.115A just to stay at 41C. That's a lot more power than I was hoping for. I could use a bigger outer enclosure, w/ more insulation. I can even use some exterior vinyl siding foam board insulation too.The tin is oversized for some basic TL431 circuit, but what's the typical power usage of some little oven like this ? Also what operating temp's are most common ? And for running a wheatstone bridge + op-amps for regulation, should I put the full WSB in the box? or just my NTC's ?On this thing, 3k ohms on each NTC is not too hot to touch, that was another heatsink. On this one, 41C has each NTC at ~3.32k

author: ch_scr
author_rank: Frequent Contributor
timestamp: August 01, 2025, 09:05:29 am
content: Scroll down here:https://techlib.com/electronics/ovenckts.htm, recommend to expand the shown TL431 oven controller with external drive transistor. No additional op-amp needed!

author: tszaboo
author_rank: Super Contributor
timestamp: August 01, 2025, 09:36:40 am
content: Oven controlling a TL431 is like racing tires for a Trabant.Just get a better reference.

author: MathWizard
author_rank: Super Contributor
timestamp: August 01, 2025, 10:10:03 am
content: Yeah thanks I found that site yesterday. I have some scraps of copper pipe, I'll have to try a smaller version. I might use SMD resistors too.This is more of a learning experience tho too, and even if I ordered some today, that takes too long. This is already a sidetrek of a sidetrek. I just want a better Vreference than what I made for an op-amp test jig. To measure the Aol of an op-amp, the jig used a 1Vref, and then you have to measure the difference between 2 configurations, that are only a few mV difference for common op-amps. But with an 8Vref, it might be a few dozen mV that needs measuring (not all op-amps like 8V tho).In open air, my emitter follower 1Vref derived from a TL431, moved by 1-2mV too easily. Then when I realized 8V would be much better for the jig, I used that 1V to get 8V, but it would move by +/-5mV or more over the run of a day.I've altered the compensation on the Vdrive op-amp, so the open-loop gain from the NTC to the BJT's would be ok at least. And I'll probably use a driver BJT too. I have lot's of NE5532's, they have 1-2mVos, and ~30-40nA Ip,n. LTS seems to have some 6.8uVos, and ~500nA Ibias, so that's not the final circuit or anything either.

author: PCB.Wiz
author_rank: Super Contributor
timestamp: August 01, 2025, 07:48:56 pm
content: Quote from: MathWizard on August 01, 2025, 10:10:03 amThen just buy a better reference !There are shiploads these days to choose from, you can easily get < 0.1% and good long term stability.Buy some good auto zero opamps, and you should be good to sub 10uV

author: MathWizard
author_rank: Super Contributor
timestamp: August 01, 2025, 11:50:27 pm
content: I want to make a big LCSC order sometime soon, but I feel overwhelmed with all the choices for everything, including resistors. I started making a short list of cheapish but high accuracy and low ppm temp-co Vref's, that I want to have a bunch of in my boxes.Like REF5025, and the improved and low power TL431's, and some better 0.1%, <20ppm/C ones that are <$1 when buying 10-20. Nothing too special tho, just more general purpose stuff that I want on hand. I need NTC's with a datasheet too, not these ebay ones.But yeah I'm not buying boxes of low temp-co resistors, so I'll make lower grade versions of some things, before maybe actually ordering an SMD PCB.Now I built the TL431 circuit, and bridge circuit, all on 1 PCB and put them in the tin box, and it's sealed pretty good. And then I packed a 750mL (~26oz) plastic jar w/ pink insulation, and have the box in the middle. And I have taped the lid on the big container, and have a little hole for all the 28AWG wires, out to header pins. So with 118mA total on the heater, the losses balance, and it's holding around say 46-47C on the surface of the box (going by the BJT currents, rather than the NTC's). I forgot to attach the temp-coupling to the tin box, so I poked it into the insulation, and it's around 31C, ~ 7C above ambient.And so far, w/ a 100R pot on a breadboard to adjust the TL431 gain for 8V, I'm holding ~8.00000 with a span of 650uV over 140 100PLC cycles. It's way more stable than the 8Vref on my op-amp jig, short and long term.So now to build the regulation part, and hook up a scope, but I better hook up the k-couple to the can. Can I solder tack it to something, like the TO-220 tab ? Otherwise I'm just using electric tape, and the tip never stays down on the metal, and I'm taping the metal.I've read the NE5532 is low noise, and that you aren't supposed to go over 600R parallel resistance at either input. But at least for the basic configurations I tried, that seemed to load the TL431 way too much, and make the Vout worse. I don't really know any noise eqn's, nor do I use the noise analysis in LTS yet. So IDK how bad my circuit is, except that above 100k resistors adds Johnson? noise, and Shot noise is bad for low frequency stability. I know R18 was in the wrong place in that picture, no wonder I had 10u there.For my E-load project, I want to make some really good Vref, and some good log-amps, so I'm becoming much more aware of temperature effects lately.My latest old PSU I bought, uses a simple LM336 circuit, w/o any direct adjustment of the 2.5V output. And yet that whole PSU barely drifts over a day idling, and not much with a modest load either. But I might upgrade it too.

author: Vgkid
author_rank: Super Contributor
timestamp: August 02, 2025, 03:07:40 am
content: Here in a lm723 derived oven controller.https://www.eevblog.com/forum/metrology/small-oven-controler-for-voltage-reference/

author: MathWizard
author_rank: Super Contributor
timestamp: August 03, 2025, 03:49:41 am
content: Yeah I saw that one too, I made a model of it already in LTS, and I already own some SMD uA723. So I'll probably try 1 of them too, w/ SMD resistors.Meanwhile my through-hole TL431A version is doing better, I have the k-probe inside the tin, and have little holes in the tin box and plastic outer case for the wires. And now with op-amps regulating, it's currently holding at 47C inside the tin can, and BJT heater's use about 0.5W total. The whole circuit uses 0.79W, to hold steady at 47C.Looking at it now, it's doing great, 80uV span over 100 samples, and it's been holding around there for quite a while once it warmed up, overshot some, and settled. But yeah I'm playing with the op-amp resistors and output range, and slope with temperature. I'm not running it like a comparator, it has a much gentler Vout/(temp difference) than the LM723 model I tired.The breadboard is noisy tho, if all that was in an insulated tin can with another Temperature control loop, and a battery at least running the control side, then it would be even better.I keep thinking my SDM3065X has gone to sleep, and nothing stands out in FFT mode besides radio stations, and 60Hz at -70dB, the rest is usually below -90dB.

author: Doctorandus_P
author_rank: Super Contributor
timestamp: August 03, 2025, 04:57:58 am
content: There are quite a lot of threads regarding the many different versions of the TL431.Here is my own old thread about voltage references:https://www.eevblog.com/forum/projects/voltage-reference-precise-but-cheap/My own project has stalled for an indefinite time. Later I realized it's much better to start with a reliable reference (maybe a few of them to compare them with each other) and then maybe build a test jig which can compare and track a bunch of TL431 variants.For getting started, cost is not such a problem, so you don't have to think long about buying some good references. Later you can think about production, and whether it can be worth it to reduce BOM costs.And some other links I found worthy of archiving.https://www.eevblog.com/forum/metrology/tl431/https://www.eevblog.com/forum/projects/low-cost-voltage-reference-experiment/https://www.eevblog.com/forum/metrology/is-tl431-good-for-reference-in-19-999-counts-voltmeter-any-variants/msg1821833/

author: MathWizard
author_rank: Super Contributor
timestamp: August 05, 2025, 04:16:24 pm
content: Well this seems to do good enough making 8V for my op-amp test jig. Today after 5,940 100PLC samples, the average is 7.99987V, and the span is 1.67mV. A lot of the big deviations are from me moving the desk/floor, and even when the furnace cuts in/out.Here's a shot of the op-amp output, and the NTC voltage on warm-up from room temp. It over heats to about 53C, before dropping back down and settling into 47C.Now to make the mini-version w/ TL431 and zener, and an SMD uA723 version. I could also try a LM1117 version, I know I have a few of those around.I made an adjustable CV-CC linear PSU with a TL431 at it's core, I'll have to make a mini-oven for it, and see how much it improves it.But yeah, I really have to get my list together and make a LSCS order.

author: ch_scr
author_rank: Frequent Contributor
timestamp: August 05, 2025, 06:12:57 pm
content: Now that is a nice "well-behaved" looking transient response. I have observed 10MHz OCXO's from multiple manufacturers with their warm-up current draw and (once warmed up) how stable they are (judged by the freq stability) when given an "external thermal upset" (putting a thumb on the metal case).My observation was that some have a nice turn-on transient response current, while others have much more gain (the turn on transient slams the heater into cut-off, then into it's internal current limit for a moment, cut-off again, .. a few times before calming down & drawing a near constant current).Generally speaking the ones with the "wild" turn-on current respond a lot better to the "thumb-test" though!In a way it makes sense, an OCXO is not made to be turned on&off all the time anyway.For the controller to "regulate out" small influences once at steady state it needs all the P gain it can get, IMHO.

author: Doctorandus_P
author_rank: Super Contributor
timestamp: August 06, 2025, 09:28:15 am
content: For the TL431, there are many variants, and when you get into details such as this, there are important differences. In some of the datasheets there are also graphs that strongly suggest that the temperature coefficient is dependent on the reference voltage at room temperature.But overall, it's probably better to skip the oven and put that budget into a better reference. But I do agree it's fun to play with.

author: Kleinstein
author_rank: Super Contributor
timestamp: August 06, 2025, 09:52:23 am
content: An oven for a reference can be a fun project. However I would not use the TL431, but more a good zener diode (for low noise maybe 1N825 / 1N827) or JFET based reference. I got surprizingly low noise with sime 5.6 V BZX79 zeners, though this could be a lucky batch. A zener may want a regulated oven.The TL431 that I tested so far were more a bit noisy and with a plastic case they are inherently sensitive to humidity.Zeners and JFETs (2N4391) can be get in a glass / metal case.

author: MathWizard
author_rank: Super Contributor
timestamp: August 07, 2025, 04:35:05 am
content: My TL431A's are from ebay, I was too lazy to test a bunch of them, but the 1 I used was pretty close to 2.495V at 24C. But yeah I never added a wire right on the 2.5V to see where it is on the temperature charts. I might put a fan or bag of frozen veggies on the plastic container, to see how well it can keep up the temperature. Now I should solder up the control circuits and put them in some shielded box, and put it and the oven in cardboard box.I've been looking at some of the PTAT and CTAT circuits for log amps, so I'll be trying some of that on the breadboard soon. I'll have to try and hook up my Siglent DMM to my computer, and get some real data.I just made a big LCSC order, lots of through hole 1% +/-100ppm resistors, some good MOSFET's for an E-load project, a few more sub 10MHz general purpose op-amps, and various 2.5V and 5V Vref's, like REF5025, REF3425, and some better spec'd SMD and through-hole TL431's. And some dual/matched BJT's and JFET's in SOT363 packages

author: trobbins
author_rank: Frequent Contributor
timestamp: August 07, 2025, 07:36:33 am
content: Why did you choose to raise the device temp to 41 or 47C ?Were you able to get some example plots of incremental tempco (for the 431 and associated resistors) for say 5C spans from ambient up to say 50C ?Do you have an aim for the ambient temp range you want to cope with - eg. 15 to 30C ?

author: PCB.Wiz
author_rank: Super Contributor
timestamp: August 09, 2025, 03:44:24 am
content: Quote from: MathWizard on August 05, 2025, 04:16:24 pmIf you are going to the trouble to make an oven...For Zener + Oven designs, Nexperia still have PLVA2668 which is a dual Zener, that specs a noise figure and average tempco.With this dual part you can combine Diode+Zener TC's to choose where the parabola flat portion is, and then oven-ize that.That leads to musings aboutwhat if a single part can do reference sense, and heating ?- like the uA723, but modern and much smaller.There are more choices of multiple Zener parts :ROHM RSA6.1U5 in SOT26 5 Zeners ~ 6.736V well spec'd but unclear how 'good' that is thermallyROHM EMZT6.8 4 Zeners SOT-553 ~6.850VROHM RSA6.1J4 4 Zeners SOT-553 ~ 6.684VDiodes inc DMF05LCFLPA in U-DFN1616-6 5 Zeners ~ 6V ?TI TPD5E003 X2SON6 1.00 mm × 1.00 mm 5 Zeners ~ 6.5VVishay VESD05A6-HAF LLP75-7L (DFN1616) 6 Zeners ~ 6.7VVishay VESD05A6A-HAF LLP75-7L (DFN1616) 6 Zeners ~ 6.7VVishay GMF05C-HSF LLP75-6L (DFN1616) 5 Zeners ~ 6.7VVishay VESD05A5A-HSF LLP75-6L (DFN1616) 5 Zeners ~ 6.7VThe ones with a metal PAD suggest all the die are bonded to that pad, so thermal coupling should be good.Something with 6 Zeners could allocate outer 4 for heating, and use inner 2 as Diode+zener for reference and Correction.A thin PCB with isolation slots could make this very smallPerhaps something like Digikey masach-tech-ltd MS087-10S can even create a thermal box ?QuoteJust up at TI, I see an enhanced REF30E, 39c/1k, that offers 0.1% and < 20ppm (typ 7ppm/K) and 25uAhttps://www.ti.com/lit/ds/symlink/ref30e.pdf

author: Kleinstein
author_rank: Super Contributor
timestamp: August 09, 2025, 07:09:59 am
content: There is not absolute need to control the temperature directly at the ref. element. With a constant supply and thus constant power consumption it is good enough to control the temperature around the reference.The idea of using a zener diode instead of the ref. chip (usually bandgap) is that one can get quite a lot of types in an old style THT glass case. This avoids the humidity issues with plastic cases. With zeners one can also fine tune the TC. One gets a more positive TC at higher currents and a more negative with less current. With a small linear TC the oven around the reference does not have to be that accurate. Even a crude oven is quite effective against the higher order parts (usually T-T_0)².With much of th effort for the oven it may be worth having more than 1 reference in the oven to have some cross check.For the reference it is still the question what it should be used for and what are the target specs.

author: iMo
author_rank: Super Contributor
timestamp: August 09, 2025, 09:09:11 am
content: With the 723 in metal or ceramic (or worst case in plastic) you will get a single chip "7V reference inside the oven". Search for the thread where we elaborated the "723 self heating ovenized reference"..https://www.eevblog.com/forum/projects/playing-with-723-voltage-regulator-oldtimer/Wolfgang did as well:https://electronicprojectsforfun.wordpress.com/silly-circuits/silly-circuits-a-heated-lm723-reference/

author: MathWizard
author_rank: Super Contributor
timestamp: August 11, 2025, 03:00:23 am
content: Yeah I need to go back to basics and try some of these diode, zener and BJT versions, and read more theory on it. I've been skimming a couple of good, old books on low voltage and current measurements, and making math functions w/ circuits. I'm not sure if any of those PTAP/CTAP external circuits, can make the TL431 version, better or not, or how much that's already built into a TL431. I've seen in the datasheet how the tempco varies from chip to chip with the different Vref values at 25C. Trying to type into a spreadsheet Vout, temp, and time to any accuracy by hand with this circuit, is very hard, it moves way too fastIDK why I chose 47C, and I'm not sure what the Vout tempco of my chip really looks like. I don't remember much thermodynamics, I got out a big physics book, but never opened it yet for this. But I still haven't made the controller PCB, I've been trying to upgrade the design of my op-amp test jig.Yeah I really should try a uA723 version, over the past few years I've read and heard of x723 chips, but I've seen mixed feelings on it. If I'm using the datasheet right, it gives the max tempco of the output, as percentage/degC, and if the output was 7.15V like the Vref, it can drift upto 1.07mV/degC. But I guess the TL431 changes some 10's of mV getting up to temp too, so it must be similar.

author: Kleinstein
author_rank: Super Contributor
timestamp: August 11, 2025, 08:25:56 am
content: The 723 regulators from different manufacturer and maybe also with age differ in the reference circuit. Some were OK low noise reference, but others are not worth the effort (e.g. comparable to TL431). So not all the 723 are the same, especially when it comes to the reference part.

author: MathWizard
author_rank: Super Contributor
timestamp: August 13, 2025, 06:14:24 pm
content: I think the uA723 I have are genuine T.I. from Digikey

author: iMo
author_rank: Super Contributor
timestamp: August 13, 2025, 06:50:23 pm
content: Based on the TI's datasheet its TC is typically 30ppm/C or around 0.22mV/C (max 150ppm/C).If the DS is true (and your chip is from TI) it is almost the same as the non-heated 399 afaik..