Heated housing for voltage references - Page 1
Forum section: electronics / general

author: F64098
author_rank: Regular Contributor
timestamp: February 18, 2018, 07:11:46 pm
content: Hello everybody,after working for some hours i just finished my newest project.Last year i cracked a double-oven OCXO by reversed power and opened it, to replace the burnt parts.The inner housing was heated and held by 4 MOSFETs and contains also a MAX6350ESA to produce a stablereference voltage for the frequency trim.So the idea was born to build a combination of an oven control and a low noise power supply to take my small voltagereferences (AD587) in a heated housing and also supply them by a low noise regulator from varying power sources.The pcb will fit for a TEKO 371-16 housing which is made of tinned steel. The transistors are mounted with M3 screwsand a silicone pad on the bottom side provides thermal and mechanical contact to the resistors on the pcb.The quick and dirty prototype runs at ~43°C (fixed resistors), consuming 86mA @20V when mounted in a double-walled coffee cupmade from stainless steel. Maybe a glass made insulated cup works better. I will find it out...You can see the schematic and pictures of the pcb and i would be grateful for suggestions and ideas, or any kind of criticism.Best regardsFrank

author: Vtile
author_rank: Super Contributor
timestamp: February 18, 2018, 07:28:45 pm
content: I would say that you have too much volume vs. radiating area allowing drift and radiant heating to form. ....Just thinking out loud, zero experience to back it up.

author: Andreas
author_rank: Super Contributor
timestamp: February 18, 2018, 07:57:00 pm
content: Hello,just some thoughts:- the LT1763 is specced only for maximum 20V. So I would not use 22V.- the temperature distribution with only some single resistors could be improved.I prefer heater foils. But you could also use a bed of SMD resistors.- I would place the temperature sensor directly at the point of interest (inside the metal housing).- to get a stable temperature controller I often use 2 temperature sensors.one for the I-Part at the point of interest (usually thermally coupled bad to the heater).one for the P-part near the heat source to avoid overshoots.- I would never use sockets for precision references like the AD587- And shurely I would start with something like the SVR-T cirquit to further reduce the T.C. of the AD587.with best regardsAndreas

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: February 19, 2018, 06:09:10 pm
content: Steel is a dreadful heat conductor. I'd use an aluminum box to even out the heat and give less delay. It makes sense to put the sensor at the point of interest, but you can often do better by closely coupling near the heater because of the reduced delay, as long as the box is highly conductive.

author: GigaJoe
author_rank: Frequent Contributor
timestamp: February 20, 2018, 01:23:54 am
content: That the thing i did about week ago:R6 - 10K thermistor, 5K around +32C, it makes around -1Kohm / 1C slope2x2K - hooked to hp3456 to measure temp deviation, in sequence resistance changing around -75 ohm / 1C , at 33 Cthermistors from aliexpress, defined as MF58.heater 2 mosfets, pick 2 from around 6 with close characteristics, mounted on opposite side.when ON consumed 200ma total, around 1 sec on, 5 sec off, cycle. ( the hysteresis produce very small level of a temperature jumping, thermistor sensor changing at 0.15 ohm much less then 0.001 C )the whole board tossed inside the case, so it relatively stable ....What I'm observing:assuming 2x2K thermistors are stable - my resistance deviation for 6 hours 2 ohm == 0.05 C; over 24h == 8 ohm -> 0.1 Cand drifting per day approx. 0.08C, it drift up and down, around 0.2-0.3 C per week. ( i had just a week of running)when I use single 2K thermistor, (1K at +43C) due to much less slope i had 0.3C per day stability.it basically my first attempt, so I wonder:about overall direction, are thermistors as sensors are stable enought, or I should think about to use diode instead ...2K and 10K - has a differ material , guess, in my understanding, wonder if 2k are more stable, then schematic need a change to sense much less slope, same for diode.To decrease hysteresis more , i'm thinking a rectangular aluminium plate , drill a hole for thermistors or diode , and use it between heaters and the frame,also thinking 2 transistors, but only 1 are monitoring it may produce more uncertainties ... so to change to one transistor only,And I didn't count a self-heating effect of thermistors at all - even don't know if it matter ( when I manually switch resistance range on HP, a diff voltage apply, and self heating was very-very visible)So here we go again ... assumed it was a finger snap but .... like ... drifting a 2C per year - at the best , it will spoil everything ...schematic:the box:10K thermistor as sensor:2k thermistors:

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: February 20, 2018, 03:20:51 pm
content: Thermistors can be very stable. How much is very? Not a clue, but I used to use a digital thermometer, a Thinc I think, that read out to 0.001 degrees and had no obvious drift problems over a week or so. Even over years it didn't move much.

author: Vtile
author_rank: Super Contributor
timestamp: February 20, 2018, 04:18:34 pm
content: Anyhow we have been bastards for the OP. He created something nice and all we talk is how it might be improved.

author: F64098
author_rank: Regular Contributor
timestamp: February 20, 2018, 05:39:42 pm
content: The heat conduction may be not good as with aluminum or copper housings but it's compensated by the use of 4 transistors.And the material is tinned, so it's very easy to solder it.The heat distribution to the inner pcb will be done by 4 brass standoffs (6mm) which are soldered to the housing.I also still plan to fill the housing with molten paraffin to get a better heat distribution and reduce the humidityproblem to nearly zero.The maximum input of the LT1763 is a problem, i haven't looked at. Thanks for the note!In the first version i had the same tracking pre-regulator as for the 5V-supply on the input of the LT1763,but there is not enough headroom, when using 3 6V Pb-accumulators, so it was rejected.Maybe it's possible to reduce the voltage drop of the LM317, when using an LDO as pre-regulator.Best regardsFrank

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: February 20, 2018, 07:45:07 pm
content: Quote from: Vtile on February 20, 2018, 04:18:34 pmHey, it's in our DNA, it's what we do!

author: GigaJoe
author_rank: Frequent Contributor
timestamp: February 21, 2018, 12:54:35 am
content: F64098,I'm really like to see your thing as a Temp. stability in overall, seems 10k glass thermistors would be an extremely sensitive to a minor ( like 0.01 C) degree of a temp change.I'm still thinking: a sensor should be with direct connection to the heater, eliminating hysteresis (overshooting) .

author: montemcguire
author_rank: Regular Contributor
timestamp: February 21, 2018, 04:16:47 am
content: Having the thermal sensor in close contact with the heater will allow the heater's control loop to operate faster, making it easier to stabilize, but what's not clear is whether you're reading the right temperature. Moving the sensor closer to the thermal target will require a slower control loop, and while that's less desirable, the fundamental goal is to have a control system that attains its set point at the place where the temperature needs to be controlled. Controlling the output side of a heater is useful, but it thwarts the overall control loop, whose goal is to stabilize the 'load', not just the heater. Stabilizing the heater alone is an open loop system.So, if you're using a control loop, the thermal mass of the controlled stuff needs to be minimized in order to minimize the loop response time. The thermal conductivity between the heater, the thermal target, and the sensor needs to be as high as possible, while minimizing overall thermal mass. And, in the end, regardless of the thermal mass and any thermal sensor lag, the control loop needs to be stable, without overshoot and other easily avoidable problems. The details of a mechanical / materials design will determine the speed of a stable, optimal control loop, so the goal should be more focused on minimizing thermal mass while providing sufficient thermal conductivity from the heater, the thermal target, and the thermal sensor. Given all of those constraints, an optimal control loop can be created, and if that's not sufficient, the materials and mechanics of the thermal system need to be re-designed to increase thermal coupling and decrease thermal mass.

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: February 21, 2018, 06:21:43 pm
content: Air is a great thermal insulator, but the thing I rely on is that anything in a highly conductive closed box of small size, say of aluminum, will be at the temperature of the box... eventually. And if it's not dissipating significant heat. Building a long time constant control circuit that's well behaved on start-up and doesn't drift, is way harder than a short time constant one. The really serious systems will sometimes use two boxes and two control circuits. Because the outer box does the bulk of the work, the inner box can be just a degree or three above the outer box, making it very stable.

author: GigaJoe
author_rank: Frequent Contributor
timestamp: February 22, 2018, 03:09:02 pm
content: 2 2K thermistors, NTC, connected to the multimetera difference on air or covered by styrofoam brick is -190 ohm, I measure slope around -70 ohm / C ; self heating by multimeter

author: thermistor-guy
author_rank: Frequent Contributor
timestamp: February 23, 2018, 12:33:09 am
content: Quote from: Conrad Hoffman on February 20, 2018, 03:20:51 pmBIPM states that "the most stable thermistors are bead thermistors encapsulated in glass". See section 4.6 in:https://www.bipm.org/utils/common/pdf/ITS-90/Guide-SecTh-Thermistor-Thermometry.pdf

author: F64098
author_rank: Regular Contributor
timestamp: February 25, 2018, 07:05:57 pm
content: I happened to find today the circuit diagram of the OCXO (Morion MV-180) that I damaged. I had to reengineer it, because there were no schematics available.Nevertheless, I got from Morion the information which types of transistors are used, because they can't be identified (SC-70 housing).This is only the heater of the inner housing. In this inner housing there is a second heater circuit fo the crystal housing.There they used a thermistor in 0402, directly soldered to the housing and connected to the pcb with a ultrafine wire,so i thought it's just a solder blob and tried to remove it, cracking the thermistorMaybe this schematic could help someoneBest regardsFrank

author: CatalinaWOW
author_rank: Super Contributor
timestamp: February 25, 2018, 07:17:57 pm
content: While aluminum conducts heat much better than steel, they both conduct much better than a good vacuum bottle. The thermal time constant of the stuff inside the container will be short wrt the time constant to the outside world.I have seen wide variations in the stainless steel coffee cups that are available on the market. Some are near vacuum bottle performance, others seem little better than a solid ceramic cup. Performance of the ones I have evaluated correlates only weakly with price. Unfortunately none of the good ones I have encountered has stayed on the market so I can make no recommendations on brand or model number. It may even be serial number variations (poor control on sealing, the ones that don't leak are good). If the one you are using doesn't work so well I would try another brand or sample of stainless steel before I went to glass.

author: cellularmitosis
author_rank: Unknown
timestamp: February 25, 2018, 08:03:18 pm
content: Quote from: thermistor-guy on February 23, 2018, 12:33:09 amIt looks like the study he's referring to is "Stability of NTC thermistors", published in "Temperature: Its Measurement andControl in Science and Industry, Vol. 6"Code:Siwek W R, Sapoff M, Goldberg A, Johnson H C, Botting M, Lonsdorf R and WeberS (1992a) Stability of NTC thermistors, in Temperature: Its Measurement andControl in Science and Industry, Vol. 6 (Edited by J. F. Schooley), New York,American Institute of Physics, pp. 497-502Google knows about that book:https://books.google.com/books/about/Temperature.html?id=rfFAAQAAIAAJUnfortunately, it looks like UT only has volumes 4 and 8:http://catalog.lib.utexas.edu/search~S29?/XTemperature%3A+Its+measurement+and+control+in+science+and+industry&searchscope=29&SORT=D/XTemperature%3A+Its+measurement+and+control+in+science+and+industry&searchscope=29&SORT=D&SUBKEY=Temperature%3A+Its+measurement+and+control+in+science+and+industry/1%2C8%2C8%2CB/frameset&FF=XTemperature%3A+Its+measurement+and+control+in+science+and+industry&searchscope=29&SORT=D&8%2C8%2Chttp://catalog.lib.utexas.edu/search~S29?/XTemperature%3A+Its+measurement+and+control+in+science+and+industry&searchscope=29&SORT=D/XTemperature%3A+Its+measurement+and+control+in+science+and+industry&searchscope=29&SORT=D&SUBKEY=Temperature%3A+Its+measurement+and+control+in+science+and+industry/1%2C8%2C8%2CB/frameset&FF=XTemperature%3A+Its+measurement+and+control+in+science+and+industry&searchscope=29&SORT=D&2%2C2%2CVolume 3 is up on Library Genesis:http://gen.lib.rus.ec/book/index.php?md5=A371D84737B38CC372E4A4317B91FE1BBut I haven't found a source for volume 6 anywhere.Could you gents search your local university online catalogs for it?

author: cellularmitosis
author_rank: Unknown
timestamp: February 25, 2018, 08:07:28 pm
content: Here's section 4.6, in case that PDF goes offline:4.6. StabilityThere are several intrinsic effects causing instability in thermistors [Zurbuchen andCase 1982], including: mechanical cracking of the thermistor body with temperaturecycling, drift at high temperatures due to ingress of atmospheric gases, changes in thecrystallographic structure, and changing contact resistance between the leads and thethermistor body. The most stable thermistors are bead thermistors encapsulated inglass. Within the range –20 °C to 60 °C, selected and pre-aged thermistors may bestable to better than a few tenths of a millikelvin per year. Thermistors withresistances in the range 2 k? to 10 k? also appear to be the most stable. Glassencapsulateddisc thermistors and epoxy-encapsulated bead thermistors are also verygood but not quite as stable as the glass beads. (See stability studies by Siwek et al., Wise , Edwards , La Mers et al. , Wood et al. ,and Strouse et al. .)The extrinsic sources of instability relate to the instrumentation: insulationresistances, stability of the measuring current and stability of the electroniccomponents used in the measurement.

author: GigaJoe
author_rank: Frequent Contributor
timestamp: February 26, 2018, 03:47:24 am
content: Gentlemen,MV-180 schematic ; Sorry but I dont get it .... Can, please, someone explain OP2 - meaning, current limiter for a heater? And NTC2 - it it really NTC, means negative coeff, then NTC2 - R7 drive to increase voltage output on OP1 , when temp rising ... ( or I'm wrong ? )Thanks ...

author: F64098
author_rank: Regular Contributor
timestamp: February 26, 2018, 08:52:45 pm
content: Quote from: GigaJoe on February 26, 2018, 03:47:24 amYes, inrush current has to be limited. This means a little extension of the heating time, but you do not need an oversized power supply.QuoteIn the next days i will look again at the real circuit.Maybe i made an error...Best regardsFrank