oven temperature controlled ANALOG circuit, wow design - Page 1
Forum section: electronics / projects

author: oz2cpu
author_rank: Frequent Contributor
timestamp: May 03, 2022, 08:53:17 am
content: oven temperature regulated circuits are very well known in timing circuits, like xtal osc,and also seen more and more in voltage references,but how often do you see this in analog circuits ?in my latest teardown of a very special analog amplify measure producti found a very well designed oven for a few opamps, transistor pairs,see all details here, please feel free to commenthttps://youtu.be/yJha9LdtfVA

author: David Hess
author_rank: Super Contributor
timestamp: May 03, 2022, 11:20:04 am
content: Quote from: oz2cpu on May 03, 2022, 08:53:17 amTemperature control used to be more common to get differential pair drift lower than 1uV/C, but now chopper stabilized amplifiers and the best precision bipolar parts can achieve this without temperature control.In the 70s Fairchild produced the uA727 which was a temperature controlled differential preamplifier with 0.6uV/C drift.

author: oz2cpu
author_rank: Frequent Contributor
timestamp: May 03, 2022, 11:56:53 am
content: good point David, This unit could be from 1973-74 due to ic markings.so i would assume this method was the only way to go, else they would not go as far,it is a very expensive and complex solution.

author: David Hess
author_rank: Super Contributor
timestamp: May 03, 2022, 03:37:03 pm
content: Quote from: oz2cpu on May 03, 2022, 11:56:53 amIn the 1980s you could buy PTC constant temperature "hats" for metal TO-99 parts like operational amplifiers. Before the OP-07 became available, they were one way to increase precision of common parts like the 301, and of course they were later used on parts like the 308 and OP-07 anyway.Another way to achieve precision is with one transistor of an array used as a heater and another used as a sensor to hold the temperature of the chip constant.

author: oz2cpu
author_rank: Frequent Contributor
timestamp: May 03, 2022, 07:11:17 pm
content: i would love to get my hands on some of them heater hats :-)I tried to use normal PTC fuses for this, and they do help and regulate and all that,but not as tight as i like it to be. I like the simple solution.

author: David Hess
author_rank: Super Contributor
timestamp: May 03, 2022, 11:12:09 pm
content: Quote from: oz2cpu on May 03, 2022, 07:11:17 pmI searched for them a couple years ago but could not find any reference to them, including datasheets. I might have one laying at the bottom of a box somewhere.QuoteI think I would use a block of aluminum with one or two TO-220 or TO-225 packages mounted to it as heaters. An NTC thermistor can then be easily attached for temperature sensing.

author: oz2cpu
author_rank: Frequent Contributor
timestamp: May 04, 2022, 07:12:56 am
content: correct, i made a tiny circuit that does exactly that : NTC, OPAMP, FET, POWER RESISTORthe pcb is so tiny i can clue it on the top of IC´s i like to perform more stable, less affected by ambient temp changes.by the way : todays release, oven xtal oschttps://youtu.be/6oYjtm0BNzw

author: SeanB
author_rank: Super Contributor
timestamp: May 04, 2022, 10:34:14 am
content: 80C so that it will be well above ambient, even if the outside of the equipment is at 50C, and the inside is at 70C, still 10C to regulate with. Power transistor is heater and sensor is likely one of the transistors in close contact with the brass shield, they use the Vbe drop to extract temperature of the junction, and then this lower voltage is used to control the bigger power device. Likely using the same reference voltage used to make the oscillator power supply as well. Tantalum bead is fine at high temperatures, they are not going to degrade with high temperature like electrolytics. Can be replaced with a wet slug glass sealed tantalum for more reliability, as they are fine for long exposure at 125C.

author: Fraser
author_rank: Super Contributor
timestamp: May 04, 2022, 10:52:31 am
content: This topic caught my eye for a different reason to the OP’s interest in the design. I am playing with Blackbody temperature references for thermal imaging targets and have been looking for simple analogue heater control circuits that could be built into compact designs as quick test sources. I thought that analogue loop heater control circuits would be quite common, but this is not the case. I suspect modern designs will all use clever, highly integrated controllers or one of the modern miniature ARM processors that are now so inexpensive. I like the idea of the PTC ‘hats’ as I am already looking at common PTC heaters that are available cheaply from China. I had not considered using a PTC fuse though ?.. that is a really interesting ideaThanks for raising this topic …. Very interesting

author: SeanB
author_rank: Super Contributor
timestamp: May 04, 2022, 01:13:30 pm
content: Fraser, problem with the PTC fuses is that they have a lot of hysteresis and also tend to drift in temperature switching point with time. You could probably use the PTC heater as a way to get up to temperature rapidly, and then control temperature more closely to a lower point than the set point of the PTC device, relying on it's higher resistance around transition to act as a limiter of input energy to the system, improving accuracy, and reducing overshoot. The normal application for PTC heaters has a broad range of temperatures it will settle to, not great for a precision black body, but great to get it to 80% of input power fast.

author: mawyatt
author_rank: Super Contributor
timestamp: May 04, 2022, 08:33:21 pm
content: Quote from: Fraser on May 04, 2022, 10:52:31 amBack in 1980 we worked on a new battlefield sensor called XM-21, this was a passive remote sensing chemical agent detector intended to give early warning of an impinging chemical warfare attach. To self calibrate the instrument an internal blackbody made from alumina and coated was flipping into the field of view. This BB was servoed to set temperatures above and below ambient, ambient and absolute temperatures to calibrate the instrument. The BB had a solid state temperature sensor and Peltier device in a closed loop analog servo, where a DAC issued temperature commands and the control loop servoed the BB to that temperature.Recall the servo loop was just a simple dominate pole type integrating loop with a lead network in the feedback for stability, classic analog type. These tend to be somewhat slow due to the long thermal time constants involved. Maybe a more non-linear loop would be quicker, where a table is utilized and the temperature setting element is driven hard for a fixed time scale, then the loop is closed for traditional closed loop analog type control after the BB temperature has ramped close to the desired temperature?Anyway, think you have a number of options available to consider, and good luck on your projectBest,

author: oz2cpu
author_rank: Frequent Contributor
timestamp: May 05, 2022, 06:56:43 am
content: there are a few ways to do thermal , even in xtal ovens..the two i see the most : direct, (heater directly connected to xtal)and indirect (a heated box, round the xtal and all parts, with out direct contact)the new video release of today, show an indirect heated xtal oschttps://youtu.be/E2utGpBx25Aviewer discretion is advised !! since this one contain anglegrinder !!:-)

author: David Hess
author_rank: Super Contributor
timestamp: May 05, 2022, 05:45:41 pm
content: Several times I have gone the other way and used a simple control loop with one operational amplifier to control temperature with variable fan speed. For a temperature sensor I usually use a diode, but an NTC thermistor is a better solution because it will not require any calibration. I have thought of making a little board that uses the delta Vbe method but I just do not require it often enough for it to pay off.

author: james_s
author_rank: Super Contributor
timestamp: May 05, 2022, 06:51:35 pm
content: I think most of the circuits for this sort of precision temperature control are analog. They often use the power transistor itself as the heater too.

author: magic
author_rank: Super Contributor
timestamp: May 06, 2022, 09:32:04 am
content: 741 opamp maintains constant die temperature if its inverting input is connected to ~1.8V (with -7mV/°C setpoint coefficient) and the output is shorted to the negative rail through a few diodes(direct connection would result in continuous 25mA regardless of anything).It can also work as a thermometer by simply wiring it for unity gain with the noninverting input floating or tied to the negative rail.

author: oz2cpu
author_rank: Frequent Contributor
timestamp: May 31, 2022, 08:21:13 am
content: todays video release is another ovenhttps://youtu.be/UpAvgAQBId4this one is a case in a case in a casewith no isolation used, so that design was new to me :-)also what i saw : curved crystal ?? any one seen that before ?

author: twospoons
author_rank: Frequent Contributor
timestamp: May 31, 2022, 11:47:22 pm
content: The round crystal is the same as what I was using at Rakon, when I was working on their double oven oscillator. Its an SC-cut crystal, which has a more pronounced third-order (cubic) f/t curve, as compared to the more usual AT-cut. This results in two temperatures on the f/t curve where the temperature coefficient is zero, and the oven will be set to one of those temperatures.In our product the crystal temperature was stable to a few mK, and the resulting oscillator was stable to 0.2 ppb over a -40C to +70C ambient. For a 10Mhz oscillator than means a variation of just 0.002Hz.The control circuit was just a few opamps, thermistors and BJTs.

author: oz2cpu
author_rank: Frequent Contributor
timestamp: June 01, 2022, 11:40:57 am
content: thanks for the comment, i lost my 10MHz HP osc, i loaned to someone, many years ago and now cant remember who,i think 10MHz is a much more usefull frequency, since all instruments accept that freq,cool to know the "lens" cut shape is seen before, i just have not seen it before