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| Temperature time constant of chip capsule epoxy? |
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| David Hess:
--- Quote from: MT on August 21, 2019, 11:35:31 pm ---Whats a "moderns solution" to exponating a current whos cost equaling 2 trannies and a NTC/PTC and some scaling resistors set aside Zeloof's garage home cooked integrated solutions? --- End quote --- For low changes in current, the change in power dissipation is minimal so all that is required is good thermal coupling. This goes double when the external signal conditioning holds Vce to a very low value. The common alternative is to use an integrated transistor array with one or two transistors operating as active elements, one transistor operating as a temperature sensor, and one transistor operating as a heater. This reduces the thermal time constant to milliseconds or lower. |
| MT:
--- Quote from: David Hess on August 22, 2019, 10:29:53 am --- --- Quote from: MT on August 21, 2019, 11:35:31 pm ---Whats a "moderns solution" to exponating a current whos cost equaling 2 trannies and a NTC/PTC and some scaling resistors set aside Zeloof's garage home cooked integrated solutions? --- End quote --- For low changes in current, the change in power dissipation is minimal so all that is required is good thermal coupling. This goes double when the external signal conditioning holds Vce to a very low value. The common alternative is to use an integrated transistor array with one or two transistors operating as active elements, one transistor operating as a temperature sensor, and one transistor operating as a heater. This reduces the thermal time constant to milliseconds or lower. --- End quote --- Im perfectly aware of sensor/oven solution e.g LM3046 (used it many times) for instance but thats one are obsoleted (thanks TI) not to mention uber obsolete Ua723 , no not the regulator. My problem , and i apologize if not explained in clear is board space and component count to fix a thermal drift in the case of expo and range issue in the case of linear. The linear one requires redesign and more control lines and eats more components thereof the expo type, both gives different problems to what has to be done by the actual circuit. Other exotic solutions are available such as double expo's/multipliers one counteract the other tempco but thats only reasonable integrated and what was once available in 1980 is obsolete since then. Also a temp sensor in each current sink (6 of) feedback to MCU as mentioned previous, or linear current sink with switchable ranges, all cost more in all aspects of the initial design thats why the NTC/PTC request for that tiny fickle current that shift just so tiny much. --- Quote --- --- Quote ---Any idea about a 0603 NTC that have 0.9-1sec response time? --- End quote --- --- Quote ---Such a rating would be pointless: the thermal mass of a chip component doesn't much depend on the material it's made from. --- End quote --- --- End quote --- Sorry dont understand your point, if the NTC are made of concrete it would ofcourse react very different then what a NTC usually are made of would. I aslo see package reaction times between DIP14 and SOT23-6 beeing quicker. So if NTC/PTC have one time constant and the current sink another it not going to work right? Also there is tolerances on NTC B and PTC t.c.r who usually is 10% some 5% but anyway, while el cheapo NTC can be had at 1%. --- Quote ---A smaller component altogether will have a shorter time. --- End quote --- yes, thats my point! I thought! --- Quote ---It's not really a thermal time constant, it's more about thermal diffusion through the board material and traces; it's a bit messy of a process. So you wouldn't necessarily expect an 0603 to have half the time constant of a 1206, despite being half the length and width. --- End quote --- Well, looking at the NTC/PTC time constants specs are all over the place, some manufacturer dont even specify, some manufacturers of chip termistors also not mention self heating.Some even say chip types is not on par with wirewounds while they have their problems as well. --- Quote ---Beats the hell outta me, what are you exponentiating and why? --- End quote --- A tiny control current point that needs to be compensated due to environmental temp change. --- Quote ---Point being, the solution lies not in solving superficial problem Y, but underlying problem X. Tim --- End quote --- Uhum yes, and thats what im trying to do and i dont have many options then a PTC or NTC. So there are no 0603 or 0805 NTC 3300ppm i can epoxy blobbing together to fix temp drift? Are there any linear current sink/source , low pin count chip that either have automatic current range switching or can be modified to have switchable current ranges? |
| David Hess:
--- Quote from: MT on August 23, 2019, 01:22:15 am --- --- Quote from: David Hess on August 22, 2019, 10:29:53 am --- --- Quote from: MT on August 21, 2019, 11:35:31 pm ---Whats a "moderns solution" to exponating a current whos cost equaling 2 trannies and a NTC/PTC and some scaling resistors set aside Zeloof's garage home cooked integrated solutions? --- End quote --- For low changes in current, the change in power dissipation is minimal so all that is required is good thermal coupling. This goes double when the external signal conditioning holds Vce to a very low value. The common alternative is to use an integrated transistor array with one or two transistors operating as active elements, one transistor operating as a temperature sensor, and one transistor operating as a heater. This reduces the thermal time constant to milliseconds or lower. --- End quote --- Im perfectly aware of sensor/oven solution e.g LM3046 (used it many times) for instance but thats one are obsoleted (thanks TI) not to mention uber obsolete Ua723 , no not the regulator. My problem , and i apologize if not explained in clear is board space and component count to fix a thermal drift in the case of expo and range issue in the case of linear. The linear one requires redesign and more control lines and eats more components thereof the expo type, both gives different problems to what has to be done by the actual circuit. Other exotic solutions are available such as double expo's/multipliers one counteract the other tempco but thats only reasonable integrated and what was once available in 1980 is obsolete since then. Also a temp sensor in each current sink (6 of) feedback to MCU as mentioned previous, or linear current sink with switchable ranges, all cost more in all aspects of the initial design thats why the NTC/PTC request for that tiny fickle current that shift just so tiny much. --- End quote --- Another method which might apply to your situation is to use a PTC as a temperature regulator and heater. They used to make little PTC ovens which fit on TO-99 cans and worked very well to make very low drift operational amplifiers. |
| ajb:
You could certainly stick an NTC to an IC or transistor package to get the thermal parameters of the board out of the equation. Time constant is then dependent on the mass of the NTC and the total thermal resistance between the die and the NTC. Would that get you a fast enough response? I dunno. I guess that if you wanted an even faster response you could weld a thermocouple onto a lead, that would give you minimal additional mass and intermediate thermal resistance. Then you just have to deal with rejecting whatever signal is present on that particular lead from the TC signal (plus all the normal annoyances of using thermocouples!) |
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