Measure temperature with diode - Page 1
Forum section: electronics / projects

author: facumedica
author_rank: Contributor
timestamp: June 25, 2014, 02:47:17 pm
content: Hi guys! I'm building a project that needs temperature measurements up to 230°C. I can use diodes to measure temperature but I don't know if they will act as suposed at those temperatures (The working range of the 1N4148 goes up to 200°C). Can it be done? Or I must go with thermocouples?Thanks!

author: jlmoon
author_rank: Unknown
timestamp: June 25, 2014, 03:04:00 pm
content: Hello Facumedica,If you will look at the 4148 datasheet you will find the Max Tj is 200C, not going to work!, not to mention the leakage currents go off the charts on anything above 140C.I suspect a type J or K thermocouple (chromel – alumel) would much more reliable, depending on required temperature range, your atmosphere or environment. Corrosive / Non-Corrosive.. etc.JLM

author: dannyf
author_rank: Super Contributor
timestamp: June 25, 2014, 03:13:41 pm
content: Two issues:1) calibration: while the slopes (temperature coefficient) are highly similar within the same batch, the starting point differs wildly. Without calibration, it is essentially unusable.2) the temperature range is usually less than 100c. 200c+ is quite extreme.

author: facumedica
author_rank: Contributor
timestamp: June 25, 2014, 03:41:55 pm
content: It will go into a plastic extruder, so corrosion won't be a problem (I think...). Now, what about thermistors? I have never worked with these things, how high they can measure? I don't need too much precision, so +-1/2°C or maybe up to 4°C error won't be an issue.

author: ejeffrey
author_rank: Super Contributor
timestamp: June 25, 2014, 03:51:38 pm
content: NTC thermistors (the common $0.05 / 10 kohm devices) don't have that kind of range -- they have a strong non-linear response and only work well over a relatively narrow range, usually around room temperature. A Pt100 platinum RTD will work very well in that regime, and is quite easy to use.

author: C
author_rank: Super Contributor
timestamp: June 25, 2014, 04:18:07 pm
content: Have you thought of using a soldering iron heater?The better ones have both the heater and the temp sensor.Some here are probably thinking too messy or expensive?Look at the big pictureIf you try to heat cold plastic too fast you burn the outside while the center is still cold.You also need to quickly start and stop the plastic flow, a temp change can help for thisThink of existing extruder as a bulk heater,add a low mass tip that you can change the temp faster. Would allow the start and stop faster or cleaner.

author: facumedica
author_rank: Contributor
timestamp: June 25, 2014, 04:49:55 pm
content: I don't think buying a soldering iron to get the heater and the sensor would be possible, I can't even buy a decent one for my selfMaybe I just go for the thermocouple, they are quite cheap so...The heaters are not a problem, I have a couple of PTC heaters to "pre-heat" the material (at the begin) and then after that a lot of 7W resistors on a decagonal arrangement. I calculated the best shape with some basic math. For example my cilinder perimeter is of about 110mm, the resistors are 10mm wide, I could put 11 of them, but for practical reasons I just go for 10, so on each side of the decagon I have 11mm. So having 10 plain surfaces give me the chance to increase the thermal contact between the extruder and the resistors.If someone is interested in the project, and want to share advices, I would be glad to hear them!

author: SeanB
author_rank: Super Contributor
timestamp: June 25, 2014, 06:59:56 pm
content: Quote from: facumedica on June 25, 2014, 03:41:55 pmYou will be best off with a Type K thermocouple and an amplifier or dedicated thermocouple amplifier. If you are working with plastics you will find out that they are very corrosive when hot, so get a thermocouple and get spare ones as well, as they will fail after a while when the leads corrode off. Typically you have a sheathed thermocouple in a thermowell in the injector barrel, one per section, so you likely will have at least 5 thermocontrollers, typically one doing the inlet feed, 3 doing the ramp up of temperature along the extruder shaft and then a final one at the injector end. You also will have to control the temperature of the die set as well with more complex mouldings.You probably will be best off buying 5 separate DIN 48x48 sized thermocontrollers, preferably a PID type with autotune, as these are a lot easier to set up. Either get the SSR drive types or use the relay drive ones with external contactors to control the heaters. In any case you will need typically a minimum of 10kW of heater for a typical small injection moulder. Put circuit breakers per heater as well, as they also tend to fail with aging.Blow moulding is so much easier, only 6kW of heater for a 150 by 150 sheet, and 2 temperature controllers with PT100 sensors, as they will work to 200C. For extrusion you will get up to 250C in places down the barrel to get the plastic to flow. I have seen some run up to 350C for some plastics, though there you keep a fire extinguisher very close by, as if it stays there too long it will catch fire.

author: C
author_rank: Super Contributor
timestamp: June 25, 2014, 07:08:45 pm
content: soldering iron heater not the whole ironhttp://stores.ebay.com/trustyhl/Heating-Elements-/_i.html?_fsub=3442149016

author: mikerj
author_rank: Super Contributor
timestamp: June 25, 2014, 07:48:56 pm
content: For 200 Celsius operation your options are pretty much limited to either a thermocouple or a resistance temperature detector (RTD), e.g. a platinum sensor such as a PT100. Semiconductor sensors (including thermistors) just aren't going to cut it.RTDs don't require cold junction compensation and only require minor linearisation. However the resistance change over temperature is quite small so some care is needed to ensure you don't swamp the result in offset errors etc. and because of the low resistance you may need to use a four wire connection to get sufficient accuracy.Maximmake an interface device with built in ADC giving an SPI output if you need it.

author: facumedica
author_rank: Contributor
timestamp: June 26, 2014, 02:42:09 pm
content: I'm trying to make something with the stuff I can get, where I live isreally hardimpossible to get parts (Yes, XXI century and I can't even buy an IC). Awful laws...Thanks for your help, I'll see what I can do. Maybe a epoxi covered type-K thermocouple?

author: David Hess
author_rank: Super Contributor
timestamp: June 26, 2014, 04:18:54 pm
content: Quote from: dannyf on June 25, 2014, 03:13:41 pmDelta Vbe temperature measurement allows uncalibrated absolute temperature measurement using a PN junction. Basically the difference in forward voltage drop changes by 198 microvolts per degree C over 1 decade of current and this is independent of the absolute value of the forward voltage drop. Typical accuracy over a range of 0C to 100C is 1 degree. Microprocessors often use this tecnique for on chip temperature measurement and there are ASICs like the LM86 which implement it.There is a good description with references starting on page 7 of Linear Technology application note 45:http://www.linear.com/docs/4136Quote200C might be possible with a transistor designed for high temperature operation like a 2N2484HR but I would at least try using a low leakage transistor in a metal can package like TO-5, TO-18, or TO-39. JAN parts are rated to 200C are readily available but more expensive. Look for one with low leakage indicating that it is not gold doped. I would also try a low leakage JFET.

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: June 26, 2014, 09:02:17 pm
content: It's always fun to see what you can do with next to nothing. Possibly you could wind up a tiny spool of fine enameled wire (need an enamel that can stand the temp.) Measure the resistance and use the TCE of copper to figure the temperature. That's how they measure the core temperature of transformers- use the winding!