Linear thermistor ? - Page 1
Forum section: electronics / projects

author: JanJansen
author_rank: Frequent Contributor
timestamp: June 24, 2017, 03:44:53 pm
content: Hi, someone told me i could linearize a thermistor ?I need a 560 ohm PTC resistor.How can i replace this ?thanks

author: DaJMasta
author_rank: Super Contributor
timestamp: June 24, 2017, 04:00:14 pm
content: Many of the major thermistor makers have app notes that go in depth into how they work, how they respond, and some example circuits. Google has many, but how about Vishay for a start:https://www.vishay.com/docs/29053/ntcintro.pdfWhile I can't link it because of google's redirect, TDK's app note has exactly what you're describing on the second page, just search "thermistor linearization app note"

author: MagicSmoker
author_rank: Super Contributor
timestamp: June 24, 2017, 04:13:33 pm
content: Linearizing NTC thermistors is relatively easy; the same is not true for the typical polycrystalline PTC thermistor, which tend to have a more abrupt change in resistance vs. temperature (a nearly hyperbolic curve). Generally speaking, if you need to measure temperature then use an NTC thermistor.560 ohms is a strange resistance value for a PTC thermistor. What was the original part number?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: June 25, 2017, 05:55:27 am
content: Quote from: MagicSmoker on June 24, 2017, 04:13:33 pmHrm, not sure what kinds you're thinking of?Ceramic NTC thermistors (the most common NTC type) have a grossly nonlinear curve, something like R = 1 / log(T) with constants (and higher order terms) thrown in. They can only be linearized in the Calculus sense: that, for a sufficiently small range of temperatures, the resistance changes proportionally (the curve locally approaches a tangent line). By placing (constant) resistors around the NTC, you can trade some gain for range, so that a wider range meets the same linearity spec (within so-and-so percent of linear), but that the absolute slope (|dR/dT|) is smaller as well (lower gain). (Here's a calculator that does the first part -- using Calculus to find the slope, and using resistors to tweak the slope -- but doesn't show range:http://seventransistorlabs.com/Calc/Tempco.html)There are two kinds of PTC resistors: metallic RTDs (doped Si, Ni, Pt and others), and resettable fuses based on conductor-plastic composites (the plastic melts and expands, and the metal particles separate, leaving a suddenly high resistance). The latter are nearly hyperbolic, and useless for temperature sensing applications (but handy for thermal and current protection, hence Polyfuses). The former are quite accurate, with lower noise than ceramic NTCs, and a wide linear range. The resistance is nearly proportional to absolute temperature, while minor tweaks can be done with resistors (linearize over a given range -- again, effectively Calculus at work), or by correction factors or lookup table (the approximating Taylor polynomials are usually given for these types).I would think the OP has this type, but it's not clear why it would need to be linearized if it's replacing a PTC that's presumably already had that solved...Tim

author: MagicSmoker
author_rank: Super Contributor
timestamp: June 25, 2017, 11:59:07 am
content: Quote from: T3sl4co1l on June 25, 2017, 05:55:27 amAnd yet it is still easy enough to linearize them with 2 resistors (one in parallel with the thermistor, one in series) with sufficient accuracy for applications like derating current in a motor drive or power supply. If you need to report temperature with an accuracy that satisfiesNOAAorNWSthen you probably shouldn't use a thermistor in the first place (an RTD would be far more appropriate, and speaking of...)Quote from: T3sl4co1l on June 25, 2017, 05:55:27 amYes, I was thinking of the polyfuse type of PTC, just not being used as a fuse (ie - running sufficient current through it to cause self-heating); I consider RTDs (e.g. - type Pt100) as a completely separate type of temperature sensor though, broadly speaking, they are also PTC thermistors.But getting back on topic, I found some PTC thermistors that are 560 ohms at 25C:https://www.digikey.com/products/en/sensors-transducers/temperature-sensors-ptc-thermistors/550?k=ptc+thermistor&k=&pkeyword=ptc+thermistor&pv866=45&FV=ffe00226&mnonly=0&newproducts=0&ColumnSort=0&page=1&quantity=0&ptm=0&fid=0&pageSize=25http://www.mouser.com/ProductDetail/KOA-Speer/LT732ATTD561J3900/?qs=CS4loa5Mb0aN%2fXICw7XtKQ%3d%3dSadly, though, none of the above parts are in stock, which seems to be the crux of the OP's problem...

author: JanJansen
author_rank: Frequent Contributor
timestamp: June 25, 2017, 01:33:46 pm
content: Nice, thanks for finding, to bad i cant solder SMD, i use thru hole only.Its for a VCO in a old synthesizer.http://privat.bahnhof.se/wb447909/dinsync/service_manuals/TB-303.pdfmainboard R100 top left a bit.Sometimes they use 2times a 1K tempco instead also :http://www.ladyada.net/media/x0xb0x/mainboard_beta.pngSomeone also gave me the tip to not use a tempco resistor, to just use a 1% resistor instead.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: June 25, 2017, 01:57:41 pm
content: That's at D 15, by the way. Hard as hell locating anything on these old schematics, without coordinates...It's in a voltage divider. A different value will be fine. If it's outside the adjustable range of TM5, change R106.It's on a exp converter, so you want PTAT correction, otherwise the tuning will be horribly off as the ambient temperature changes just a little.Tim

author: JanJansen
author_rank: Frequent Contributor
timestamp: June 25, 2017, 02:23:59 pm
content: Ok, thanks, will a thermistor with changes will be linear enough for this VCO tuning ?