Extracting individual Thermistors in Parallel - Page 1
Forum section: electronics / beginners

author: AQUAMAN
author_rank: Regular Contributor
timestamp: January 10, 2019, 04:46:24 pm
content: If I have 4 thermistors in parallel, is there a way to extract the values from each of them without using a multiplexer?See problem:4 thermistors in parallel:https://imgur.com/a/Jn3W6eTThe best way I can think of is to perhaps split them into two groups using a PMOS and NMOS transistor and then vary the input voltage from -5V to +5V (see below). This only splits the reading into 2 groups, is there a way to split it into 4? Is there such thing as a 'bandpass' switch that only lets current flow between certain voltages? Then you could set the voltage to 4,5,6,7,8V to cycle through each thermistor.https://imgur.com/a/YDYseLF

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 10, 2019, 05:24:01 pm
content: If you don't actually need to know which is which, the smallest value (hottest, assuming these are NTC) dominates, which can be handy for a crude protection system.Otherwise, you're asking for some kind of mux, whether it's an explicit digital one or not. For example, your voltage selection proposal is equivalent to sending that voltage to an ADC, and using the digital output to set a mux. Whether there's a worthwhile simplification with discrete parts, who knows, but you will find it very, very difficult to compete with, say, an 8-pin MCU.Or to put it another way, you're proposing a mux with a multi-nary logic level selector input. That is, instead of binary 0/1, the voltages 4 to 4.99 = 0, 5-5.99 = 1, etc."Bandpass" could be used, if each thermistor is driven with a voltage at a different frequency, and the resulting currents are summed into an output. But then you need to do signal analysis to tease them apart, and you're still looking at an MCU as the best option (ADC + FFT + peak tracking) but with a hell of a lot more steps!Which still isn't a terrible method, actually, but with a lot more considerations, and really only for applications that absolutely demand going to such serious extents. The telephone system went through just such a process back in the 1930s: frequency division multiplex, where individual phone channels are modulated into separate frequency channels, and all those channels are transmitted at once down a single wire (instead of dozens of wires -- a huge savings at a time when most telephone lines were overhead messes!).(But yeah, unless there are really specific requirements, probably not necessary to do that here.)So, really what you need is to step back and consider what IO pins (and types, digital/analog) you have available to perform the selection, and how much hardware you can afford (due to space, cost and design constraints) to implement it.Tim

author: MrAl
author_rank: Super Contributor
timestamp: January 10, 2019, 07:33:05 pm
content: Quote from: T3sl4co1l on January 10, 2019, 05:24:01 pmHi,I dont know what kind of thermistors you are working with nor what kind of accuracy you are after, but a sort of crude method would go as follows.First, enumerate the thermistors 1 through 4.Then, connect silicon diodes in series with each thermistor, the number of diodes for each thermistor is to match its enumeration. This means 1 diode in series with thermistor 1, 2 diodes in series with thermistor 2, etc.Now when you ramp the supply voltage up little by little, you will get a voltage and current reading that correspond to only the first thermistor and diode, then ramp up a little more and you'll see the voltage current reading that corresponds to the first and second thermistors and diodes in parallel. As you ramp up you'll see all four sets in parallel at some point. In this way you'll get four main readings that you can correlate to the thermistor values.Keep in mind that the diode temperature characteristics come into play, but you might be able to correlate that too along with the thermistor readings by calibrating the system using different value resistors first.Might take some good solid experimentation and algorithm to correlate the actual readings to the actual thermistor values, and might turn out a little crude, but should work if you have the supply voltage required to get them all biased at some point.Of course part of the algorithm has to take into account that as the voltage ramps up the readings will correspond to units in parallel so you have to extract the most recently biased thermistor based on that.Also consider that using diodes you might be able to also Charlieplex 2+2 thermistors. That would mean at most you'd only have two thermistor/diode pairs in parallel which would make the algorithm and measurements a little simpler. This would work if you have the ability to reverse the polarity of the test voltage.