Query on Trimming circuits - Page 1
Forum section: electronics / beginners

author: tumblebomb
author_rank: Newbie
timestamp: January 26, 2021, 09:28:51 pm
content: So I'm building a new PC and adding an airtemp snesor and a water temp sensor (As it's watercooled). They are all 10K ohm @25'C standard, but theres a bit too much variance I've seen.What I'd like to do is make a small PCB to go between the motherboard and the sensor with a trimming pot so I can calibrate them (Some PC software does allow for an offset to be applied but I can't be sure which i'll use). It sounds like a fairly quick and easy project to prototype and then build a few boards for.What I'm not sure of is value of Trimpot to use, or even circuit layout, should it be in parallel or serial to the sensor? If anyone had any guidance or links to examples that would be amazing. I don't have any of my old textbooks to refer to.Thanks

author: ledtester
author_rank: Super Contributor
timestamp: January 26, 2021, 09:56:22 pm
content: How much are your readings off by?And what is the typical temperature range the thermistors will be operating in?A resistor in series will add resistance and for a NTC thermistor this will lower the temp. reading. A resistor in parallel will wildly distort the readings.Every thermistor comes with a temperature-to-resistance lookup table -- for instance this one for a B3950K 10K thermistor:https://cdn-shop.adafruit.com/datasheets/103_3950_lookuptable.pdfFrom that you can get an idea of the effect of adding resistance. Note that the graph is not a straight line -- so adding 5K of resistance, for instance, will result in a different delta depending on the what the temperature is.

author: ve7xen
author_rank: Super Contributor
timestamp: January 26, 2021, 10:04:49 pm
content: What adjustment range are you aiming for? What do the sensors connect to?In series it is only possible to increase resistance; in parallel it is only possible to reduce it. What you may do if you want to have a +/- range is add a fixed parallel resistor to drop the expected value, and then add a pot in series that will allow you to adjust back up to the desired value.For example, if you want to adjust from 5k-15k, you can add a 10k fixed resistor in parallel with the sensor (which will bring its nominal value to 5k), then add a 10k pot in series (which will allow you to adjust from 5k+0 to 5k+10k).It isn't going to be that simple though, as this will also affect how the measured parameter changes the output; e.g. it will affect the temperature coefficient of a thermistor, which would be relatively difficult to recalibrate, especially in the parallel case where it's not simply adding a constant resistance to the unknown. To preserve this, how this would normally be 'calibrated' out would be to adjust the measurement circuit, not the sensor itself, either adjusting the measurement voltage, a balance resistor in the measurement bridge, or with digital calibration, depending on how the measurement is done.Are you sure there is too much variance? These are sensors, they are meant to vary. Even cheap thermistors are generally pretty accurate, like 1% for nominal value.

author: tumblebomb
author_rank: Newbie
timestamp: January 26, 2021, 10:37:32 pm
content: Wow I didn’t realise it could be so in depth. It is a very interesting topic.I’ve only seen 1 pc part manufacturer have the lookup table available, the rest don’t seem to bother, accuracy isn’t too much of a concern for many it seems.Currently sensors will connect to 2 pin headers on motherboard, I expect adjustment range could be 1-10 degrees C max.I’ve seen examples of 2 different sensor giving +1 and +5 degrees compared to a reference. Expected measurement range would be from room temp to 60C. Not that absolute value is the most important, often just interested in the deltas but having an alarm for anything too low/high would be useful. PC motherboards not always known for being good at this but I might be able to find out what chip they are using for it. Down the road I could always look at a micro controller, and I do have a couple of Sensors that have a chip and I2C connection laying around which I’ve never used.I’ve haven’t got anything for testing with currently, just working on the idea in my spare moments.

author: ve7xen
author_rank: Super Contributor
timestamp: January 26, 2021, 11:06:52 pm
content: Quote from: tumblebomb on January 26, 2021, 10:37:32 pmI'd guess that the variance is probably more down to the measurement side than the sensors. You could test this out with the same fixed resistor connected to the various inputs and comparing readings. It may also be that the lookup table / transfer function / linearization being used doesn't match the thermistors. In such a system that isn't really designed for accuracy, I wouldn't really expect better than +/-5°. What is the resolution of the ADC, how is the measurement done, how bad are the power supplies, etc. Then add the uncertainty in the thermistor itself. And since none of this is ever well documented for a PC motherboard, I'd prefer to just implement it myself if I cared. You can use digital sensors like I2C or 1wire, or an integrated analog sensor like LM35, or you can use the thermistors with a resistor, constant-current source or bridge.

author: David Hess
author_rank: Super Contributor
timestamp: January 27, 2021, 12:46:47 am
content: At 25C, the change in resistance is about 450 ohms per degree. From that, a series and parallel combination of resistors can be calculated to trim the response but accuracy will suffer further from the nominal temperature. So for instance a 2k trimmer in series yields 11k nominal, which requires a 110k resistor in parallel to restore the 10k resistance at 25C and allows about a +/-2C trim range.An alternative way which has less effect on the response is to add or subtract a current in parallel or voltage in series but the implementation will depend on how the circuit works.