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I need to sense temperature for a lithium battery pack and a TO-220 package transistor. Would an NTC with voltage divider be ok or is it better to use a sensor like the LM36?

Note:

  • Measured temperatures are between 65-95 C.
  • It is no problem if accuracy differs a bit.
  • All would be tied to the ADC pin of a microcontroller.
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    \$\begingroup\$ So why do you think an NTC is not a proper temperature sensor? \$\endgroup\$
    – Justme
    Commented Sep 5, 2023 at 12:45
  • \$\begingroup\$ Do you think if I know the reason I would ask the question?? \$\endgroup\$ Commented Sep 5, 2023 at 16:05
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    \$\begingroup\$ In the title of your question you write "NTC or a proper temperature sensor", implying that an NTC is not a "proper temperature sensor". Justme (and others) are asking what leads you to make that distinction since many (including myself) are of the opinion that an NTC is a "proper temperature sensor". Whether or not a particular NTC might be suitable for your application depends entirely on your requirements. How much is "a bit" when referring to your desired accuracy? \$\endgroup\$
    – brhans
    Commented Sep 5, 2023 at 16:42
  • \$\begingroup\$ @brhans thanks for the explanation, I ment by proper like you get a signal from a sensor not kind of resistors you see, when you use a 1M ohm NTC with a voltage divider will not be same with 10k ohm NTC (speaking in regarding to sensitivity of the MCU). The sensor is just fixed you use it as the datasheet states. And because I thought there would be a great difference between both. \$\endgroup\$ Commented Sep 5, 2023 at 18:30

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There's nothing wrong with an NTC thermistor, it's as 'proper' as anything. Silicon sensors maybe be better or worse than in some regards. As may RTDs or thermocouples or other more exotic sensors.

As well as accuracy and stability (which are usually quite good for an NTC thermistor, at least over a modest temperature range) consider how well you can couple the sensor to what you are trying to measure. Most temperature sensors do an excellent job of measuring their own temperature, however that reading may lag behind (due to poor heat transfer and large heat capacity- especially an issue with still air temperature measurements) or have a large offset to what you are actually intending to measure due to heat loss down the leads, self-heating etc.

Thermistors are cheaply available with very tight tolerance thanks to consumer mass markets. Something like an LM35 or LM335 is accurate to around +/-1°C. A thermistor could be more like +/-0.2°C - 5x better for maybe 1/20 the cost.

For a battery temperature measurement, something like a 10kΩ NTC thermistor is an appropriate solution and is the standard approach. It doesn't need to be a 1% type but you can use one if you like. If you'd prefer (for some reason) to avoid any analog electronics and care little about cost effectiveness, monolithic digital output sensors are available.

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  • \$\begingroup\$ What will happen if I used a ntc 100k with a voltage divider? Would it affect the ADC? \$\endgroup\$ Commented Sep 5, 2023 at 16:15
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    \$\begingroup\$ If your ADC is happy with the source resistance (if it's a voltage divider then then the maximum source resistance will occur at minimum temperature and will be equal to the Rs*Rth(max)/(Rs+Rth(max)) where Rs is the fixed series resistor's resistance and Rth(max) is the resistance of the NTC thermistor at the minimum operating temperature) then it will be fine. If it is not, you might have to add a buffer. \$\endgroup\$ Commented Sep 5, 2023 at 17:05
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A NTC thermistor is actually a pretty good sensor if implemented well. I've done something similar to yours (keeping an eye to the temperature of a Li-Ion cell and adjusting charge current to avoid overheating, of course via MCU code). This works remarkably well and is really easy to implement, as you say, just a voltage divider (but keep an eye on the ADC impedance requirements).

In a different scenario I've used a similar 10K thermistor to measure the temperature rise of a LED PCB, with the thermistor attached to a decent DMM with a serial interface to a PC, and some python code to convert the resistance to temperature, this apparently measures temperature to sub °C accuracy. When left to rest, the apparent measurement noise is again in the <0.1°C range.

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  • \$\begingroup\$ What will happen if I used a ntc 100k with a voltage divider? Would it affect the ADC? \$\endgroup\$ Commented Sep 5, 2023 at 16:14
  • \$\begingroup\$ I'd suggest that you use a 10k NTC and 10k fixed resistor. This would get by the ADC requirements I believe. If you only have a 100k NTC and want to use it: then your divider is a 100kNTC+100k fixed resistor and the impedance is too much. Place a small capacitor - 10nF..100nF - from the ADC pin to GND. The capacitor will be parallel to your NTC or the fixed resistor (it doesn't really matter much whether the NTC will be on the "top" or on the "bottom" of the divider, the conversion code will be different, but the accuracy will be the same). \$\endgroup\$
    – anrieff
    Commented Sep 6, 2023 at 10:56
  • \$\begingroup\$ interesting, but why is using a capacitor would make the accuracy better? \$\endgroup\$ Commented Sep 6, 2023 at 22:50
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    \$\begingroup\$ ADCs cannot measure any voltage input. If that input is too weak - the ADC itself will pull it in some direction, distorting the reading. For those ADC internal operation quirks, read here if interested. 100k+100k is a weak signal. Placing a capacitor to ground will make it a bit beefier, as the 100nF capacitor is orders of magnitude larger than the 10-20pF capacitor in the ADC, so when the ADC samples the signal, the voltage in the 100nF capacitor will only decrease a tiny bit... \$\endgroup\$
    – anrieff
    Commented Sep 6, 2023 at 23:10
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    \$\begingroup\$ ... this of course assumes you're sampling the ADC rarely enough, and won't work if you're sampling it thousands of times per second. For a temperature sensor, it probably does not make sense to sample it more than a few times per second. \$\endgroup\$
    – anrieff
    Commented Sep 6, 2023 at 23:13
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Would an NTC with voltage divider be ok or is it better to use a sensor like the LM36?

Choose the device that meets your accuracy requirements (data sheet information), is easiest to implement, is a decent price, is reliable and, is sourced from a recognized dealer in electronic components. Whatever device you choose it should have a decent data sheet and be made by a recognized manufacturer.

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There is no abolute "proper" or "better" solution in electronics, those notions exist only in relationship with your spec.

Quantify what you mean by "acceptable", then choose accordingly.

It is no problem if accuracy differs a bit

What do you mean by "a bit"? 1°? 10°?, 0.1°?

Once you know that, you can compute the accuracy of your temperature sensing circuit, and choose the solution that fits you precision requirement best, while still optimizing as many other parameters (cost, for example) as possible.

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