How fast it follows
Touch it and the reading takes seconds to catch up: the thermistor and the copper around it have to warm first. The electrical side settles in half a millisecond, and the divider puts at most 625 µW into the thermistor, too little to matter next to the board beside it.
Heat takes seconds
A fingertip is at skin temperature the instant it touches. The reading is not. The thermistor, its solder and the copper under it have to warm up first, so the reading climbs quickly at first and then more and more slowly. The curve is drawn to show that shape, not recorded: the part's thermal time constant is in the datasheet that could not be fetched. For an 0805 part on a small board, seconds is the right order.
Take the finger away and it falls back the same way.
The capacitor is not the delay
The 100 nF capacitor on SIGNAL does slow the voltage, but by half a millisecond: 100 nF behind the two resistors in parallel, 5 kΩ at 25 °C. That is thousands of times faster than the heat. What sets how fast this block follows a change is thermal, not electrical.
The current does not warm it
Every reading passes a current through the thermistor, and a current through a resistor makes heat. Here it is small. The current is never more than VCC over 10 kΩ, 0.5 mA from 5 V. The power in the thermistor is largest when it equals the fixed resistor, at 25 °C, and then it is VCC squared over 40 kΩ: 625 µW from 5 V, about 270 µW from 3.3 V.
That is under a milliwatt. The board the block is plugged into is a far bigger heater: an ESP32 running Wi-Fi turns hundreds of milliwatts into heat a few centimetres away. If the reading is high, move the block away from the board on a cable before blaming the current.
When it does not work
After moving it somewhere new, a minute or two, until the number stops creeping. Its thermal time constant is in the datasheet that could not be fetched, but for a part this small soldered to a board it is seconds, and the board and the air around it take longer.
Hardly. The most the divider can put into the thermistor is VCC squared over 40 kΩ: 625 µW from 5 V, about 270 µW from 3.3 V. An ESP32 board running Wi-Fi a few centimetres away turns hundreds of milliwatts into heat. That is the one to keep it away from.
The block is measuring the board. A regulator, a USB chip or a busy processor warms the air and the pins around it. Put the block on a cable, a few centimetres away, and the reading settles lower.
Not much: the part is already small. Keep it in moving air rather than still air, and do not box it in. Averaging in the sketch adds only milliseconds; it is not what makes it slow.
The raw count says which wire, and the short list of what else it can be.
When the reading is wrong →Edit this page — content/books/ntc-thermistor/how-fast-it-follows.mdx
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