A PT100 is a platinum resistor that measures 100 ohms at 0 °C and gains 0.385 ohms for every degree. Here it sits in a Wheatstone bridge on 3.3 V, next to three fixed resistors. The meter in the middle shows the difference between the two midpoints A and B. The tiles show what two different converters make of that voltage. One is the 10-bit ADC (analog to digital converter) in an Arduino Uno. The other is an ADS1115, a small 16-bit ADC module that can measure the voltage between two pins.
Things to try:
- Drag the temperature below 0 and watch what the Arduino shows.
- Switch the load to 1 kΩ and see the output voltage drop.
- Compare the curve with the dashed straight line. The gap grows toward 200 °C.
In short
At 0 °C the bridge is balanced and gives 0 V. Away from 0 °C it gives a few tens of millivolts, which is too little for a plain Arduino input and plenty for an ADS1115.
Formulas used
- Resistance and temperature R₂ = R₁ · [1 + α(t₂ − t₁)] Formelark Eltek LVF 2026, p. 1
- Ohm's law U = R · I Formelark Eltek LVF 2026, p. 1
- Resistors in series R_tot = ΣR Formelark Eltek LVF 2026, p. 1
- Voltage divider U₂ = U · R₂ / (R₁ + R₂) derived from Ohm's law and resistors in series
- Thevenin equivalent Uₜₕ = voltage with no load, Rₜₕ = R seen from the terminals with sources shorted derived from Ohm's law, resistors in series and resistors in parallel