RTDs are really very simple devices: just a small strip of Platinum that measures 100Ω or 1000Ω exactly at 0°C. Bonded to the PT100/PT1000 are 2, 3 or 4 wires.

## 4-Wire RTDs

We'll explain the 4-wire version since that's the most complex. Normally if you want to measure a resistor you just connect your multimeter to each side of the resistor. The multimeter puts a small current through the resistor and measures the voltage generated across it (remember V = I * R). This works great for just about all resistors. However, for very precise readings of low-resistance resistors, you also have to account for the wires connected! For basic resistors, they are only good to 5% anyways so we don't mind the resistance of the wires.

For RTDs, the wires, especially the 1 meter long ones, are 1, 2 maybe even 4Ω of extra resistance! That can add up to half or even a full °C! No good, we want to make sure that resistance is not included in our measurement

Thus, the 4-wire RTD. Each side of the RTD has two wires attached. Each wire is maybe 1Ω of resistance. When connected to the amplifier, the smart amp will measure the voltage across the RTD and also across the wire pairs.

For example, here's the approximate resistances of a 4-Wire PT100 RTD at 0°C (for a PT1000, the middle resistance would be (1002Ω rather than 102Ω)

(Remember that the middle resistance - 102 or 1002 Ω - will vary with temperature, but the 2Ω wires will not) When the amp measures this sensor, it will measure the resistance between one set of red and blue wires. It will then measure the resistances between the red wires and blue wires. Then divide those resistances by half - since there's two wires and we just want the resistance of one wire. The final result is 102 - 1 - 1 = 100Ω

## 3-Wire RTDs

These are very similar to the 4-wire type but there is only one 'pair' of connected wires. The reasoning for this is that the wires for the RTD are all pretty much the same gauge and length, so rather than having two pairs, the amplifier will just read one pair and use that resistance as the same for both wires.

## 2-Wire RTDs

These are as simple as it gets, only one wire per side. You may need to calibrate the sensor by putting it in an ice bath to get the resistance at 0°C (say 102Ω) and then subtracting 100Ω to figure out the collective resistance of the connection wires!

# How To Wire Up!

## 4-Wire Sensors

Connect the four wires to each of the pads. Use a multimeter to determine which wires connect together directly (2 ohms or so between them) and which connect through the RTD. Chances are the wires that connect together are the same color. The two pairs connect so that the ones that are connected together go into the two matching terminal blocks on left or right. It doesn't matter which of the matched pair is on the outside or inside. It doesn't matter which of the match pairs are on the left or right.

Do not solder closed any jumpers or cut any jumpers. Use as is!

# 3-Wire Sensors

Connect the three wires to the three right-most contacts. Use a multimeter to determine which wires connect together directly (2 ohms or so between them) and which connect through the RTD. Chances are the wires that connect together are the same color. The two wires that are connected together should go in the right-most blocks (labeled F+ and RTD+). It doesn't matter which of the matched pair is on the outside or inside. The third wire that is on the other side of the RTD connects to the left (marked F- or RTD-). It doesn't matter which slot it's in!

You will have to cut the thin trace in between the 2-way jumper on the right side of the board, and then solder closed the blob on the right side.

Then next to the terminal block on the left, solder closed that jumper as well. Alternatively you can put a piece of wire into the terminal blocks to 'short' them

# 2 Wire Sensor

This is the easiest wiring, you can just use either terminal block slot on the sides for each wire. Then either solder closed the jumpers next to the RTD terminal block or put little wires in the right and left terminal blocks to short them together.

This guide was first published on Nov 15, 2016. It was last updated on Nov 15, 2016.

This page (RTD Wiring & Config) was last updated on Aug 13, 2022.