MAX31865 — PT100 and PT1000 probes¶
Being prepared
Chosen for the catalogue; the driver has not shipped yet. The figures below still need checking against the datasheet.
What it does¶
Reads a platinum resistance thermometer — the probe on the end of a cable that industry actually uses for temperature. A PT100 changes resistance with temperature in a way that is stable for years, works over a wide range and can be several metres from the electronics, which is exactly what a semiconductor sensor cannot do.
This is the part for measuring something hot, something far away, or something that has to still read correctly in five years.
Parameters¶
| Measures | Resistance, converted to temperature |
| Probes | PT100 and PT1000, in 2, 3 or 4-wire connection |
| Converter | 15 bits |
| Interface | SPI |
Bus and addressing¶
SPI, so there is no address — each chip gets its own chip-select line. That, rather than an address, is the limit on how many you can fit.
Wiring¶
A reference resistor sets the measurement range and must match the probe: the usual values are 430 Ω for a PT100 and 4300 Ω for a PT1000. Boards are sold fitted for one or the other, and a PT1000 on a PT100 board reads nonsense.
Use three or four wires unless the probe is right beside the board. In a two-wire connection the resistance of the cable is added to the resistance of the probe, and a few metres of thin wire is worth a degree or two.
In a project¶
To be written.
Traps¶
| Trap | What to do |
|---|---|
| Wrong reference resistor | The commonest mistake with this part: a PT1000 probe on a board built for PT100. Check the resistor before blaming the probe. |
| Two-wire on a long cable | The cable becomes part of the measurement. Three wires cost nothing extra and cancel it. |
| Running probe wires beside a motor cable | A resistance measurement over a long cable picks up everything nearby. Separate them, and use screened cable with the screen earthed at one end. |