Convert PT100 Daughter Board to PT1000?

[Topher]

I was looking at the documentation for the MAX31865 used in the PT100 daughter board. I have long leads on my sensors so i think 100ohm sensors might be problematic for my application and also, I have lots of 1k sensors. Could I change out R1 and R3, and R2 and R4 with the appropriate resistors to compensate for the 1k sensor? I think I could leave the filter caps larger and it should be fine as I dont need super fast readings.

[Stephen6309]

@leckietech PT1000 connect as thermisters.

[zapta]

What duet board are you using? Generally speaking, PT1000 doesn't require a daughter board and can connect directly to the duet. You do need to tell the duet though that this is a PT100.

As for using the MAX31865 for PT1000, I wonder if it will be significantly more accurate than the duet alone.

https://www.digikey.com/en/datasheets/maxim-integrated/maxim-integrated-max31865

[mendenmh]

@zapta I would strongly expect this to be more accurate than the A/D input of the Duet. It won't suffer channel-multiplexing induced charge injection, and other sources of error in multiplexed A/D system. It very likely uses a precision constant-current source to drive the sensor, so the accuracy of the output isn't affected in the same way it is with the output in a bridge circuit.

@LeckieTech It seems as if there is no reason switching resistors and capacitors wouldn't work. The chip is designed to do this.

[achrn]

@leckietech said in Convert PT100 Daughter Board to PT1000?:

I was looking at the documentation for the MAX31865 used in the PT100 daughter board. I have long leads on my sensors so i think 100ohm sensors might be problematic for my application

Long leads is what four-wire connections are for. The four wire connection eliminates the error due to lead resistance (provided all four wires are the same and at the same temperature) so the lead-length-induced error is only that due to the portion between the point where the leads come together and the sensor element - which is normally (or should be) very short.

[dc42]

@leckietech yes that should work. I'm not sure whether you could get away with not changing the capacitor, because I have a feeling that the MAX31865 doesn't supply voltage to the sensor continuously.

To reiterate the comments made by others:

1. You can eliminate wiring resistance from the measurement by using a 4-wire connection (even if the sensor itself is 2 wire);
2. You can connect PT1000 sensors directly to thermistor inputs, although accuracy may not be as great as when using the MAX31865.

[Topher]

@achrn I have a 1mm pitch 300mm long FFC driving my hotend. The wiring path for the sensor has to travel through this.

[Topher]

@zapta said in Convert PT100 Daughter Board to PT1000?:

What duet board are you using? Generally speaking, PT1000 doesn't require a daughter board and can connect directly to the duet. You do need to tell the duet though that this is a PT100.

im using Duet 3 mini. I need 2 additional sensors for my build. One for the cooling fan air intake and one on the LDO motor feeder.

[Topher]

@dc42 Thank you, this is what i needed to know to add additional sensors to my printers.

[mendenmh]

@leckietech do you have enough conductors to put a ground on each side of the PT100/PT1000 wiring? It will help hugely on the noise floor.