Adapting 2 wire PT100 to 4 wire



  • After reading David's (dc42) blog regarding modding the 2 wire PT100 I've decided to go ahead with it as the genuine 4 wire PT100 appears to be a little on the rare side.

    https://miscsolutions.wordpress.com/2016/06/25/more-delta-printer-upgrades-wifi-and-silence/

    In the above blog it isn't clarified what the connections are to the Duet PT100 daughterboard. It would seem sensible that the four wires go into the two connectors the same as a two wire unit would be connected. I'm also assuming that the increase from 2 wires to four wires is simply to reduce the resistance through the connection, if this is the case wouldn't it be neater just to use a heavier gauge wire which would have the same effect as adding two wires to the original two wires, if of course that resistance reduction is the aim of adding the extra wires.

    Sorry for making this sound a little complicated !

    Keith



  • pretty much covered in the docs?

    https://duet3d.dozuki.com/Wiki/Connecting_PT100_temperature_sensors

    as to the why kelvin connection style is prefered is covered in detail here https://en.wikipedia.org/wiki/Four-terminal_sensing



  • @bearer Thanks very much for your input, I think I may have complicated my question.

    I have previously read both those articles and neither cover modding a two wire to four wire or it's connections to the Duet 2 Ethernet. I've also read lots of other articles and searched the forum before making this post, I've not found an answer to my question.

    The mod was made by David and shown in his blog ( the link in my previous post) but he doesn't clarify what the connections are to the PT100 daughterboard.

    David only talks about adding the extra two wires to his hot end connector which was a 2 x 4 connector and so became a 2 x 5. This connector is an addition by David to his machine for connecting simplicity or rather disconnection simplicity I assume πŸ™‚

    Using my electronics knowledge ( 50 years + ) I would assume that the extra wires are simply to reduce the resistance through the connection because any extra resistance affects the reading received by the controller. Am I right in this assumption, if so I connect the modded PT100 two wire as a standard two wire unit.

    Any thoughts?



  • David (dc42) could you comment please, very grateful if you could.

    Keith



  • Which version of the Duet PT100 boar do you have? The Wiki should have you covered in any case.



  • I have the Duet 2 Ethernet. Unfortunately the Duet wiki doesn't cover it, it's a simple codge up devised by David to make the 2 wire PT100 more reliable in the case of not being able to get hold of a 4 wire version which appear to be a bit rare and of course the 2 wire is cheaper πŸ™‚

    Keith



  • Okay, you want to turn the cheap 2 wire into a 4 wire like he did?

    Then do it like the picture, and wire the outer most wires to the outermost terminals, and the innermost wires to the innermost terminals.

    The 2x5 connector is just a 10pin connector for easy disassembly and doesn't affect 2 or 4 wire connection.

    Edit: maybe a copy/paste collage form the blog/wiki helps?
    0_1552580616694_f1dd7a3c-75f2-455f-9f9d-1fef4bb437e9-image.png



  • (Duet 2 Ethernet doesn't have PT100 inputs, you need the PT100 daughter board, which is what version I was asking about)



  • @bearer

    That's it, thank you ! It's exactly what I've been looking for and haven't found. Can you link me to it please.

    Keith



  • @bearer Yep, I have the appropriate daughterboard πŸ™‚



  • It is described in the wiki; but ref picture in post above.

    However depending on the version of the PT100 board you may have to remove jumpers, cut bridges or even desolder bridge/0R resistors for it to work as a 4-wire input. (All of that is also described in the Wiki)



  • @bearer Yep, I'm up to date with the daughterboard, I actually have 3, a pre-production where you cut the track, the first production with the solder bridge jobby and the current version with the jumpers, which I'm using in my latest printer.

    What I'd like to see is the page where you found the pic, I never came across that after searching for sever hours across the net. Could you give me the link to the page you got the pic. Thanks

    Keith



  • At the end of the day there is absolutely no difference between a 4 wire and 2 wire sensor itself. The probe is the same. All they do is add the extra wires like in the photo above when they make them. I work in the irrigation industry and we use them for our submersible pumps. Generally we use 3 wire pt100’s which we assemble ourselves. We have pumps that can be over 200m from the controller and 3 wire sensors are accurate enough. We are working on a 5-10 degree temp increase from standard running temp to trip temp (most motors will run at 35 degrees) so we do need some accuracy. All it is doing is measuring the resistance down the pair of legs to help compensate to make the reading more accurate. When you actually look at the lengths of the runs we use for the printers, it is not necessary to run a 4 wire sensor. At the temps we run 0.4 ohms equates to about 1 degree (I think it is slightly bigger as it gets into the negatives) and I don’t think you would even have that much resistance down the cables so it won’t effect your reading enough to warrant to do it. Plus I have sausage fingers and trying to get the wires into those tiny terminal blocks is hard enough just with 2 πŸ˜‚

    Kind Regards,
    Sam



  • @kayjay said in Adapting 2 wire PT100 to 4 wire:

    What I'd like to see is the page where you found the pic, I never came across that after searching for sever hours across the net. Could you give me the link to the page you got the pic. Thanks

    I copied the hotend picture from Davids blog, and the PT100 picture from the wiki, and placed them next to each other. It is only described with words how to do the wiring in the wiki. Hopefully the collage makes it clearer.



  • @bearer that's what's odd about the pic with the hot end. If you follow that link I put in the first post it leads to David's blog but that pic isn't there, that's why I wanted the link, so I could see the page it came from. I've searched high and low to find that info, I'm amazed you found it for me, you certainly have my thanks but can you give me the link please, I'd like to get to the bottom of why I can't see it and you can.



  • @samlogan87 Thanks Sam, I read somewhere on here about the 0.4 ohms equating to 1 degree, I suppose an extra foot of wire needed to reach the controller wouldn't make that much difference as you say. I think I'll forget all about adding the extra wires, I don't have sausage fingers but I have 67 year old eyes that don't work all that well even with my reading glasses on πŸ™‚

    Keith



  • @samlogan87 said in Adapting 2 wire PT100 to 4 wire:

    When you actually look at the lengths of the runs we use for the printers, it is not necessary to run a 4 wire sensor.

    It's not just about the length, it also helps reject the electrical noise interference from being run alongside other wires like the heaters and endstops and steppers.



  • @phaedrux I would be very interested to know how much of an effect it has. We have 3 wire ones running parallel to sometimes 500A continuous loads and we don’t tend to have to many issues with temperature.

    Kind Regards,
    Sam



  • Well going from reports here on the forum switching to 4 wire has helped some people.

    I'm not an electrical engineer though. My understanding stops at 2 wires good, 4 wires better.

    For what it's worth I run about 2 meters of 4 wire PT100 in a bundle to the hotend along with the heater, BLTouch, 2 fans, and stepper wires, and haven't had any issues.



  • @kayjay said in Adapting 2 wire PT100 to 4 wire:

    @bearer that's what's odd about the pic with the hot end. If you follow that link I put in the first post it leads to David's blog but that pic isn't there, that's why I wanted the link, so I could see the page it came from. I've searched high and low to find that info, I'm amazed you found it for me, you certainly have my thanks but can you give me the link please, I'd like to get to the bottom of why I can't see it and you can.

    I combined to two for you to help illustrate the instructions in the wiki.

    0_1552593601698_63db95dd-5d43-4392-96ca-8a7edaba29de-image.png


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