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RTD Wiring issues


(@hemicharger99)
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Joined: 4 years ago
Posts: 10
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The last piece of the puzzle is getting all the RTD temperature sensors to work. 

I have 5  RTD temperature sensors total, 4 on one MEGA 2560 (Group 1), 1 on another MEGA 2560 (Group 2). Each interface is running Firmware 43 SR, since the boards are wired via USB.

All RTD sensors have individual amplifier boards (MAX31865) inside a junction box about 18" from the panel.

Group 1 sensors have the SCLK, SDI, and SDO from Interface 1 run to them in parallel, and plugged into pins 50-52 on the interface accordingly.

Group 2 sensors have the SCLK, SDI, and SDO from Interface 2 run to them in parallel, and plugged into pins 50-52 on the interface accordingly.

I have a dedicated 5V power supply inside the panel providing power to each individual board. Each board is verified to have +5VDC, and appropriate wiring to each CLK,SDI,SDO pins. Each CS pin goes to a corresponding pin on its respective interface (D14-D17).

Each resistor was measured for the proper resistance value (varies between 427 and 431 ohms) and entered into the calibration of each sensor. The problem I am having is that each and every temperature sensor is reading zero.

Each sensor is connected to F+, RTD+ (blue wires), RTD- (red wire), or equivalent wiring.

I have pulled the MEGA from the panel and direct wired up the sensor using short cables with no luck.

 

Any ideas what I am missing?


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(@pbruno3)
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Joined: 5 years ago
Posts: 312
 

Hmmmm... you seem to have most of the bases covered... What amplifier is it? Adafruit/PlayingWithFusion/knock-off? If a knock-off, please provide a link.

Did you prepare the amplifier according to the instructions from the manufacturer? Assuming you have 3-wire probes, which it looks like you do, then you will need to cut traces and/or solder pads together.

Short wires are the key between the MEGA and the amplifier as it is a high-speed bus. I think it is important to test one amp with short interconnect wires to get it working correctly. Of course, make sure the device port matches the CS pin wiring on the MEGA. Make sure you have the wiring correct: SDO <-> MISO, SDI <-> MOSI, SCLK <-> SCLK, CS <-> pin per interface wiring map.

If you power the amplifier from a separate power supply, the grounds must be tied together so they reference the same voltage levels between the MEGA and the amplifier board.

I recommend a MEGA reset following each wiring change too. 


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(@hemicharger99)
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Joined: 4 years ago
Posts: 10
Topic starter  

I am using the MAX31865 boards from diymore. I have 6 of them in total, one as a backup, soldered but uncut for the trace.

I had one working before, but that was using the +5VDC from the MEGA. Having 5 or more, i ourchased a DIN mount 5VDC power supply that powers the sensors only. The MEGAs are powered off of a separate 12VDC power supply.

 

You mention that the grounds should be tied together. Did you mean between the mega ground puns and the 5vdc power supply?

 

The image pictures is the unused and uncut trace board. It is soldered for a 3 wire, but the trace isn't cut (wasn't sure if I was going to go with 4 wire ones).


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(@hemicharger99)
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Joined: 4 years ago
Posts: 10
Topic starter  

Here is a diagram of how it is right now.

If I understand correctly, I need to tie in the -V/GND from the 5V power supply, to the -V/GND of the 12V supply, and then connect ground pins from each MEGA 2560 to the same common ground.


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(@hemicharger99)
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Joined: 4 years ago
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Topic starter  

And lastly, the proposed solution, if I am understanding correctly.


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(@pbruno3)
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Joined: 5 years ago
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Yes, correct... tie the grounds of the two power supplies together. If this isn’t done, the 5V from one and the 12V (therefore 5V logic on the MEGA) from the other have no reference to each other. 

You do not need to double ground the MEGA (meaning running a ground from the header to the terminal block) that ground happens through the main power jack as you have it drawn. In fact you do not want to do this as it could set up a ground loop (introduces noise). All grounds should be tied to one common point. 


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(@hemicharger99)
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Joined: 4 years ago
Posts: 10
Topic starter  

Thanks for the assistance, it worked great! I knew it was something simple I was missing. All temperature sensors are reporting accurately. This weekend it will be time for a few test runs. Thanks again!


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(@pbruno3)
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Joined: 5 years ago
Posts: 312
 

Happy to help! Have fun!


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