Rejoining 24V power through converters into sensor?

Thread Starter

lali0020

Joined Dec 20, 2012
3
I have an application of reading two 4-20 mA outputs from a sensor. Each 4-20 output will be read by an individual inline converter which takes a single 4-20 input and converts it a modbus output. The converters are each powered by 24V split from the same DC source and have a pass through of the 24V and ground. For simplicity of cabling (since they are all M12 connectorized), can I split the 24V and ground to the converters only to rejoining the 24V and ground after the converters into a single cable to power the sensor with the dual analog output? I'm always warry of creating a ground loop or imbalanced circuit. The converters should be identical but cant be guaranteed. Is this ok to do or likely to cause issues? Thanks in advance
 

MisterBill2

Joined Jan 23, 2018
12,678
Questions that require answers before an answer can be given: Are the sensors powered separately from the 4-20 mA connection? OR is the sensor simply a two-wire hookup to the converter? IN some systems the sensor is separately powered and the current loop is isolated only running into the DAQ input resistor, which feeds a differential input A/D.
For the other kind of arrangement the sensors all have their supply loop positive to one common bus, and then the loop return goes to the input plus and the iput negative returns to the supply negative. Quite different.
 

Reloadron

Joined Jan 15, 2015
6,813
I have an application of reading two 4-20 mA outputs from a sensor. Each 4-20 output will be read by an individual inline converter which takes a single 4-20 input and converts it a modbus output. The converters are each powered by 24V split from the same DC source and have a pass through of the 24V and ground. For simplicity of cabling (since they are all M12 connectorized), can I split the 24V and ground to the converters only to rejoining the 24V and ground after the converters into a single cable to power the sensor with the dual analog output? I'm always warry of creating a ground loop or imbalanced circuit. The converters should be identical but cant be guaranteed. Is this ok to do or likely to cause issues? Thanks in advance
Do you mean something like this?
Sensors.png

A single 24 VDC supply powers all sensors. R1, R2 and E3 in this example are just resistors to convert the individual 4~20 mA outputs to a voltage for an ADC. Just a crude image but yes, it is done this way.

Ron
 

Thread Starter

lali0020

Joined Dec 20, 2012
3
Questions that require answers before an answer can be given: Are the sensors powered separately from the 4-20 mA connection? OR is the sensor simply a two-wire hookup to the converter? IN some systems the sensor is separately powered and the current loop is isolated only running into the DAQ input resistor, which feeds a differential input A/D.
For the other kind of arrangement the sensors all have their supply loop positive to one common bus, and then the loop return goes to the input plus and the iput negative returns to the supply negative. Quite different.
In this system the two converters are 5 pin M12 inputs with 2 of those pins being 24V/Gnd and 1 being the 4-20 input. The sensor is wired downstream of the converters and power is supplied through the converters which use the 24V and pass it through on the same 2 pins. So a junction box could be made to drops those two pins from one of the converters and only have the 2nd 4-20 go to that converter BUT for simplicity of installation it would be able to use standard 5 pin M12 cables if the 24/Gnd pins from each converter could be rejoined.

Think of it simply as 24V and Gnd coming from a source being parallel split in a cable T splitter to pins 1 and 3 of 2 devices in parallel. Those devices also pass through 24V and gnd after using it to power themselvws to pins 1 and 3 of their other downstream connector. Both devices now connect to a splitter cable down stream which combines both pin 1s (24v) together for each device and both pin 3s(gnd) together from each of those device back together.
 

Thread Starter

lali0020

Joined Dec 20, 2012
3
Do you mean something like this?
View attachment 272055

A single 24 VDC supply powers all sensors. R1, R2 and E3 in this example are just resistors to convert the individual 4~20 mA outputs to a voltage for an ADC. Just a crude image but yes, it is done this way.

Ron
Not quite, I'm doing this on my phone right now so I don't have a good way to make a diagram but the first 2 devices are converters that read in the dual output 4-20s individually from the sensor which is downstream from the converters. Those converters are each powered from the same source in parallel and passthrough the 24v and gnd to power a sensor. Normally the converter are intended to be used as a 1 converter per sensor setup but since this sensor has 2 4-20s instead of just one I need a second converter to read that extra 4-20. The converters use the 24v to not only read the 4-20 input but to power a chip for configuring and converting it to a Modbus output so multiple devices can be added to the same comms bus as each converter gets addressed with a unique ID.
 

Reloadron

Joined Jan 15, 2015
6,813
Not quite, I'm doing this on my phone right now so I don't have a good way to make a diagram but the first 2 devices are converters that read in the dual output 4-20s individually from the sensor which is downstream from the converters. Those converters are each powered from the same source in parallel and passthrough the 24v and gnd to power a sensor. Normally the converter are intended to be used as a 1 converter per sensor setup but since this sensor has 2 4-20s instead of just one I need a second converter to read that extra 4-20. The converters use the 24v to not only read the 4-20 input but to power a chip for configuring and converting it to a Modbus output so multiple devices can be added to the same comms bus as each converter gets addressed with a unique ID.
This would go better with a schematic, even a rough schematic. I want to just answer yes to your question but less a drawing am a little reluctant to just say yes, no problem that will work.

Ron
 

MisterBill2

Joined Jan 23, 2018
12,678
Recall the definition of a ground loop, which is a segment of a signal conductor circuit that has some current other than the signal current passing through it. It is a segment of the signal loop that has some other current flowing through it. The additional current is the cause of the error signals.
 
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