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Connecting a 4-20mA loop powered sensor to USB-6255

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Hello all,

 

I have a USB-6255 DAQ box.  I also have some sensors that are loop-powered and have a 4-20mA output (and at least one sensor that has an NPN/PNP output).  My question is, how would I wire the sensors so I can read the output?  Some of the sensors run off of 24V (2 wire sensors).

 

Thanks!

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Accepted by topic author JoeKellyATI

For the 4-20mA signals, use a 500 Ohm resistor to terminate the signal.  This will convert it to a 10V signal.  It wouldn't hurt to add in an op amp to act as a buffer as well.

 

For the 24V signals, use a voltage divider to divide by 5.  That will cut it down to 5V.  Again, an op-amp as a buffer would be a good idea.  Alternatively, you might find a chip to do the logic conversion for you.


GCentral
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For more detailed suggestions, please provide links to  (or add) the datasheets of your signal sources .

Depending on how accurate and fast you need to measure, the buffers are needed.

For digital IO it also depends on timing requirements and noise level ...   sometimes an optocoupler is a good idea, sometimes a single resistor will do it...

 

Can you build up small circuits?  Otherwise industrial signal converters (20mA->10V, ...) are a 'screw together' solution...

 

Greetings from Germany
Henrik

LV since v3.1

“ground” is a convenient fantasy

'˙˙˙˙uıɐƃɐ lɐıp puɐ °06 ǝuoɥd ɹnoʎ uɹnʇ ǝsɐǝld 'ʎɹɐuıƃɐɯı sı pǝlɐıp ǝʌɐɥ noʎ ɹǝqɯnu ǝɥʇ'


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Additionally, please refer to this too: http://zone.ni.com/reference/en-XX/help/371361G-01/lvmeasapps/measuring_current/

 

Could prove useful as an example.

Regards,

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Thanks Crossrulz!  I was wondering if it was that simple, just wanted confirmation.

 

For those that are interested, attached is a datasheet for one of the thermometers we're looking at.  It's loop-powered and the supply voltage has to be in the 9-32VDC range.

 

I'm also looking at one of these multifunctional transmitters to simplify the process: http://www.prelectronics.com/?altTemplate=showproduct&id=2270

 

Thanks for all of the help,

Joe

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Hello all, 

I was looking a bit closer at your answers, and I have a few more questions.

 

I have two different sensors:

  • Air thermometer Loop-powered sensor that runs off of 9-32VDC and outputs a 4-20mA signal
  • Pressure switch sensor that runs off of 12-24VDC and outputs a PNP or NPN signal

For the first sensor, I think I figured out, from your guys' descriptions, how to wire it to my USB-6255 DAQ:

Air Thermometer.png

For the pressure switch, it's a 2-wire sensor (not a normal 3-wire NPN/PNP sensor).  Is it even possible to wire up a 2-wire sensor that requires 12-24VDC from the DAQ?  Do I need to use a boost converter to convert from 5VDC (digital output) to 12VDC?

Pressure Switch.png

 

Thanks,

Joe

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No, the DI shouldn't see more than 5V, so some resistors (and or Z-Diodes/LEDs)  are needed. Can you post (a link or) the datasheet of that digital sensor? 

Greetings from Germany
Henrik

LV since v3.1

“ground” is a convenient fantasy

'˙˙˙˙uıɐƃɐ lɐıp puɐ °06 ǝuoɥd ɹnoʎ uɹnʇ ǝsɐǝld 'ʎɹɐuıƃɐɯı sı pǝlɐıp ǝʌɐɥ noʎ ɹǝqɯnu ǝɥʇ'


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@Henrik_Volkers wrote:

No, the DI shouldn't see more than 5V, so some resistors (and or Z-Diodes/LEDs)  are needed. Can you post (a link or) the datasheet of that digital sensor? 


The more I think about this, I think a simple Optocoupler will do the job.  Just add a resistor or two to the diode input to limit the current.  Something like this should work for this situation: TLP250H(F)


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Attached is the datasheet for the digital sensor.  It's just a simple pressure switch by SMC.

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@crossrulz wrote:

@Henrik_Volkers wrote:

No, the DI shouldn't see more than 5V, so some resistors (and or Z-Diodes/LEDs)  are needed. Can you post (a link or) the datasheet of that digital sensor? 


The more I think about this, I think a simple Optocoupler will do the job.  Just add a resistor or two to the diode input to limit the current.  Something like this should work for this situation: TLP250H(F)


Without needing the use of any protoboard (for this project), would something like this work? https://www.digikey.com/products/en?mpart=2964296&v=277

 

I found a few schematics online and have come up with this:

Pressure Switch.png
I don't think I'll need any resistors, since the sensor is such low current.

 

Do I have the sensor wired up backwards to the diode?  I'm not 100% confident of that.

 

Thanks all,

Joe

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