Mixing loop-powered and externally powered sensors on SNAP-AIMA-iSRC module

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I have two sensors that I want to connect to a SNAP-AIMA-iSRC module; one is loop powered and the other is externally powered with its own power supply. Is this possible?
Thank you.

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Hi Metasequoia,

Welcome to the Opto 22 forums!

To get my head around your question, I had a close look at doc 1182, the isolated analog input module data sheet.
Page 5 to be precise.

It makes it clear that BOTH channels are powered from the single 24v supply when its hooked up.
So in your case you will end up with the two power supplies fighting each other on the channel that has another externally powered sensor.

That said, since you must have two supplies anyway (one for the loop powered sensor and the other with its own supply), I would just wire the loop powered one to the sensor and then to the module.
In other words, don’t put the supply on the module pins 1 and 2, but rather follow the diagram that shows the external loop supply for the first sensor. Then your other self powered sensor just wires straight to its input (pins 5 and 6)
Since both channels are isolated, you will have no problems.

Cheers,

Ben.

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Hi Ben

Guess what? You have just invented a SNAP-AIMA-i module! :mad:

The SNAP-AIMA-iSRC does simplify wiring and reduce installation time, but at a premium cost. A lower cost alternative is to use field wiring terminals and a common field power supply for SNAP-AIMA modules and where necessary, SNAP-AIMA-i modules for channel isolated instrumentation.

Here is my dummy guide for understanding the wiring scenarios.

[B]Self Powered.[/B] An instrument that uses an internal power supply to provide 24VDC loop power. Often found when the instrument requires higher power or additional electronics. Instantly recognisable as it needs 4 wires. (Two for power supply and two for 4-20mA signal

[B]Loop Powered.[/B] An instrument that requires an external 24VDC power supply. (Instantly recognisable as it only requires 2 wires (Two for 24VDC over which the 4-20mA is imposed)

[B]System Powered[/B]. An instrument that “expects” that the I/O module provides the 24VDC power. (In reality it has no idea if the 24VDC is from the I/O module or a loop power supply)

  • The only SNAP current module which can actively supply 24VDC loop power is the SNAP-AIMA-iSRC.
  • All other SNAP current modules are passive and require the use of a 24VDC loop power supply or self powered instruments.
  • Do not mix power supplies on a non isolated module.
  • On a SNAP-AIMA-iSRC, do not use supply terminals 1&2 unless both transmitters are system powered.
Its that simple!
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@Arun (my apologies if this is the wrong Arun) - I think this is pretty much what I was asking you about in the Chat today. Everybody keeps saying you can’t mix power supplies. I went back and looked at the wiring diagrams provided by the company that did the panel for us, and I guess all the “self-powered” instruments were still ultimately powered by our 24 vdc power supply. Oh well. I don’t know how they found all instruments that did that – whenever I try to buy a scale, it’s always powered by 120 VAC.

FYI, their wiring diagrams were interesting, because they hooked a bunch of stuff up BACKWARDS – i.e. everything was scaled -4 to -20 mA. My brain wasn’t used to going that direction. Anyway, there you go.

I’m still stuck with the isolated AIMA modules.

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The “right” Arun @ktgaston . Thanks. Pantea also gave me a heads up.

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