Summing module reading different weighted 4-20mA signals. Hardwired solution to get correct sum? 1

[bdn2004]

Current monitors sense large dc currents from separate paths. They sum into a summing module whose output value reports on a meter and a PLC. We have a new DC current path will have more than twice the current as any of the old paths.

We want to pull this info into the existing summing module that can't be programmed for individual inputs. I thought about pulling this info into 2 separate inputs on the summing module... Is there a hardwired solution here?

See the attached sketch.

 

[Skogsgurra]

A current source is a current source. That means that the internal resistance is infinity (in theory, and very close to infinity in practice).
All your current sources are floating and that means that you can just wire them together and they will sum up nicely.

It is the Thevenin equivalent of adding DC voltages in series.

So, all you need to do is wire all current sources together and read the resulting current.

Of course, you need to scale all signals so that 1 mA means the same thing. That condition is not satisfied in your sketch, where the lower current source is scaled differently from the other ones.

You also need to make sure that the sum of your currents doesn't exceed the range of the summing node (which is probably 20 mA).

There are loop insulators if you want to keep the current sources isolated.

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[VEBill]

"...wire them together and they will sum up nicely. It is the Thevenin equivalent of adding DC voltages in series. So, all you need to do is wire all current sources together and read the resulting current. ..."

Clarification: In parallel, right?

 

[Skogsgurra]

Right. Can there be any doubt?

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[IRstuff]

Clarification is always better than people reading whatever they think they want.

TTFN
faq731-376

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[Skogsgurra]

OK. I just can't see any alternative reading here. The Thevein equivalent is all about going from series (the batteries) to parallel (or the opposite). Should have mentioned the Norton thing also.

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[btrueblood]

Trying to think of wiring the current sources in series made me pull a tendon in my brain...ouch.

 

[VEBill]

"...current sources in series made me pull a tendon in my brain..."

 

[DRWeig]

I'm right there wih summing them by wiring in parallel. But the OP is having an issue with the third transducer's different ratio.

Change the two low ones from 15,000A:20 mA each to 35,000A:20 mA each. All the same ratio, no need for a summing / scaling device.

Best to you,

Goober Dave

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[Skogsgurra]

Yes. Noticed that
Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[DRWeig]

Of course you did, Gunnar. My reading-retention skills seem to be waning.

Best to you,

Goober Dave

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