4 wire vs 2 wire 4-20ma loop question

ganutenator

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I'm probably going to be called stupid for asking this.
I have only ever used 2 wire sensors. we would connect one wire to 24 dc+ and the return to the analog input card.

Recently, was using a banner ultra sonic sensor. Had to connect the brown to 24+, blue to 24-, the black to 24- and the return (white) to the input card.

question, is the always the case?

reason for the question. panel drawings: customer hasn't decided on which ultrasonic sensor they will be using.
 
You may well be dumb as a duck, but I would never hold it against you. :p

All the ultrasonics I have hooked up want constant 24V supplied to them with a separate set for the loop. I'm sure there are exceptions, but none that I'm aware of.

Bubba.
 
You may well be dumb as a duck, but I would never hold it against you. :p

All the ultrasonics I have hooked up want constant 24V supplied to them with a separate set for the loop. I'm sure there are exceptions, but none that I'm aware of.

Bubba.

you a better man than me. i can't stand stupid people.

but i probably wasn't clear w/ my question.
I have to connect one wire to 24- and the return to the input, where as the two wire loops like my pressure transducers and temp sensors i have to connect 24+ to them.
 
reason i asked, i told my customer that we needed to change the panel drawings to reflect this. he replied w/ we aren't sure we going to use this sensor.
 
I agree with what Bubba said. I haven't seen any two-wire ultrasonics.

I guess the problem is that the current draw for the electronics is greater than 4mA. It must be lower than 4mA to work in a two-wire setting.
 
I've used 2wire Flowline transmitters. As far as is the next brand of 4 wire going to wire the same? Well basically yes. Although the next brand could be a 3 wire or 2 wire.
 
With a 4-wire sensor, the extra 2 wires is 24DC power. Third wire to analog card. 4th wire, who knows? They should call it 2-wire 4-20mA or 3-wire 4-20mA.
2-wire, we draw current from the analog signal. We go 4-wire if our current draw is too high to get away with that.

Not sure about your wiring for the Banner Ultrasonic regarding the black wire. Look at schematic on page 8 here.

Let's simplify for your panel drawing.
2-wire is brown to +24, blue to Analog input.
4-wire is brown to +24, blue to -24, white (or vendor-specified color) to Analog input.
 
Those are analog and discrete output. If you don't use the discrete output, you should just cap the wire. That is a 3 wire analog sensor not a 4 wire.
 
As has already been posted, dimensions Siterans LU are loop powered ultrasonic devices. There are also loop powered ultrasonics from the likes of Mobrey (Emerson), and Vega. Where I work we have lots of loop powered ultrasonic level sensors.
 
Have only used Endress&Hauser ultra-sonic devices, they were also 2-wire.

If you are unsure, I would draw 4 terminals. 2 going into the PLC, the others 24VDC and 0. This way you can easily hook it up for either user case.

The way you describe you normal procedure, you would setup the PLC as 4-wire anyway (no 24 VDC feed from PLC).
 
Those are analog and discrete output. If you don't use the discrete output, you should just cap the wire. That is a 3 wire analog sensor not a 4 wire.

A 3 wire transmitter and a 4 wire transmitter are really the same thing. The only difference is that the power supply common and the current loop output common happen to be the same point electrically for a DC powered, 3 terminal configuration. Many of the 4 wire transmitters that I've worked with power up with 120 VAC. These obviously require a separate, 4th terminal to provide access to the common of their analog output.

The significant difference between the two classes of transmitters is the source of the instrument's current loop excitation power. As has already been stated, instruments that consume a relatively large amount of power for their operation aren't candidates for a 2 wire configuration, since they would overload the external current loop excitation power supply.
 
A 3 wire transmitter and a 4 wire transmitter are really the same thing. The only difference is that the power supply common and the current loop output common happen to be the same point electrically for a DC powered, 3 terminal configuration. Many of the 4 wire transmitters that I've worked with power up with 120 VAC. These obviously require a separate, 4th terminal to provide access to the common of their analog output.

The significant difference between the two classes of transmitters is the source of the instrument's current loop excitation power. As has already been stated, instruments that consume a relatively large amount of power for their operation aren't candidates for a 2 wire configuration, since they would overload the external current loop excitation power supply.

I understand, I was saying in the case of this transmitter it has 4 wire, but the 4th wire is not a ground/return but a discrete output. In fact if he grounded that wire and it is a pnp version transmitter, then it would short.
 
i wasn't clear.
the one i connect to 24 positive
the other i connect to 24 negative
both tech support guys called me stupid for asking
 

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