2 or 3 wire

[tlvaun]

Hello,

I was just wondering if anyone could give me any real advantages of a 3 wire DC proximity switch over a 2 wire? Does one work better in certain applications. MY company has several machines that require NPN inputs and some other that require PNP. I have started requesting 2 wire sensors as a way to reduce inventory (and to make them interchangeable). They seem to work well. I've heard that the 3 wire provides better noise isolation, but have not been able to find any documentation that shows direct comparisons. All input would be appreciated.

Thanks

 

[Rick Densing]

A three wire prox has no leakage (or at least very small compared to a two wire, I am not sure) current. The two wire prox needs to power itself through your input. If your inputs can't tell the difference, you should be fine. As for noise, yes. Since the leakage current in the two wire is already marginalizing your signal and giving you less overhead, you are a little more susceptable to noise.

 

[Ken Roach]

I had an application once where a rolling cart was powered by an AC inverter, and sensed it's position by inductive proximity sensors (Turck, I think) that looked for steel flags embedded in the floor.

The problem was that any time the inverter (located onboard the cart) ran, the 2-wire DC proximity inputs would flicker on and off. If I filtered the input data I was able to detect the position flags.... about four inches too late.

3-wire models of the same sensor did away with the noise problem. I wish I could say what the magnitude of the noise was, and I can't say if it would have happened on a machine that was bolted down and properly grounded.

 

[Eric Nelson]

In my opinion, 2-wire prox switches are only used by lazy electricians to replace existing 2-wire devices (limit switches, etc.). The "extra" wire just confuses them... These are the same guys who use 230V for control wiring...

IOW, just use 3-wire switches like the rest of us...

beerchug

-Eric

 

[Robby]

I have found AC-two wire prox switches to be very handy in very simple cicuits where you have only a couple switches no power supply,and no plc. They are ideal for simply turning on a light when something passes before them or runing a moter when something gets full. The main advantage being that they can be used without a dc power supply.
Once you have a ligitamate dc control circuit I always go three wire.
I am a maintenance man not an engineer though (just a warning before you listen to me)

 

[ianingram]

I have had bad experiences with 2-wire prox switches that don't turn off after the metal they were sensing had gone past..I believe this problem is caused by a very high input impedance of the plc.

You can get around this by (a) using 3 wire proxs, or (b) additional loads[resistors] connected to a 2 wire prox... Method (b) is asking for trouble when these resistors fail..

 

[AllenF]

We use 2-wire proxes at 6 factories across well over
150 machines with no problems.

Mainly DC, some AC.

Allen F

 

[Agostinho Macedo]

2 wire prox. are very helpfull in many circunstances, and beside they only use 2 cables.
Schmersal prox. 2 wire 220AC and 24VDC are very secure and very reliable. I have many prox. working on 12 years and I can assure that I don´t remember of any failure. I must say too, that in a high temperatures and high humidity environments 220VAC prox. are more imune to electrical noise, than 24VDC, and in most of the times they can handle higher temperatures.

macmac (portugal)

 

[johnhopps]

NPN or PNP

Type that is used must match plc input card type. Plants shoooould
maintain a STANDARD. But....

Some perfer PNP. Pos Signal output is easy to measure, but , induced
voltages can or could be a problem giving false signalling. Voltage drops will cause problems also if wiring Cable is far enough away or extremly long.

NPN however, when on, signal using "GROUND TRUE". This can not be
induced. at Ground potential they inherintly elliminate or ground out
noise. Distance is not a concern either. If LED light goes off when wire is removed from input you will know device is signaling its "ON" state, and without removing any wires you can safely use a jumper wire to GND to force input LED "ON" to check the card. EASY! and I Think a better Choice for Plant STANDARD.

 

[rsdoran]

NPN however, when on, signal using "GROUND TRUE". This can not be induced.

WHY cant it?

 

[johnhopps]

OK, what I said is true most of the time. The RETURN (RTN) from the
power supply (usually 24vdc) will normally be referenced to GROUND by
a wire physically. But not allways, sorry.

If and when it is what I said is always true. The Signal wire which is supplying the RTN thru Output NPN type transistor and is referenced to GND, having NO VOLTAGE POTENTIAL riding on it, will effectivly ground out any noise voltages induced on it along its lenght from the Device to the Input Card in the PLC Rack. When the Device is "NOT ON", Induced voltage can occur on it because it is in effect an antena, However, The Input Card will not respond to these because it responds only to RTN and not Positive voltage values of any value.(spikes, or noise, or whatever.) Remember that I am discussing Discreate Input Signals, not 4 - 20 ma Analog, which is something totally different where none of the above would apply.

BASIC RULE: TO ELLIMINATE OR REDUCE OR DISAPATE NOISE SIGNALS THEY ARE SHORTED (SHUNTED) TO GROUND

It is Sunday, and I sure didn't want to get so involved. I only responded to the orignal request for info on PNP NPN and thoght I might help.

 

[rsdoran]

The original quextion was about 2 o 3 wire sensors and the advantages/disadvantages.

I only questioned your statement because of the obvious to me...if any voltage is induced (by any means) on a wire that is a ground reference ...wouldnt that interfere with its operation?

 

[drewcrew6]

To : johnhopps

Now I'm not defending either type, and I'm just a maintanance tech.

What happens in the rare case the sensor output wire rubs through to ground?


In a PNP sensor you will probably blow a fuse in a properly designed system, worst case you will lose all inputs when the sensor is activated and the power supply voltage will start "chattering" .

I believe the best choice is just like chosing your the way you write the logic.
Asess(?) the possibilites of a cable break and a shorted sensor (on all the time) and the associated machine action to decide what safegaurds are needed. IE: If this sensor stays on what happens to the machine and personell, and if it doesn't come on? Which one has better safety for both machine and personel?



Drewcrew6

 

[carisa]

2 or 3 prox.

hi ppl
you see, sometimes we need 2 wire prox. sensor and other times we need 3 wires prox. sensor but i think in oll the cases you need to know what you whont to do.
for plc, in my opinium , the 3 wires prox. sensor are the best, but in hader cases the 2wires prox. sensors are god to.
when you whont to do a job whith a plc use 3 wieres prox. sensor because they are better. the signal works as the same as a transistor, so you have the same power as the sorse, when it's activeted.
i hope you anderstand what i meen.

ps: sory abaut my english

Rui Silva
Portugal

 

[Tim]

tlvaun,

I asked a Omron tech about the advantages and disadvantages of a 2 wire dc prox and a three wire dc prox. Here is his response.

The advantages for the 2 wire DC variety are, simplified wiring, they can be used with a PLC, either line can drive the load and some versions do not require any specific polarity and they can drive a load like a solenoid or relay directly.

The disadvantages are, they require some small leakage current to function, cannot be used as an input device with timers and counters and cannot provide TTL logic signals.

The advantages of the 3 wire DC types are, they can be used as an input device to a PLC, can be used as an input device to timers and counters, can provide TTL logic signals (if open collector version is used), can be either positive or negative common (depending on type, NPN or PNP) and can supply voltage output for logic circuits without an external pull-up/pulldown resistor (most E2E series have it built in).

The disadvantages are slightly more complex wiring and cannot drive a load requiring current greater than about 200mA.

Hope this helps. "I asked him specifically about their E2E series, I think the specs can be used in most applications."

Tim