24VDC positive ground.

[lquadros]

We have systems, where we use 24VDC with 0VDC polarity feeding the PLC inputs and 24VDC polarity as common. The 0VDC polarity is isolated and 24 VDC polarity is grounded.But the drawing says 24VDC polarity to be 0VDC and the 0VDC polarity to be -24VDC. Does this make sense? If we connect a multimeter with red lead to + and black to common, then we would read 24VDC. Is there a reason why it is written this way in the drawing?
Your replies are appreciated.

 

[dicklaxt]

Here is a link that may help your understanding of polarities used in PLC's and why..............I'm not an electronic's type but these are quite clear with a little thought and study IMO.

dick

http://www.patchn.com/npnpnp.htm.

You might search Sinking vs Sourching as well for more info

 

[gar]

100825-0730 EST

lquadros:

I do not understand your question.

Looking at the input diagram I do not see a ground. I do not know to what 0 is referenced. But this does not matter.

What matters is whether the input module is polarity sensitive. My guess is that it is polarity sensitive. Reverse polarity may not cause damage or it might, but with reverse polarity. if polarity sensitive, then it won't work.

If pin 16 is to be more positive than any of the input terminals, then as drawn the circuit is OK.

If the inputs are isolated from the internal logic circuits, for example by optical couplers, then pin 16 could be at +500 V and what is labeled -24 could be +476 V and the device would work. Or pin 16 could be at -500 V and the -24 at -524 V and the circuit would work.

On the other hand if your input module requires inputs to be pulled high, then the -24 should be +24 relative to pin 16.

I sense your native language is not English. I hope my explanantion above is clear.

.

 

[steve66]

We have systems, where we use 24VDC with 0VDC polarity feeding the PLC inputs and 24VDC polarity as common. The 0VDC polarity is isolated and 24 VDC polarity is grounded.But the drawing says 24VDC polarity to be 0VDC and the 0VDC polarity to be -24VDC. Does this make sense? If we connect a multimeter with red lead to + and black to common, then we would read 24VDC. Is there a reason why it is written this way in the drawing?
Your replies are appreciated.

Its really the same thing with a difference in notation. Either way the voltage buss (N33) is 24 volts less than the common or ground bus.

Personally, I prefer the notation with -24 volts for the supply voltage, and 0 volts for the ground buss. By convention, its usually common to assign 0 volts to the common bus or ground bus, and calculate all the other voltages referenced to ground bus.

 

[lquadros]

24VDC discussion

24VDC discussion

gar,

I sense your native language is not English. I hope my explanantion above is clear.
You have guessed it right. You could be a detective.

Looking at the input diagram I do not see a ground. I do not know to what 0 is referenced. But this does not matter.
The ground is connected at the source. P is connected to enclosure.

What matters is whether the input module is polarity sensitive. My guess is that it is polarity sensitive. Reverse polarity may not cause damage or it might, but with reverse polarity. if polarity sensitive, then it won't work.
It is polarity sensitive. It may or maynot work otherwise. You are right again.

If pin 16 is to be more positive than any of the input terminals, then as drawn the circuit is OK.
It is more positive and can be called P or 24VDC. This is the issue I would want to clarify. It would be confusing to troubleshoot, when -24VDC is mentioned. A multimeter will read +24VDC when connected across P and N.

 

[gar]

100825-1136 EST

lquadros:

If the wire at the left labeled N33 is connected to the equipment chassis and therefore probably ultimately to earth, then I would call that wire ground, and by its connection to the machine chassis I would call it common. I would not describe it as -24 V, but rather as 0 V. Then I would label the present wire labeled 0 V as +24 V. This would make sense for trouble shooting.

In my opinion it is very confusing to label the + rail in this application as 0 V.

.

 

[lquadros]

24VDC - which is correct?

24VDC - which is correct?

gar,

We have P grounded. The reason is because the system uses N as input to PLC. If N was grounded, then we could have an input turn on in case of a fault to ground.This would not trip the circuit. As it stands, we do need P grounded to enable a breaker trip.


When it comes to wiring panels, we see all kinds of devices with P and N, + and -, 0VDC and 24VDC marked. But never -24VDC. This is the issue I am interested to discuss. Can we write 24VDC on P and still have it grounded?

 

[gar]

100825-1227 EST

lquadros:

With wire P grounded, connected to chassis, then I agree with the drawing as labeled.

With the drawing as drawn if you use a voltmeter for trouble shooting, then you should expect to read a negative voltage on N or any input with a closed contact to it relative to P..

If you use a digital meter with the common or negative terminal (I would have the black wire in this terminal) and connect black to chassis, then make measurements to the N wire or inputs to the PLC I would expect a reading of near 0 volts or about -24 volts, depending upon the state of the limit switch. This makes logical sense.

.

 

[rattus]

2 cents more!

2 cents more!

Voltages are measured between two nodes, one of which is the reference node which is usually assigned a value of 0 grounded or not--in this case pin 16. Then the voltage on pin 1 relative to pin 16 would be -24Vdc. This is conventional. Now if you swap the meter leads the voltage on pin 16 relative to pin 1 would be +24Vdc, however you would not label pin 1 as +24V.

Whatever, the positive supply terminal must connect to pin 16!

This is akin to the argument about the phase difference between L! and L2 in the 120V/240V residential service--depends on the reference.

 

[lquadros]

24VDC clarification

24VDC clarification

rattus,

In our case, P is the reference node and is grounded. On a multimeter, pin 16 with respect to pin 1 will read 24VDC as long as +V terminal is at pin 16 and common terminal is at pin 1.

Can we then write 24VDC on pin 16 and still have it grounded and write 0VDC on pin 1? Is it correct and easier to understand? In the existing system, if the ground were to be removed, then the existing drawing will not make any sense. Where as it would not mislead the user if pin 1 and pin16 are properly designated with voltages. Just a thought.

 

[LarryFine]

It doesn't really matter which rail is grounded. Whichever polkarity suits the operation of the circuit is fine. One merely needs to observe the polarity of the meters and testers.

When faced with a bi-polar supply and loads, the same thing applies to half of the circuitry. There are both positive and negative rails and signals.

It's funny how we're used to negative grounding of DC, probably because of automotive polarity, which wasn't always negative ground for many years. (It makes a better spark.)

 

[rattus]

rattus,

Can we then write 24VDC on pin 16 and still have it grounded and write 0VDC on pin 1? Is it correct and easier to understand? In the existing system, if the ground were to be removed, then the existing drawing will not make any sense. Where as it would not mislead the user if pin 1 and pin16 are properly designated with voltages. Just a thought.

No. That would be confusing. You need to understand the concept of a reference. Analog ICs for example, usually require both a positive and a negative supply measured relative to a common/ground bus. The obsolete PMOS circuits required a negative supply only.

Voltage is either positive or negative relative to the reference.

It is desirable to have a voltmeter which measures positive and negative. Then place the black (-) test lead on the reference bus, and place the red (+) on the voltage to be measured. If the voltage is -24V, so be it. That is,

V1 re V16 = -24V

If you are using a conventional analog meter, you may have to swap leads, but leave the drawing as is.

 

[dbuckley]

...more positive...

These are the really important two words in Gar's post.

When dealing with polarity sensitive components in a DC circuit, to get it right, you only need to figure out which end is "more positive" or "more negative", and then you can connect things the right way round.