2 sensors 1 input

[elevmike]

Just a couple of things here.

Second, a sinking input will sink current into the plc which means that the sensor will source the current. Or put another way, the sensor will supply current to the plc input. This is achieved by using a PNP sensor and wiring the inputs as sinking type input.

Again I stand corrected! (I told you so), anyway if anybody else has a hard time with the sinking - sourcing terms. Click here: http://web2.automationdirect.com/static/specs/sinksrc.pdf

 

[ready961]

Thanks for all the great info guys!

Yes these are 24vdc sensors. I see that Phoenix Contact makes Y connectors to do the same thing. Anyone ever use these?

 

[rsdoran]

First, Diodes DO NOT convert AC signals to DC signals. Diodes and capacitors can do this but not just diodes. A diode inline with an AC signal will produce a half wave rectified signal, not a dc signal.

What exactly is DC?
A potential difference with the electron flow in only ONE direction.

What does a diode do?
Allows electron flow in ONLY ONE direction. The most common application is rectification.

What is rectification The conversion of Alternating Current to Direct Current.

Whether half wave, full wave, filtered or not...if the electron flow is in only ONE DIRECTION then its DC.

Sources for this information:
http://www.patchn.com/Semi/SEMI_3.html
http://www.patchn.com/lessons.htm

For a PNP/NPN tutorial :
http://www.patchn.com/npnpnp.htm

 

[mrowen]

Interesting concept to say the least. So in that version, if I have a sinwave with peak to peak of 1 Volt floating on a 5 vdc bus, then my actual signal is a "dc" signal because the "voltage" nor the "current" never goes negative. I hope no one believes that.

A more formal definition of dc voltage (or current) is as follows.

"... voltage and current sources that generate either CONSTANT voltages or currents, that is, voltages and currents that are invariant to time. Historically, a direct current was defined as a current produced by a constant voltage. Therefore a constant voltage became known as a direct current, or dc, voltage. You might think that if a constant current is called a direct current, a constant voltage is called a direct voltage. However, the term Direct Current Voltage or dc voltage, is universally used in science and engineering."

Now the main point here is simple, don't confuse the term DC simply because it came about as a slang term to describe a battery suppling some resistive load. And don't confuse this concept with the limitations of your fluke multimeter.


RSDoran,
I sure would like to know the source of your definition.

 

[bernie_carlton]

To mrowen: therefore a nominally DC supply which has ANY RIPPLE AT ALL is not DC. It's sure not AC. I'm sure that all the DC in our machines has a slight amount of noise/ripple. So we are mistaken in caling it "24VDC" and should call it "24VDC with some undefind maximum amout of ripple/noise". Wow, our manuals have been wrong all this time!

 

[mrowen]

Your kidding, right. There is the theoritical pure dc signal and then there is the real world. In the real world, our powersupplies have manufacturers specs that we should actually read before we just purchase the cheapest one. Electrical engineering is all about approximating whether that is getting a voltage that is close to what we really want or dealing with a cheap power supply that has a 10% ripple voltage. Here's something neat you can do.

Put that diode in line with the ac signal and run it to your dc input card. Be sure to step the voltage down to something under 24.
Let us know how that works out for you. It's just DC, right. Or is it "alterating ON and Off". Hmmmmm, what does that really mean. Maybe it is now a digital signal. Or perhaps just a fast discreet on off with unpredictable on off transitions. Hmmmm, It sure ain't DC.

And actually, your dc signals on your machine are indeed a very complex mix of dc and ac signals. Everything on this planet shares this property. We deal with this by decoupling (at least some folks do) our circuits and filtering where needed.

I'll add this too. It may be extremely useful to learn that all signals can be represented by a complex combination of pure sinusodial wave forms. That may be a bit beyond this discussion but if you really want to understand electricity beyond the "car battery -vs- the wall outlet" you would do well to learn a bit about complex signals such as Radio, cell phones and audio signals. Not exactly light reading but it will surely enhance your understanding of a chopper circuit.

 

[Terry Woods]

mrowen,

As Ron already indicated, current flowing in one direction is DC.

That is the definition of DC.

Whether the current is pulsating between zero-current and some peak-current, as in simple half-wave rectification with no filter capacitors, or pulsating between one DC value (of a given polarity) and another DC level (of the same polarity), that current is, by definition, Direct Current.

Granted, it can be shown that the algebraic summation of a particular set of True AC Signals can produce a DC waveform.

However, it doesn't matter what complex waveforms might have been used to generate the result... if the resulting waveform is DC, then the final waveform is DC.

The "alternating", or rather, "fluctuating" portion of a DC signal can indeed be stripped off, through a filter, thus producing an AC-like signal. Of course, whether or not that ends up being a true AC signal depends on the bias signal onto which the stripped signal is applied.

Now, as far as 60Hz being applied to a simple diode, the result will produce a waveform that has equal ON-time and OFF-time; .0166 Seconds ON followed by .0166 Seconds OFF, and so on.

If that pulsating signal is applied to an LED, some of us can see the flicker, some of us can't. I have a lower retention factor. Having a lower visual retention factor means that one is more likely to see things as they really are.

At the same time, the LED has a particular "delivery retention". This is more apparent in full-wave rectification. That is, light emission from the LED does not go OFF completely before it is told to turn back ON. In this case, the light information from the LED is more like a rippled DC than a true full-wave rectified signal.

If the signal is from a 50Hz source (.020 Seconds ON follwed by .020 Seconds OFF), as in the European system, the flicker is more apparent.

Whether or not one can discern the flicker depends on ones' eyeball.

One of the aspects of visual acuity is "retention". Most people have high visual retention. That is, if a light goes ON-OFF-ON, at some particular frequency, their eyeballs do not notice the ON-OFF-ON transitions. Their retention factor is high. For those that have a high(er) retention factor, the eyeball/brain connection tends to smear the flow of light information from one video frame to the next.

For some of us, television in America (and other 60Hz nations) is easier to watch than the television in 50Hz nations (typical of Europe). Sometimes, European television gives me a head-ache.

BUT,... all of that that is neither here, nor there... at least, not in terms of a photo-detector. A photo-detector will indeed "notice" the flicker. Whether or not the "flicker" has an impact on the "reading" depends on the extent of the flicker.

But now, in terms of the original question...

Since the field devices are PNP...

I don't belive there will be any problem reading either one or two inputs being ON since the internal resistor in the PLC input should have been sized to accomodate zero-resistance on the field device (as in the case of a simple limit switch with zero-resistance).

In terms of what the PLC "sees", you don' need no stinkin' diodes.

Blah, Blah, Blah... so what else is new?

 

[elevmike]

ready961

You see, this form is kinda like talking to my wife. You ask her one question, and you get a whole boat load of something else. utoh

 

[mrowen]

I'm guessing I'm the only one that sees it the way I have explained. For simplicity of an on/off state of a plc input, you alls (ya'lls) definition of DC current is sufficient. Seems to me that someone, somewhere down the line has dreamed up a correlation of DC or DIRECT Current to UNIDIRECTIONAL flow. Was it the word DIRECT. I have a reasonably respectable Electrical Engineering degree and never have I seen or heard of such a description. Have even pulled out all the college text books to confirm this. Out of 16 Electrical based book, 3 physics, and 3 chemisty books, NOT ONE suggest unidirectional flow as a definition of DC current. Furthermore, every single one defines DC as a constant voltage or current.

And just to be clear, AC typically only refers to sinusodial signals. The rectified AC signal is neither DC nor AC. It is just some signal like countless others that can indeed be generated and represented in design consideration. And it most certainly will not be seen as being on for 50% of the time and off for 50% of the time by a plc.

As for the original question, I don't think any one was concerned with how the plc will perform. The concern was for back feeding the sensors.

It has been entertaining for sure, but I think I have robbed the thread.

 

[Eric Nelson]

...never have I seen or heard of such a description. Have even pulled out all the college text books to confirm this. Out of 16 Electrical based book, 3 physics, and 3 chemisty books, NOT ONE suggest unidirectional flow as a definition of DC current. Furthermore, every single one defines DC as a constant voltage or current.

A quick Google search for "DC Current" AND definition found this:

Direct current (DC) is the continuous flow of electricity through a conductor such as a wire from high to low potential. In direct current, the electric charges flow always in the same direction, which distinguishes it from alternating current (AC).

Source: THIS PAGE

Isn't "always in the same direction" the definition of 'unidirectional'?...

beerchug

-Eric

 

[rsdoran]

I offered links for reference in my reply. I have learned electronics different from you I reckon. My understanding of DC (direct current) is that electron flow is unidirectional...the current may vary with time in intensity (pulse) but it never reverses polarity.

More places that offer that definition:
http://whatis.techtarget.com/definition/0,,sid9_gci213659,00.html

http://dictionary.reference.com/search?q=direct current

http://en.wikipedia.org/wiki/Direct_current

http://reference.allrefer.com/encyclopedia/D/directcu.html

The Illustrated Dictionary of Electronics Fourth Edition by Rufus P Turner and Stan Giblisco
direct current: Abbreviation, dc. A current which always flows in one direction, e.g., the current delivered by a battery.

A reference I used in the other reply is from an online book by Tony R. Kuphaldt who is an college intructor. I need to update this book too but it can be find at: http://www.patchn.com/lessons.htm

I am just an old maintenance guy, not an engineer but I have studied electronics a little and can offer alot more sources that explain direct current as shown here.

I will agree that for most industrial applications a simple diode rectification is not sufficient. I think you should also take another look at how voltage is affected in a half wave circuit.

 

[juad]

hi
i sent letter but ithink somt thing wrong ok
i think we go far away from the main problem any one of us know what is AC and DC voltage he can return back to any refrence book to get the answer ...
**** ready961: where are you now for your problem
**** hesham : using realys mean increase the hard ware out side of the plc that mean more problem and more maintanance espically for realy as we find the moving contacts. yes you increase the seafty for your plc but it is better to use non moving contact ,you can use optocoupler it is more seafty and low maintanance.
**** mrowen: i hope you are not tired , is all the inputs are sinkining type ,may the sensor also behave as pooling current, so it prevent the current flow from the other sensor to go to the input module espically when it is not in contact state so we need the diode
to prevent that take in mind that may we did not have the details of the sensor or may it changes in future.


thanks to all

 

[ready961]

ready961

You see, this form is kinda like talking to my wife. You ask her one question, and you get a whole boat load of something else. utoh

How knew one little diode could cause such big problems!! I think we are losing focus on the question here. Do I need to use a diode so one sensor doesn't backfeed into the other sensor?

 

[Dragon_Man]

First, Diodes DO NOT convert AC signals to DC signals. Diodes and capacitors can do this but not just diodes. A diode inline with an AC signal will produce a half wave rectified signal, not a dc signal.

Diodes do create "DC" voltage from AC signals. DC is short for direct current which means current flows in only direction. You while normally only speak of DC by voltage level this is the average voltage level. A capacitor used with a diode only reduces the value of ripple. The other term used to describe a diode is rectifier.

Before you start posting replies that say there is more to it than that I know there is but I am not going to get into all here.

I hope this is constructive...

 

[mrowen]

Okay, For simplicity, I agree that you can say that DC means only one direction of flow. However, that property is simply one given property that happens to be true but it does not define what DC is. By defining DC as Constant Current, it is a given that the current will stay in one direction (because it is constant). However, by saying the current stays in one direction does not mean that the current is constant. This is a case of poor deductive reasoning. For example:
If you see a bird with wings flying, you would be wrong to say that all birds with wings fly. But you could say that all birds that fly, have wings.

Again, if you have an AC signal riding on a DC offset, that signal is very much so decribed as an AC signal with a DC component. Whether it crosses some reference point that you happen to call zero is irrelavant.

Man, what have I gotten myself into?

I sure would hate to go into learning about Op amps with AC and DC coupling based on this unidirectional flow thing. That would get pretty ugly I think. Heck, it would actually be much easier because you would never need to AC couple anything and impedance matching would be a cake walk. Oh yeah, problem is nothing would work.

Bottom line, if this is working for you, then more power to it.