2 sensors 1 input

[mrowen]

Oh yeah, because I apparently have nothing better to do this morning, I ran the little google search for defining direct current. Many, Many returns defined it simply as unidirectional flow. Almost as many went a step further and included reference to maintianing "nearly" contstant value. Again, one automatically implies the other while the other does not at all imply the other.

 

[elevmike]

ready961

If you can find a diagram for your sensor check it out there. Just do a little experimenting by installing the diodes, and if they cause your sensors not to work, then remove them.

However as stated before by many posters, you should really use seperate inputs for each sensor.

That's all I have to say on the subject.

Mike.

 

[hesham]

What is going on?

mrowen
You are absolutely right in all what you are saying.
To whom says that diodes produce DC from AC

As said The other term used to describe a diode is rectifier
Diodes don't produce DC.Diodes rectify the AC to be in one direction
Capacictors due to charging and discharging Cycles convert this time variant signal(Review the AC defination:Variable in both the intensity and direction)to approximate dcc level with ripples)
As long as we increase the capacitance we reduce the ripple may be we reachch a pure DC.
ready961
As you said using diodes is to prevent the two sensors to feedback each other.
Juad
Sfety is an important issue .input modules is more expensive than adding relays.how much does it cost the relays and the wiring and the space compared to the input module and in the same time how much does it cost your name with the client if your plc damged due to that you don't want to spend a more small amount of money ,and how much you will gain from providing a good quality and durable work.Think about it.
ready961
think about what will happen if one diode damage due to any spike?What will happen?
How you will install the diodes in your panel?
how the maintenance man will know id the diode damaged?
I hope that this will help
BR

 

[rsdoran]

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?

Terry said no and I would think the same way, backfeed wont hurt the sensor and if either is on you will get your input. As was mentioned though the real issue will be if one sensor fails troubleshooting it may become a problem, in the long run it would be better if you could add more inputs.

mrowen: I do not understand how you have come by your definition or understanding of DC. DC stands for Direct current, there is no implication or definition that defines the current as constant. In fact the definitions state that current may vary in time(ie source frequency change) but polarity never reverses. The total output voltage will remain constant in reference to the source voltage.

I made my original comment because you stated that a diode alone can not create DC...a diode rectifies AC to create DC...ie just one is known as half-wave rectification which produces a pulsating DC. I agree that for many industrial grade applications this is not sufficient as a power source but it is still a DC voltage. To create a more efficient form of DC output the use of bridge rectifiers, capacitors, zeners etc are needed. NOTE: To this day half wave rectification is used in many everyday devices like radios, vcr, etc.

 

[Dragon_Man]

I would say that if you have PNP device that putting a positive voltage on each end of the transitor will not hurt it. The transitor is essentially a diode that takes some sort of signal in order to let the current flow.I just don't see why putting +24Vdc on the emitter (drain) and +24 Vdc on the receiver (source) will hurt it. I would say that device that is not on would just not turn on since the base voltage would not be greater the emitter or drain, until the other device went off.
My vote is for you just try it, or better yet have your local supplier bring over a couple of sensors and try it on them.

 

[chriswarrn]

I see 2 seperate components in a DC power supply, the first being a voltage/current that would be a perfectly straight line if seen on an O-scope, and the second being an AC voltage/current ADDED TO the DC. Remove the AC and you end up with pure DC. Remove the DC and you end up with a current that travels in both directions, but together they produce a current that travels in only one direction; i.e. a DC current with an AC component.

 

[RATBAG]

Hiya,
After going thro' few books & annoying a colleague, if at all possible add a new input, if that is not an option, preferably two three wire photo cells in parallel, with diode protection will work, but you may find a few strange faults during operation. Apparently this should be the same whether PNP, or NPN.
I hope this is of use, if not, ignore it.

 

[Mike Williams]

Hey Ron
A "half-wave rectified" signal is a signal where any negative voltages are replaced by a 0 voltage. Only the positive half-cycles of the ac input voltage appear across the load. Since the output does not change polarity, it is a pulsating dc voltage.

It is DC where I come from. I rekon I am in agreement with you.

Take it easy

 

[ayman metwally]

That's it :

 

[elevmike]

How do you "un-subcribe" to a thread..... zzzzz

 

[Dragon_Man]

On the email that is sent to you there is a link to click on to unsubscribe to this thread or all threads.

Has the original questioned been answered?

 

[elevmike]

Paul,

Thanks for the tip on unsubscribing. I'll see you guys elswhere.

BTW, In my opinion the origional question has been answered...

When you guys finally determine, and all agree on what DC current really is, feel free to let me know.
banghead

Good Bye,

 

[bernie_carlton]

OK - one last for me on this thread. It's like 'real time' being defined as 'fast enough to get the job done'. A device which expects a 'DC' input usually has a minimum limit for On/High and a maximum limit for Off/Low. Any real world DC signal has some noise on it. If the DC is constant enough to get the job done (with appropriate margins) then it's DC as far as that device is concerned.

 

[ndzied1]

Sounds kind of like the difference between a scientist and an engineer....

What's the difference between an engineer and a computer scientist?

An engineer thinks there's 1000 bytes in a kilobyte, and a computer scientist thinks there's 1024 meters in a kilometer.

 

[rsdoran]

My apologies

I apologize for taking this on a tangent. As many of you know I am not an engineer, I am a maintenance tech that went to school for electronics.

Its my understanding that the theories/equations in the electrical/electronic field are based on the fact:
AC, alternating current, current/electron flow reverse direction/polarity

DC, direct current, current/electron flow is one direction.

Its this basic understanding that allows the use of semiconductor devices to convert AC to DC or vice versa.

Can one (1) diode convert AC to DC?

I will not make any more posts to this thread either.