Sink and Source / current flow - understanding

[joao felix]

Greetings.

Beg your pardon for this noob question:

Reading Electronics Club explanation on sink and source for 555 IC gets me totally confused.

They write:
"If the IC is sinking current it is flowing into the output." -> I get this. But when they write, right after that: "This means that a device connected between the positive supply (+Vs) and the IC output will be switched on when the output is low" ->What??

This is one situation where, to me, talking about electricity according to Conventional or Electron current flow is NOT indifferent. Don't electrons go from 0V to Vs? If so, when the IC is sinking, electrons have to flow from 0V into the IC. Hence it is the device connected between the negative (0V) and the IC that should be switched on! Right?

Dear lord, and I thought I had it figured out at last...

Much obliged
joao felix

 

[MikeMl]

According to the conventional current definition used on the IC maker's data sheet (and by most engineers and scientists), the 555 output pin SINKS current when the output is LOW and the load is returned to a POSITIVE voltage (same at the 555's VDD pin), and SOURCES current when the output pin is HIGH and the load is returned to the same voltage as the 555's VSS pin.

Current ALWAYS flows from a more POSITIVE voltage to a less POSITIVE voltage!

 

[joao felix]

Ok, first thanks for your reply. I salute your patience.

Now, I'm not trying to debate what the most learned/scholar way to talk about electricity is. I am aware that the Electron Flow way is viewd as a "begginner approach" (although I have read that some circuit engineers consider the Electron Flow).

What happens is that, in fact, electrons flow from the "-" battery terminal to the "+". Thanks to B. Franklin we designate as "-" the terminal wich has more electrons, when it would make more sense to designate it as "+" (because it has more).

I humbly ask you to overcome your Conventional Current Flow (CCF) view of the world and try to tell me what happens in the circuit (I was going to ask you to do it from the Electron Current Flow (ECF) view but I want the truth, without the bounds of any belief system.

In truth, the current can only flow in a way, there is no room to fundamentalism or choice; if we could watch the electrons moving, it would not matter if we used CCF or ECF spectacles/goggles.

What I desperately need to understand since I first got involved in electronics is if (and let me use this Electronics Club example), setting aside all theories and personal preferences regarding the way we look at how electricity "functions", when they write: If the IC is sinking current it is flowing into the output. This means that a device connected between the positive supply (+Vs) and the IC output will be switched on", they say it because it is according to the CCF view (although knowing that it will be the device connected between the 0V and IC that will be switched on) OR because:
1. it really will be the device connected between the Vs and the IC that will be switched on (in wich case when they say the IC is sinking current it is actually pushing electrons away from the Output and into Vs)?

Thank you!

 

[crutschow]

You are really trying to make it more complex than it is. For the 555, when it is sinking current from +Vs to 0V it means the output transistor connected to the common pin is on. It's just acting as a switch between +Vs and the circuit common. So whether you say it's current flow from +Vs to 0 or electron flow from 0 to +Vs makes absolutely no difference to the operation of the output and any load. It is unaffected by your "belief system".

The only time you really have to worry about electron flow is when studying semiconductor theory or the operation of thermionic vacuum tubes.

 

[Jaguarjoe]

The output of the 555 has 2 transistor switches. One switch connects the output of the 555 to ground. The other switch connects the output of the 555 to Vs. Only one switch is on at any time.

When the switch connected between the output and Vs is turned on, current will flow from Vs to the output, through the load and then to gnd. This is current "sourcing" because current flows out of the output. The load is connected between the output and gnd.

When the switch connected between the output and gnd is turned on, current will flow from Vs through the load, through the switch and then to gnd. This is current "sinking" because current flows into the output pin. The output is connected between Vs and the output.

Its like a SPDT switch. One pole to Vs, other pole to gnd. The armature is the output. One way its sourcing current from Vs, the other way its sinking current to gnd.

 

[Grossel]

To understand an IC output as discussed here, you can think of it as a simple switch where the load is connected between positive voltage and switch.

 

[Ratchit]

joao felix,

This is one situation where, to me, talking about electricity according to Conventional or Electron current flow is NOT indifferent. Don't electrons go from 0V to Vs? If so, when the IC is sinking, electrons have to flow from 0V into the IC. Hence it is the device connected between the negative (0V) and the IC that should be switched on! Right?

After reading through this thread, I believe there are two concepts you do not understand very well.
1) What constitutes charge movement?
2) Why do we have conventional current?

I will cover these two questions below, but first you may want to read the following thread.
This is conventional vs electron theory right? - All About Circuits Forum

1) Current is the movement of charge. "Current flow" or "current movement" are malapropistic phrases. They literally mean "charge flow flow", which is redundant and ridiculous. Although many folks and publications use that term, it is a misnomer to do so. "Charge flow" is the correct terminology, or you can say "current exists", "current existence", or "current direction". Using current flow is akin to NASA saying "space walk" when we know that astronauts float in space. They should instead say extra-vehicular activity (EVA).

2) Conventional charge flow (CCF) is a mathematical methodology, not a designation of the physical direction of charge carriers. Although it may seem intuitively correct to think of the current direction as governed by the movement of the physical charge carriers, you can really get your brain wrapped around the axle by doing so. There are just as many positive charge carriers as there are negative charge carriers in the universe, and you don't want to deal with their polarities separately. Positive charge carriers include semiconductor holes and positive ions in electrochemistry, and ofter they are moving simultaneously with negative charge carriers. The CCF method was invented by engineers to do away with that worry. CCF does not ask what the polarity of the charge carrier is. It mathematically designates charge carriers to be positive, and their flow from the positive terminals of a voltage source to the negative terminal to be in a positive direction. After the calculation is made and a number is reached, THEN, if necessary, one can reason that if the charge carrier was positive, then the number is correct. If it is negative, then the sign of the number is flipped to designate a opposite direction. By following the CCF method, you can keep your head screwed on right and tight, and concentrate of the problem instead of the charge polarity. By the way, if Ben Franklin designated electrons to be positive, then people would be bitchin' about semiconductor holes being wrong. So to sumarize, you first perform the calculation mathematically, then look at the charge carriers to determine the physical direction.

As a final note, you will notice that semiconductor manufacturers and electronic equipment manufacturers mark their diodes, transisters, ammeters as if they were conducting positive charged carriers. In other words, they use the CCF method.

Dear lord, and I thought I had it figured out at last...

The Deity will not help you in this matter. The members of this forum will.

Ratch

 

[joao felix]

First I want to thank crutschow and jaguarjoe for your constructive answers. Now let me explore them to push away some more of my ignorance regarding electronics.

crutschow, I have to disagree, because in this case I'm concearned with timing. But it was my mistake not to include a picture of the "phenomenon". This time I have inserted a picture as attachment (in a message below, sry).

The picture represents two halves of the same circuit at 2 different instants. The output is connected to Vs and 0V, and in each half there is a LED and a resistance. And here I go to jaguarjoe's answer.

In your 2nd paragraph you're describing the lower half. It seems to me that, when you say the IC is sourcing current, you are describing this phenomenon according to CCF. As the people from ElectronicsClub are certainly doing. If we were to analyze this from that veemently hated point of view of ElectronsDirectionOfDisplacement (I no longer know what to call it) we would have to describe it in the complete opposite way, right?

Is this the "secret"? When they say the IC is sourcing current, they do it under the CCF. It so happens that the electrons are flowing in the opposite direction, so from this point of view the IC is "sinking electrons".

Hence, when the output is high, electrons are moving from 0V to Output (and positive charges in the reverse way?).

Is it a mere matter of words?

Maybe now I can move on to understanding the Astable operation.

Many Thanks

 

[joao felix]

.............

 

[MikeMl]

.............

Yep, and current flows in the direction of the arrows. Now, ain't that simple?

 

[crutschow]

Is this the "secret"? When they say the IC is sourcing current, they do it under the CCF. It so happens that the electrons are flowing in the opposite direction, so from this point of view the IC is "sinking electrons".

Hence, when the output is high, electrons are moving from 0V to Output (and positive charges in the reverse way?).

Is it a mere matter of words?

It is rather a matter of words. Current flows from positive to negative. Electrons flow from negative to positive. That's all you really need to know. Which ever one you use will give you the correct answer when analyzing circuits. I use current since I find it easier to think that way and all the arrows on diodes and transistors point in that direction. But if you prefer to think in terms of the flow of the actual charge carriers than you can use electron flow.

However, one thing that does change is the "right hand rule" for magnetic fields that indicates the direction of the magnetic field in a coil based upon current direction. It becomes the "left hand rule" if you use electron direction.