# Question about LEDs and resistors 1/4, 1/5 ...

Tom, You have got the LED symbol the wrong way round.

TomGeorge: Hi,

File this image away for reference. Everybody copy it, use for future reference on the forum. (Just like a certain servo diagram.)

Tom.... :) I love Express and Paint.

but isn't the current going the opposite direction ? from + to - and the LED in opposite direction ?!

Hi, took a while to notice, I kept the arrows in one direction to show potential difference.

Tom…

MeSat: See the attached image.

Sticking with the water analogy. The LED is a one way check valve. It prevents current going the wrong direction. If you hook an LED up backwards to a battery, it won't work just like a check value only works in one direction.

Current is like the water flowing through the pipe. The amount of water flowing at one end is the same as through the other end.

Resistors are narrower pipes. The narrower the pipe, the higher the restriction on water flow.

VB - Battery Voltage - 5V VF - LED Forward Voltage - 3V VR - Resistor Voltage - 5V - 3V = 2V

CrossRoads provided a pretty good description of the maths.

These calculations are for continuous operation, not pulsed. For pulsed usage, the amount of current can be higher but until you are used to the maths, then it is best to use these values.

There are various pdf's and web sites that offer an introduction to electrics and electronics.

Hope this helps

this is how I think it is :relaxed:

but now i am confused with what Grumpy_Mike said, if there is no direction means no flow, which means what they teach us at school when we were little made this bad thinking isn't it ? :fearful: :(

Hi, current flow explained.
No further discussion about current flow will be entered into.

Tom…

TomGeorge:
Hi, current flow explained.
No further discussion about current flow will be entered into.

Tom…

thanks for this schematic, it’s great except one thing for me, the direction of the arrow of the I

I think of the circuit this way, the current goes from source, it hits the resistor first which limits this current and make a voltage drop then the current continues and hit the LED, at this point the LED won't burn because the voltage is right for it, then the current continue and get back to the source, and it happens again and again until you break the circuit ...

TomGeorge: No further discussion about current flow will be entered into.

Good on you.

It would have been far easier to simply put the current arrow on the top of the diagram, where it was supposed to be, and have it correct.

firashelou: I think of the circuit this way, the current goes from source, it hits the resistor first which limits this current and make a voltage drop then the current continues and hit the LED, at this point the LED won't burn because the voltage is right for it, then the current continue and get back to the source, and it happens again and again until you break the circuit ...

That is not the best way to think about it. Better to see it as a circuit - a complete [u]loop[/u] around which current can flow.

Once you perceive that, you forget about tracking the current and examine the items in the loop. You know the supply voltage, you know the LED voltage (near enough, though it is not precise), you thereby deduce the resistor voltage. Next, you know the resistance so you can derive the current using Ohm's law and you have the whole picture.

Paul__B: That is not the best way to think about it. Better to see it as a circuit - a complete [u]loop[/u] around which current can flow.

but the loop goes in one direction isn't it :astonished:

i need to really understand this and change my way of thinking of the flow going forward, but i need a very simple way of explaning

the LED let the current goes in 1 direction, means the current is flowing forward in 1 direction so ...

firashelou: but the loop goes in one direction isn't it

That's what you need to get away from. The loop does not go in any direction - it is just a loop. All things in the loop happen simultaneously; the current circulates because it is a loop, it does not start or end at any point. Just forget it and concentrate on the structure of the loop.

TomGeorge: Hi, current flow explained. No further discussion about current flow will be entered into.

Too late Tom you have confused the OP here.

if there is no direction means no flow,

I did not say there was no direction, I said that it didn't matter which way round you think of it flowing. In fact current flows in both directions at once. Electrons carry negative charge and flow form -ve to +ve. Where as, at the same time holes carry positive charge and they flow from +ve to -ve. These are two faces of the same thing. You only have to consider one flow direction because one is the mirror image of the other. By convention in electronics we consider current flow from +ve to -ve. Physicists have to sometimes consider electrons or holes depending on the type of material.

This concept is also used very successfully to confuse beginners. If you think the direction of flow matters to anything you are considering then you are thinking about it totally incorrectly.

Current flows, it flows in a direction, as long as you stick to one standard direction you will not go wrong. If you are in electronics that is from +ve to -ve.

So think of it in this way and forget electron flow.

If you are in physics you know these things and you know for electronics it flows from +ve to -ve.

Paul__B: That's what you need to get away from. The loop does not go in any direction - it is just a loop. All things in the loop happen simultaneously; the current circulates because it is a loop, it does not start or end at any point. Just forget it and concentrate on the structure of the loop.

so you mean once the loop is closed, electricity happens ? means like current is not circulating in any direction but it just appears once the circuit is closed ?

so what about a LED of 1 direction :-[ ?

so you mean once the loop is closed, electricity happens ?

No. Electricity needs a source, say a battery. Once a circuit is made from the +ve to the -ve current flows round it.

means like current is not circulating in any direction but it just appears once the circuit is closed ?

No please read what I wrote, current flows in both directions NOT no direction.

so what about a LED of 1 direction

What about it? A diode will only let current flow in one direction, that direction is from anode to cathode in the case of hole current ( positive current carriers ) or from cathode to anode in the case of electrons (negative current carriers)

@firashelou - please forget about electron flow when dealing with electronics, you are only confusing yourself. It is an esoteric topic that will get you nowhere.

but isn't the positive particles stay stable, and the electrons are the moving particles ?

but isn’t the positive particles stay stable,

No, the positive carrying particle in electricity, that carries positive charge, is called a hole. It is the absence of an electron, it moves, it has a mean free path and it even at times has mass, which means that at times an electron has a negative mass which is a good trick. Idiots will tell you it doesn’t exist but it is just as real as anything else. In P type semiconductor material it is the majority carrier of charge, in some other material like metals or N type semiconductors it is a minority carrier of charge. Current by the way is the flow of charge.

BUT all this is physics, you are not playing with physics you are playing with electronic engineering. Just think that current flows from positive to negative.

Guys - I hope you are happy having a laugh with some one struggling to cope with a simple resistor and LED.

well i do think of this way but when it comes to resistor, i am not being able to cope with this if we think the current flow from + to - then according to this concept the resistor should be places before the LED to limit the current coming to it which takes us back to the beginning of 5V reaching the resistor before the LED :disappointed_relieved:

i don't care if anyone laugh :) so far i learned a lot from my stupid mistakes and questions (it's the dumbest rule of any game) but in theory behind the scenes not which made me ask this question when i was making some calculation which showed that it was wrong :(

PS : we can all laugh " ... togetha or not togetha " :grinning:

well i do think of this way but when it comes to resistor, i am not being able to cope with this if we think the current flow from + to - then according to this concept the resistor should be places before the LED to limit the current coming to it which takes us back to the beginning of 5V reaching the resistor before the LED

My advice to you then would be to give up on electronics and take up knitting.

Grumpy_Mike: My advice to you then would be to give up on electronics and take up knitting.

knitting is nice :D lol

let's just take this step by step

first the current we agreed flows from + to -. Good

so the LED will let current flows in one direction so if there is noise or repulsion it will block it.

and resistor can be put before the LED and after, well this one is not clear why ?!! :slightly_frowning_face: :slightly_frowning_face: :'(

Electrons are repelled away from the NEG terminal and attracted to the POS terminal.

Reference direction Since the current in a wire or component can flow in either direction, when a variable I is defined to represent that current, the direction representing positive current must be specified, usually by an arrow on the circuit schematic diagram. This is called the reference direction of current I. If the current is flowing in the opposite direction, the variable I will have a negative value. When analyzing electrical circuits, the actual direction of current through a specific circuit element is usually unknown. Consequently, the reference directions of currents are often assigned arbitrarily. When the circuit is solved, a negative value for the variable means that the actual direction of current through that circuit element is opposite that of the chosen reference direction. In electronic circuits, the reference current directions are often chosen so that all currents are toward ground. This often corresponds to the actual current direction, because in many circuits the power supply voltage is positive with respect to ground.

Electric current

well i do think of this way but when it comes to resistor, i am not being able to cope with this if we think the current flow from + to - then according to this concept the resistor should be places before the LED to limit the current coming to it which takes us back to the beginning of 5V reaching the resistor before the LED

I think you are not "getting" it. I wire is similar to a garden hose in the sense that a restriction in ANY part of it restricts the flow in ALL of it. It makes absolute no difference WHERE the current limiting resistor is. The ONLY reason we would normally put it at the GND end (and NOT the +5V end) is that way we can measure the current with an analog input instead of needing a DMM. We COULD still do it with analog inputs but we would need TWO and obtain the resistor voltage drop by subtraction of the lesser from the greater of the two analog signals.

As far as current flow direction, the convention is to use positive hole direction (from + to - ). Not exactly sure if the above quote clarifies that, but I find it more convenient to use the conventional direction of + to -.

(we're only at 38 Replies. I wonder if we'll get over a hundred on this one...)

so the LED will let current flows in one direction

Yes.

so if there is noise or repulsion it will block it.

Not relevant in this circuit there will never be any reverse flow anyway so forget that bit.

and resistor can be put before the LED and after

Yes.

well this one is not clear why

Because you only get a current flowing when you have a complete circuit. In a series circuit like this the same current flows through both the resistor and the LED, if it did not then there would be no circuit and current would not flow. Therefore it does not matter if the current first flows through the LED or the resistor because the second thing it flows through is the other component. The current has to pass through both components or there would not be a circuit. So the concept of flowing through any component first or last has absolutely no meaning, because current flows through all components and that current is the same in all components in a series circuit.

You can not have different currents flowing in different components in a series circuit, electricity simply does not work like that.