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Using Arduino => General Electronics => Topic started by: Sohail_Siraj on Oct 26, 2019, 10:03 am

Title: resitor used in an led circuit.
Post by: Sohail_Siraj on Oct 26, 2019, 10:03 am
I have a simple question over here,

for a simple led circuit to lighten it up.There are 2 ways for it to work


1, connect the 5v dc power source to a resistor and then to a anode of the led and then connect the ground to the cathode,

2,connect the 5v dc power source to the anode of the led and connect the cathode of the led with a resistor and then connect the resistor to ground,


in both ways the led gets lit up.Now this is kind of confusing for me .How does the current /voltage flow in this case since the led is being lit up in both the ways.Please answer this question .This question has been buzzing me for quite a while of time,
Title: Re: resitor used in an led circuit.
Post by: slipstick on Oct 26, 2019, 10:18 am
In either case you have 2 things, an LED and a resistor, connected in series between 5V and ground. As they are in series the same current goes through both things and so some of the voltage is dropped across the LED and some across the resistor. Where do you see a problem?

Steve
Title: Re: resitor used in an led circuit.
Post by: PerryBebbington on Oct 26, 2019, 11:06 am
For things in series, which is what you have with an LED and a resistor, the order they are connected does not matter*, the current goes through them all and each one drops some voltage according to its characteristics and Ohms law.

*Does not matter at low** frequencies
**Definition of 'low', as a person who knows about radio!
Title: Re: resitor used in an led circuit.
Post by: Paul__B on Oct 26, 2019, 12:12 pm
Basic study (https://en.wikipedia.org/wiki/Series_and_parallel_circuits).
Title: Re: resitor used in an led circuit.
Post by: PerryBebbington on Oct 26, 2019, 12:57 pm
Paul__B, that link is just to your logo / icon / avatar / whatever it is called.

[EDIT: Paul__B has since corrected the link]
Title: Re: resitor used in an led circuit.
Post by: Sohail_Siraj on Oct 26, 2019, 03:04 pm
fine lets take the 2nd case for example .I was under the assumption that the resistor is connected to ground .So electons flow through the led first and then the resistor and then to ground (where all the electrons are dissipated to the ground ) so there is no way we are limiting current flow to the led .

so now in a series circuit it its the same right.
Title: Re: resitor used in an led circuit.
Post by: PerryBebbington on Oct 26, 2019, 03:22 pm

Quote
So electrons flow through the led first and then the resistor and then to ground
Firstly, just to confuse you, due to historical accident electrons flow the other way; they flow from 0V to the positive. However, for convenience we usually think of the current as flowing from positive to negative because it's easier to think that way and, as long as you consistently think that way round it doesn't make any difference.

Electrons have no sense of where they are.


Quote
I was under the assumption that the resistor is connected to ground.
Your assumption is wrong. Why do you think that?

Does this help:
You go on a long journey on the motorway by car. There is an accident on the motorway that makes your journey take an hour longer than it should do, because of the delays caused by the accident. Does it make any difference where on the motorway the accident is? Near the start of your journey? In the middle? Towards the end? Wherever it is, the accident delays you by 1 hour. The accident is like the resistor, the delay is like limiting the current flow. It makes no difference where you put the resistor.



Title: Re: resitor used in an led circuit.
Post by: Paul__B on Oct 26, 2019, 03:34 pm
Paul__B, that link is just to your logo / icon / avatar / whatever it is called.
Threads mixed up!

Electrons have no sense of where they are.
Not only that, but they actually "flow" at only a walking pace (drift) in most cases.  :smiley-eek:
Title: Re: resitor used in an led circuit.
Post by: Grumpy_Mike on Oct 26, 2019, 10:06 pm
Quote
So electons flow through the led first and then the resistor and then to ground
No they don't flow through anything first. They flow through the whole circuit at the same time.

Before a voltage is applied to make them flow, they are hanging about, just chilling out in every component of the circuit distributed evenly. When the voltage is applied they all start to move, all at once, all at the same speed and direction.
Title: Re: resitor used in an led circuit.
Post by: Sohail_Siraj on Oct 27, 2019, 04:05 am
alright .Thanks a lot everyone I understood . !!!!!!!!!!!!!!!!!!!!!!!!!
Title: Re: resitor used in an led circuit.
Post by: MarkT on Oct 27, 2019, 08:47 pm
fine lets take the 2nd case for example .I was under the assumption that the resistor is connected to ground .So electons flow through the led first and then the resistor and then to ground (where all the electrons are dissipated to the ground ) so there is no way we are limiting current flow to the led .

so now in a series circuit it its the same right.
No, there is no "first" - the electrons all flow in concert throughout the entire circuit in lock-step, the current is the same everywhere (otherwise charges would pile up somewhere and generate millions of volts like in a Van-de-Graff generator)

Think of the flow of liquid in a pipe - the flow is determined by all the pipework's resistance to flow, the flow is the same everywhere along the pipe, and is a global property of the circuit, not a local one.
Title: Re: resitor used in an led circuit.
Post by: PerryBebbington on Oct 27, 2019, 09:23 pm

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The flow is the same everywhere along the pipe, and is a global property of the circuit, not a local one.
Interesting point because it sounds exactly like a non-local quantum state. I can't help thinking that somewhere in descriptions like that lies the answer to the mysteries of quantum mechanics.

Title: Re: resitor used in an led circuit.
Post by: MarkT on Oct 28, 2019, 02:16 pm
Except this is just a classical treatment for the steady-state conditions in the large, no quantization involved (outside the light emission of the LED!)
Title: Re: resitor used in an led circuit.
Post by: PerryBebbington on Oct 28, 2019, 05:34 pm

Quote
Except this is just a classical treatment for the steady-state conditions in the large, no quantization involved
Indeed, and I am no expert on QM. Just sometimes I see something that hints at something that might be going on with QM, but I do not have the expertise to investigate it.

Enough of hi-jacking this discussion!