Go Down

Topic: LEDs (Read 1 time) previous topic - next topic

fpiro07

Hello everyone, I'm a complete newbie to the electronic world so I have some very basilar questions.

1) What lights a LED up? Voltage or current?

2) What does a resistor limit? Voltage or current? I mean, if I don't put a resistor in my circuit the LED bursts down: why? Too much volts or amperes?

3) What remains constant in a circuit? Voltage or current? I mean, what is the same at the start and at the end of a circuit? Volts or amperes?

4) How should I put resistors when I have more than one LED?

5) How should I put resistors when I have series or parallel circuits?

fungus


Hello everyone, I'm a complete newbie to the electronic world so I have some very basilar questions.

1) What lights a LED up? Voltage or current?


You can't have one without the other.


2) What does a resistor limit? Voltage or current? I mean, if I don't put a resistor in my circuit the LED bursts down: why? Too much volts or amperes?


Usually current.


3) What remains constant in a circuit? Voltage or current? I mean, what is the same at the start and at the end of a circuit? Volts or amperes?


Amperes.


4) How should I put resistors when I have more than one LED?

5) How should I put resistors when I have series or parallel circuits?


One resistor on every route to ground.
No, I don't answer questions sent in private messages (but I do accept thank-you notes...)

seanz2003

1) once the forward voltage is reached brightness is determined by the amount current flowing through. ( current)
2) current- too many (milli)amps
3) constant current source
4) always in series. if you have enough voltage you can string one resistor to several series diodes though this is not always ideal.
5) I'm not sure I understand what exactly you are asking. example or schematic, please?

JimboZA

Quote
I'm a complete newbie to the electronic world


..... have a look at this site for a good intro to the whole world of electronics
Repeal Ohm's Law

No PMs for help please

Brandeaux

#4
Jan 30, 2013, 06:42 pm Last Edit: Jan 30, 2013, 06:44 pm by Brandeaux Reason: 1

1) What lights a LED up? Voltage or current?

2) What does a resistor limit? Voltage or current? I mean, if I don't put a resistor in my circuit the LED bursts down: why? Too much volts or amperes?


This should get you started as far as answering questions 1 and 2: http://www.sparkfun.com/tutorials/219


3) What remains constant in a circuit? Voltage or current? I mean, what is the same at the start and at the end of a circuit? Volts or amperes?


It depends on your circuit. I asume that you are familiar with series and parallel circuits. In a series circuit, current is constant (assuming none of your resistors change values due to heat or other factors) and voltage will change from place to place. If you have a battery connected to two resistors (motors, resistors, anything really...) then you can use Ohm's Law to calculate everything about the circuit. Let's say we have a nine volt battery and two 50? resistors.

Ohm's Law: Voltage = Current x Resistance
We know Voltage (before the battery starts to die) is 9V. We also know the value of both of our resistors in the circuit. In series, resistors simply add their resistances to get a total. 50? + 50? = 100?, so now we have:

9V = I x 100?

To find current, divide each side by R:

9V/100? = (I(100?))/100?

0.09 = I

So the current is 0.09A, or 90mA. When working with many things currents will be listed in mA, so it's worth it to convert.

To find the voltage across a certain resistor you already know that the current (constant in series circuits) is 90mA, and you know the resistance of the part, here 50?. So:

V = 0.09A x 50?

V = 4.5V

The most important thing to get out of this is Ohm's Law. It is used everywhere and flipping it around, you can get almost whatever you want to know.

As far as parallel circuits go, you still use Ohm's law, but there are some important differences. Look here and see if you can get them figured out.

http://www.allaboutcircuits.com/vol_1/chpt_5/3.html

Here is another good source: http://www.electronics-tutorials.ws/dccircuits/dcp_2.html


4) How should I put resistors when I have more than one LED?


It will depend on your circuit. If all of the LEDs are coming on at once in series, you could get away with a single resistor calculated to supply the correct amount of current. (This is referred to as a current limiting resistor.)

If the lights are independent of each other then you will need a resistor for each LED.


5) How should I put resistors when I have series or parallel circuits?


Once you understand the various parts of series and parallel circuits, this will become easy for you to decide.

Hope that helps!


James C4S

See if this helps:
http://www.cmiyc.com/tutorials/led-basics/
Capacitor Expert By Day, Enginerd by night.  ||  Personal Blog: www.baldengineer.com  || Electronics Tutorials for Beginners:  www.addohms.com

Krupski

#6
Feb 02, 2013, 07:11 am Last Edit: Feb 02, 2013, 07:14 am by Krupski Reason: 1

Hello everyone, I'm a complete newbie to the electronic world so I have some very basilar questions.

1) What lights a LED up? Voltage or current?

2) What does a resistor limit? Voltage or current? I mean, if I don't put a resistor in my circuit the LED bursts down: why? Too much volts or amperes?

3) What remains constant in a circuit? Voltage or current? I mean, what is the same at the start and at the end of a circuit? Volts or amperes?

4) How should I put resistors when I have more than one LED?

5) How should I put resistors when I have series or parallel circuits?



Picture a short piece of string. This is the LED. In order to light the LED, the string must be pulled taut, but not too hard lest you break it. So, to connect the string between two points and insure (1) that you don't break it and (2) that it actually is taut and (3) it's at the PROPER tension - you would attach it with a SPRING in series.

The string and spring in series, stretched and connected between two points represents an LED (string) in series with a resistor (spring) attached to two mounting points (positive and negative).

If the spring is too weak (resistor too large), the tension in the string is low (the LED is dim). If the spring is too strong, the string is VERY tight and may even snap (LED is bright, getting hot and failing).

Now, imagine you had to tie TWO strings together in series (end to end). Where would the spring go? On either end, or in the middle... doesn't matter.

Imagine you had to tie several strings together side by side (in parallel). Each string may have a SLIGHTLY different length, so you can't just tie them all together in parallel... one string will get tight, the others barely tight or not at all. Solution: Use a spring in series with EACH STRING so that each one gets it's own independent "tension regulator".

Now back to reality... normal LED's use 0.02 amperes (20 milliamps) typically. When they are lit up, they drop about 2.5 to 3 volts across them.

So, figuring out the "spring" (resistor) for an LED is easy.

Say you have a 5 volt battery and the LED has a 3 volt forward drop. You have to "get rid of" 2 volts in the resistor, and do so while 20 milliamps of current flow.

R = V / I, R = 2 / 0.02, R = 100 ohms.

See? Easy, right?
Gentlemen may prefer Blondes, but Real Men prefer Redheads!

MarkT


Hello everyone, I'm a complete newbie to the electronic world so I have some very basilar questions.

1) What lights a LED up? Voltage or current?

To be pedantic: neither, carrier recombination lights it up.  Both voltage and current are needed to cause forward conduction which leads
to more carriers in the pn-junction to recombine.
Quote


2) What does a resistor limit? Voltage or current? I mean, if I don't put a resistor in my circuit the LED bursts down: why? Too much volts or amperes?

Resistor relates current to the applied voltage - higher voltage will drive more current, the only limit is when it burns up.
Quote


3) What remains constant in a circuit? Voltage or current? I mean, what is the same at the start and at the end of a circuit? Volts or amperes?

I think you mean current - assuming certain things (such as its not a transmitting antenna), the current is everywhere the
same around a circuit.  This isn't really true, since if parts of the circuit are changing their voltage some charge is used
to charge that part up, but for most practical circuits that aren't antennas it holds well.
Quote


4) How should I put resistors when I have more than one LED?

One per LED to limit current individually (unless they are in series which means they all have same current
Quote


5) How should I put resistors when I have series or parallel circuits?

That's a meaningless question - what are you trying to achieve?  All non-trivial circuits have series and parallel elements.
[ I won't respond to messages, use the forum please ]

A4kash

#8
Feb 02, 2013, 04:27 pm Last Edit: Feb 02, 2013, 04:46 pm by A4kash Reason: 1
http://electronics.howstuffworks.com/led.htm
Some informations on how the leds work..

and this is good one too
http://www.kpsec.freeuk.com/components/led.htm

fpiro07

Ok, I almost understood everything, thanks  :)

Anyways, reading here and there on the web, 3 more questions have come up to me, and I was hoping that you could answer them:

1) How can current remain constant in my circuit if I limit it with resistors?

2) In one of the websites you've linked me I found some schematics of a circuit in which the resistor was after the LED. How is this possible? Shouldn't the LED burn down before the current reaches the resistor?

3) Considering the previous question, it has come up to me that I could in some way calculate the total resistance of my circuit and then place a big resistor at the end of it right before going to GND. Is this possible?

Thanks again in advance

JimboZA

I've put my 2c's worth in below in green....


Ok, I almost understood everything, thanks  :)

Anyways, reading here and there on the web, 3 more questions have come up to me, and I was hoping that you could answer them:

1) How can current remain constant in my circuit if I limit it with resistors? Well, you can drive at a constant speed on the road, even when there's a speed limit  8). But with the LED example specifically, why would the current change? It's not like a motor trying to move a load and need ing more current as it stalls. When you subtract the LED voltage from the total available, then that's the voltage you have to lose over the resistor... so you have a given voltage drop, a required current, and hence a particular resistor which will keep that current thru it.

2) In one of the websites you've linked me I found some schematics of a circuit in which the resistor was after the LED. How is this possible? Shouldn't the LED burn down before the current reaches the resistor? Not sure what you mean by "burn down", but you can't think of the current "reaching" the resistor. The current thru the resistor and its series-connected LED have to be the same- where else can it go.... so it doesn't matter which way round you order them.

3) Considering the previous question, it has come up to me that I could in some way calculate the total resistance of my circuit and then place a big resistor at the end of it right before going to GND. Is this possible? You mean with a whole load of "legs" of a circuit joining up?- well with one big resistor you wouldn't have any say over the current in an individual leg.

Thanks again in advance
Repeal Ohm's Law

No PMs for help please

Go Up