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

James C4S

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.
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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.
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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.
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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
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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

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