LEDs

Using Arduino General Electronics
7

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?

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?