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how do you know what resistor to use with LEDs

how do you know what resistor to use in a LED circuit? I salvaged a bunch of resistors,diodes capacitors and LEDs from several objects. how do i classify the resistors(i know they have different colors/sizes/stripes but what do they mean?)

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winrow8 years ago
You have to use Ohm's Law which is V=ir
v=voltage
i=current
r=ohms

when you find 'r' use a chart to find out which resistor to use http://www.engplanet.com/content/resistorinfo.html

if the resistor has a gold or silver band at the end of it, start at the end opposite from that side.

say the number you got was 200 ohms.
you look at the first number.
the first number is 2.
look at the chart and find 2.
2 is red so the first color on your resistor should be red.

the second number is 0.
look at the chart and find 0.
0 is black so the next color on your resistor should be black

the 3rd band is the multiplier
Brennn109 years ago
First off, the different colors are just different ways of classifying the amount of voltage that can run through them. Resistors restrict or lower the amount of current flowing through the circuit. Resistance is measured in Ohms. Higher resistance results in less current.

The first two colors give the base value of the resistor; the next gives the multiplier, and the final band the tolerance. You find the number of Ohms(which resistors are measured in) by Ohms Law. Which is
  • Electrical current = Voltage/Resistance
Plug your colors in to this calculator, and it should give you the Ohms that are needed for you to implement into your circuit.
http://www.dannyg.com/examples/res2/resistor.htm
ledzep567 (author)  Brennn109 years ago
starting from which end? how do you know which one is the first... thanks though
On simple resistors, usually there is always a gold ring. Start from the opposite end of the gold ring.
ledzep567 (author)  Brennn109 years ago
thanks lol
Gold or silver, depending on the "tolerance", i.e. level of accuracy with which the resistors were manufactured. Resistors salvaged from old and/or cheap electronics may be missing a metallic band altogether - those are the lowest level of accuracy. In that case, you start counting from the rings closest to side.
ledzep567 (author)  Brennn109 years ago
that only tells me what the specific kohms are for that resistor. i need to know also which one to use
Here is a better calculator. It lets you input all of the parts of Ohms law to find out which resistor to use.

http://www.sengpielaudio.com/calculator-ohmslaw.htm
ledzep567 (author) 9 years ago
lol have you seen the "sequel" with the grandmother?