What resistor do I need with an LED?

Choosing a resistor to work with an LED is fairly simple, but does require some knowledge of the LED and a small amount of maths. Some LEDs such as colour changing LEDs, flashing LEDs and 5V LEDs are designed to run off a 5V supply and therefore don't need a resistor.
A.

Which side of the LED is positive?

Diode and LED Polarity. Diodes only allow current to flow in one direction, and they're always polarized. A diode has two terminals. The positive side is called the anode, and the negative one is called the cathode.
  • What led you there?

    But when it's a verb, “lead” is the present and “led” is the past tense. The problem is that the past tense is pronounced exactly like the above-mentioned plumbing material (“plumb” comes from a word meaning “lead”), so people confuse the two.
  • Which lead me?

    So whenever the word is used as a noun, it's always spelled l-e-a-d. Used as a verb, “lead” (pronounced like greed) means “to guide or direct.” “Led” comes into play as the past tense of the verb “lead.” Derek Jeter led the New York Yankees to a World Series victory.
  • Is a striped wire positive or negative?

    The solid/dashed lines on wires like the ones pictured in your question are used to indicate polarity e.g. for the "wall wart" power supplies. Usually* the wire with the white stripe or the dashed lines carries the "positive" (+) end, while the other, unmarked wire carries the "negative" (-) end.
B.

What voltage to use for LED?

Many circuits operate LEDs at less than the recommended maximum current, to save power, to permit the use of a standard resistor value, or to reduce brightness. Typically, the forward voltage of an LED is between 1.8 and 3.3 volts. It varies by the color of the LED.
  • What happens when LED is reverse biased?

    If we further decrease the voltage, then we enter another region of operation known as the breakdown region. We generally operate a diode in either its forward or reverse biased modes. An LED is a light emitting diode. The LED emits light when it is forward biased and it emits no light when it is reverse biased.
  • What is the forward current of a LED?

    The forward current of an LED, IF, is the current which flows across the LED's leads, from anode to cathode, in order for the LED to receive sufficient current to power on. As you can see above, positive voltage must be applied across the LED from its anode to its cathode.
  • What is a capacitor in a circuit?

    A capacitor is a passive two-terminal electrical component that stores potential energy in an electric field. The effect of a capacitor is known as capacitance. Capacitors are widely used as parts of electrical circuits in many common electrical devices. Unlike a resistor, an ideal capacitor does not dissipate energy.
C.

Why is it necessary to use a resistor in series with an LED?

The ballast resistor is used to limit the current through the LED and to prevent that it burns. If the voltage source is equal to the voltage drop of the LED, no resistor is required. The resistance of the ballast resistor is easy to calculate with Ohm's law and Kirchhoff's circuit laws.
  • Are diodes and resistors the same?

    A diode allows current to only flow in one direction. A resistor is essentially a load where higher resistance reduces current flowing in a circuit based on ohms law which says Voltage = Current x Resistance. So if Voltage remains constant and resistance increases, Current decreases.
  • Are led AC or DC?

    Depends on the led light bulb. Some are direct dc bulbs. These are meant for 12v lamps, not AC lamps. These are smaller, lack the ac rectifying parts, normally set up with 3 leds + resistor or small current controlled ic, in a ceramic casing.
  • Can you replace halogen bulbs with LED?

    ANSWER: Yes, there are LED bulbs you can use in your fixtures. The LED equivalent to the 50-watt halogen bulb will probably burn only about six or seven watts. These LED bulbs cost more, $30 to $35, but because they will last for years, they are worth the price. And LEDs give off relatively no heat at all.

Updated: 29th September 2018

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