The simplest circuit to power an LED is a voltage source with a resistor and an LED in series. 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.
Determining LED Polarity:LEDs have a positive and negative terminal, also know as the anode and cathode. The cathode should be connected towards the ground or negative side of the driving voltage source, and the anode toward the positive side.
Basics: Picking Resistors for LEDs
| Power Supply Voltage | LED Color | Resistor (calculated) |
|---|
| 4.5 V | Red, Yellow, or Yellow-Green | 36 Ω |
| 4.5 V | Blue, Green, White, or UV | 48 Ω |
| 5 V | Blue, Green, White, or UV | 68 Ω |
| 5 V | Red, Yellow, or Yellow-Green | 128 Ω |
Since this resistor is only being used to limit current through the circuit, it can actually be located on either side of the LED. Placing the resistor on the positive (anode) side of the resistor will have no differing effects from placing the resistor on the negative (cathode) side of the LED.
LED (right) stands for "Light Emitting Diode". A diode is an electronics component that only lets electricity flow in one direction. One of its leads is longer than the other: that's the positive lead, the one that goes to your 5v connection. The shorter wire, the negative lead, goes to ground (Gnd).
LEDs are electrically polarised and will only operate correctly when their positive terminal (also known as the anode) is connected to the supply positive and their negative terminal (also known as the cathode) is connected to the supply negative. LED connection polarity must be strictly observed!
Electrolytic capacitors have a positive and negative side. To tell which side is which, look for a large stripe or a minus sign (or both) on one side of the capacitor. The lead closest to that stripe or minus sign is the negative lead, and the other lead (which is unlabeled) is the positive lead.
The LED symbol is the standard symbol for a diode with the addition of two small arrows denoting emission (of light). Hence the name, light emitting diode (LED). The "A" indicates the anode, or plus (+) connection, and the "C" the cathode, or minus (-) connection.
Resistors are circuit elements that resist the flow of current. Resistors don't care which leg is connected to positive or negative. The + means where the positive or red probe of the volt meter is to be placed in order to get a positive reading. This is called the "positive charge" flow sign convention.
The electricity flows into the "cathode" (negative side) of the diode and then out the "anode" (positive side) toward the protected circuit. Knowledge of electronics standards is a must when installing a diode.
LEDs are “directional” light sources, which means they emit light in a specific direction, unlike incandescent and CFL, which emit light and heat in all directions. That means LEDs are able to use light and energy more efficiently in a multitude of applications.
The positive or anode side of the capacitor is marked with a “+” symbol. Since electrolytic capacitors are polarized, I use a symbol (shown below) on my schematics.
An LED has two terminals, one called the anode and the other the cathode. The longer lead is the anode, but the best way to test them is to carefully look for the flat mark at the bottom of the LED. On some LEDs, it is difficult to see the flat area or it may not be apparent, so you must use all clues.
If the multi-colored wire is black and red, the black wire is the negative wire, while the red one is positive. If both wires are black but one has a white stripe, the striped wire is negative, while the plain black wire is positive. Look in the owner manual to determine which wires are negative in a car.
LEDs, being diodes, will only allow current to flow in one direction. And when there's no current-flow, there's no light. Luckily, this also means that you can't break an LED by plugging it in backwards. A reversed LED can keep an entire circuit from operating properly by blocking current flow.
Car batteries, like regular AA batteries, have positive and negative ends. The positive (+) end is identified with a red plastic cover or connected to red wires, while the negative (-) end will have black markings.
LEDs must always be connected in series with a resistor. LEDs are diodes which means that current can only flow through an LED from the anode to the cathode and not the other way around.
Series components have the same current through them but fluctuating voltages. Generally speaking, most LED lighting uses a series-parallel combination. Ideally, for reliability and lighting consistency, it would be best to have one strip of LEDs all wired in series to a constant current driver.
Polarity is the direction the electrical current runs. The reason this is important in an LED light is because a light emitting diode and any electrical diode is a semiconductor. This means if you have the direction of the current wrong, electricity will not flow through and your LED will not light.
