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.
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.
- It depends if current is moving from negative to positive or positive to negative. (Sorry, it was another thread) In reality, no, it doesn't matter which side the resistor is on. The two components together make up a Series Circuit, and current is the same at all points, but voltage changes.
- Resistors are blind to the polarity in a circuit. Thus, you don't have to worry about installing them backwards. Current can pass equally through a resistor in either direction. In schematic diagrams, a resistor is represented by a jagged line, like the one shown in the margin.
- The ideal diode has zero resistance for the forward bias polarity, and infinite resistance (conducts zero current) for the reverse voltage polarity; if connected in an alternating current circuit, the semiconductor diode acts as an electrical rectifier. The semiconductor diode is not ideal.
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.
- In a wire, negatively charged electrons move, and positively charged atoms don't. Electrical engineers say that, in an electrical circuit, electricity flows one direction: out of the positive terminal of a battery and back into the negative terminal.
- N-type silicon (red) has extra electrons (black). Battery connected across the p-n junction makes the diode forward biased, pushing electrons from the n-type to the p-type and pushing holes in the opposite direction. Electrons and holes cross the junction and combine.
- 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.
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.
- If I understand your question, then yes - resistors are reversible, in the sense that they can be connected to the circuit in either direction. Resistors are not like diodes or capacitors. They do not have a polarity. The conduct (or resist) current equally in both directions of current flow.
- 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.
- Many LEDs have a rated life of up to 50,000 hours. This is approximately 50 times longer than a typical incandescent, 20-25 times longer than a typical halogen, and 8-10 times longer than a typical CFL. Used 12 hours a day, a 50,000 bulb will last more than 11 years.
Updated: 3rd October 2019