Blue LEDs typically have a higher voltage drop, which means a lower resistor value. If you assume 5 volts applied and 3V across the LED, that leaves 2V across the resistor. If you want 20mA, that's 2V/0.02A = 100 Ohms. Thomas499 January 6, 2015, 5:05pm 3. I bought two starter packs so I have plenty of led's. Using the typical numbers presented in this graph for this LED, you have about 1.9V Vf at 20mA. The series resistor would therefore be (12- (5*1.9))/0.02 ---> 125 Ohms. With the voltage reduced to 9V you could measure the Vf of each LED in the string and the voltage drop across the 125 Ohm resistor. Placing a resistor in parallel is generally a bad idea. It is like making your car go slower by breaking while not releasing the gas/accelerator. To make the LED last long: Use a RED LED as there require the lowest voltage meaning the LED will still light up as the battery depletes. Use a LED with a high efficiency. There are two things you need to consider.. 1. Without a resistor, will it cause the output unnecessary strain and overheat the device. 2. Without a resistor, how much current are you pulling through the LED and how will it effect the life of it. If the answer to both is acceptable, then don't use a resistor. There are, in fact, LEDs that have the resistor for a particular operating voltage already built-in, so you don't need an external one. For instance, here's a spec sheet for LEDS that can be directly attached to a 5V pin. I have a few of these directly connected to my Arduino Nano (5V operating voltage) - no problems so far, and they're on Each of the lines above represent a different voltage bin. To achieve 60 mA for all of the LED bins, different resistor specifications must be used to achieve different forward voltages needed to achieve the same 60 mA. 2) Current limiting resistors protect against voltage increases. We saw above the LEDs have a nonlinear relationship between Most LED's are DC voltage and are in the 2-3 volt range. Which means you will need to reduce the voltage down to meet the LED's requirements. Most people use a resistor. Thats all thats needed. No Ballast no nothing. unless you get into some Hi-perf LEDs then theirs something known as and led driver but thats another story. However, more often than not, a push button will require a resistor. A push button will need a resistor when it is used in conjunction with a microprocessor or microcontroller as an input. The main reason for using it is to ensure a defined state (0 or 1) is present at the micrcontroller’s input, otherwise the input could be left floating Vay Tiền Trả Góp 24 Tháng.

do i need a resistor for led