If the power source was exactly 3V and the voltage drop across the LED was 3V then you would not need a current limiting resistor. The datasheet for the 0402 SMD LED shows Vf from 2.1V for Red to 3.5V for green. Given the way the question was worded and the fact that the color was not specified, the LED could work. This calculator is based on the Ohms Law Calculator, but takes into consideration the voltage drop from the LED. To use the calculator, enter any three known values and press "Calculate" to solve for the others. Click "Calculate" to update the fields with orange borders. Calculate resistor values for LEDs using this simple calculator.
Equation. R = V s − V led ∗N I led R = V s − V l e d ∗ N I l e d. Where: V s V s = Supply voltage. I led I l e d = LED current. The usual operating range of common 3 mm and 5 mm LEDs is 10-30 milliamps. If access to an LED's datasheet is impossible, 20 mA is a good guess. V led V l e d = LED voltage drop. The voltage drop on a LED
1 Answer. Sorted by: 3. You will definitely need resistors for each LED connected to MCP23017 circuits. This is necessary because without resistor the following is likely to happen: burn each LED that is lit on as it would be fed too much current. burn the MCP23017 because it is rated for 125mA maximum (20mA per output pin)
Step 2: With the Pull-up Resistor. To prevent the unknown state a pull-up resistor will ensure the state on the pin is low. Add a resistor of 4.7k* (check in step 4 the calculation of the resistor) to the circuit, and try the below code. See the led working properly with the two states LOW and HIGH. I don't know if I need to put a resistor in series to protect the LED, as it is needed with 5 mm standard LEDs. Doing some maths, the needed resistor would be calculated using R = (Vs-Vi)/I, being I the desired Intensity current. As the LED is rated at 10 W, its max intensity would be P = V x I; 10 W = 12 V x I; I = 10 W / 12 V = 0.83 A.
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The teensy is a 3.3v controller and the data from it is 3.3v, the LEDs expect 5v data, some LEDs will work with 3.3v data, yours obviously do. When you add a resistor it drops the data voltage, stopping the LEDs from working. The best practice guide that recommends a resistor is for 5v MCUs which have 5v data, you don't need them.
Each LED consists of the ideal LED plus a 0.7 Ohm series resistor so the current demand from the battery is now (3 * 0.325) = 970 mA. If we consider the offset voltage fixed the total series R required is: (3.1 - 2.55) / 0.970 = 0.57 Ohm . Since we have approximately (0.7 / 3) Ohms in the LEDs themselves we need to add an external resistor of: In Example A, we can drive a led (albeit a cheap, low power) by simply attaching a resistor to it and getting a constant current. Moving up, to more powerful LEDs, we drive them via Constant Current using a transistor, same idea, but we use the Transistor to set the current with the resistor R2. Example B is the one that I want to use. 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. The LED load resistor creates a voltage drop so the LED turn signals blink at a proper speed. If the LED load resistor was not installed, the LED turn signal would blink too rapidly, and would eventually be destroyed by the high voltage. Need to upgrade? Check out this post to learn how to make the switch to LEDs. Common Resistor Applications tkgLX.
