The formula to calculate resistance in a circuit is: R=V/I or, more relevant to what we're doing: (Source Volts - LED Volts) / (Current / 1000) = Resistance*. So if we have a 12v battery powering a 3.5V 25mA LED our formula becomes: (12 - 3.5) / (25 / 1000) = 340ohms. 5. vin17285. • 7 yr. ago. Resistor restrict the flow of electricity. They are really useful when trying to prevent sensitive electronics from breaking. We can use resistors to lower the voltage and also the current if we need to. There's are loads of videos that explain the concept and the math. NeoMarxismIsEvil. Load Resistor Kit for LED Turn Signals - 50 Watts - 6 Ohm - Qty 2. (23 reviews) Code: DI34ZR. Retail: $23.41. Our Price: $8.08. Add to Cart. Accessories and Parts. Trailer Lights. Load-Resistor Kit. Any particular LED will work with a precisely set power supply, but the next LED may or may not work. The resistor is a mechanism for reducing the voltage across the LED as the current increases. If the LED requires more current, then that increased current will lower the voltage across the LED. A further increase of the voltage will then cause a drastic increase in current. This is exactly the point where you need the resistor: The resistor will define the current above the voltage where the LED starts to conduct heavily. You choose a resistor for the difference between supply and "LED voltage". \$\endgroup\$ – Just to add a little background on *why* you need a resistor A LED is a forward biased diode. That means that--electrically--it looks like a short circuit from a fixed voltage drop across the PN junction (around 0.7V, depending on the color of the LED). Sono resistor, lots of current. Blown LED and/or blown power source. rYksC7s. Decide the end you wish to work on - whether you want the light to shine UP or DOWN. Mark out 2 rows of 7 points, 3/8" apart on masking tape. Center it on the end you decided on and start 14 holes with a 1/16" bit. Expand, using a 3/64" bit. Smooth the holes SLIGHTLY - it should hold the LEDs snug. Resistor Voltage = Input Voltage – LED V f. Voltage Across Resistor = 4.5V – 3.4V. So about 1.1V will be dropped across the resistor. Now that we have this, we can use Ohm’s Law to calculate the needed resistance! Resistance = Voltage / Current (in Amps) Resistance = 1.1 / .02 (20mA) Resistance = 55 Ohms. Depending on the LED, the Technically, the LED and resistor(s) don't "use" current, because there is no drop in current (the amount of charge passing through the LED or resistor(s) in a unit of time). This is because of the conservation of charge applied to a series circuit: there is no loss in charge throughout the circuit, hence no drop in current. If you don't mind the motor slowing down under load and don't need full torque then a 100Ω resistor might be OK. Using a 20Ω resistor the motor would get the correct voltage when fully loaded, but higher voltage when the load is reduced. at no load the 20Ω resistor would only drop 20*0.06 = 1.2V, so the motor would get 18-1.2 = 16 Or do I say that the red LED needs less voltage than the green LED to glow as brightly, and the resistor soaks up voltage because of V=IR? You could say, the red led needs less voltage for the same amount of current because its voltage drop is lower than a green led. You now need to subtract the 10.8 volts from the 12 volts. The result, 1.2 volts, is therefore the extra voltage drop required for the LED Christmas light circuit. On the other hand, to calculate the resistor’s wattage, you have to multiply the 1.2 volts by 0.25 amps. Since the answer is going to be 0.03 watts, a ¼ watt resistor would be

do i need a resistor for led