(Whenever using a resistor on an LED it should get placed before the LED on the positive electrode). Low and behold, the LED lit up once again. The 150 ohm resistor stopped enough of the 4.5V power supply from reaching the 1.7V LED that it lit up safely and kept it from burning out. You need to know the maximum current that the LEDs are intended to run at. Call this Imax amps. Let the battery voltage be Vbat, and the combined voltage of all the LEDs be Vled. The resistor you need is given by. R = (Vbat - Vled) / Imax For a safety margin, round the resistor up to the next commonly available value. If fitted to the Chassis, a bad earth connection is possible and can result in problems with the lighting system of the vehicle. Further, LED Load resistors must be mounted on a metal surface, the resistor can reach temperatures of up to 170 Celsius. The most common functions requiring Load Resistors are generally the indicator and brake circuits. 168. The Brain said: You can directly hook an LED upto 110VAC, provided you use the proper resistor to limit current. However, as you have identified this would be a little more dangerous then some other solutions. The snag with the resistor method is, you're wasting over 100V at say 15mA average (50% duty), over 1.5W. 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 bulb you posted is designed for 12V, so you don't need a resistor, they've already taken care of that for you. You have to add a resistor when you're using a bare LED that has leads and needs to be soldered in, not for a plug-in replacement like what you're looking at. 1. I understand that when connecting an LED and a resistor in series; the resistor can be placed on any side of the LED as the current will remain same in a series connection. Please correct me if I'm wrong, but I was under the impression that conventionally the electrons flow from positive to negative. So shouldn't the resistor be placed So by putting the 150 ohm resistor in there there will be a maximum current of 20 mA through the LED. Let's also say that we want the LED current to go down to 1 mA. Unless the pot has a super high resistance, it won't go down to 0 mA, and 1 mA seems like a reasonable lower limit. To make that work, our pot needs to be about 2K Ohms. .

do i need a resistor for led