Installing LED in car.Using blue w/3.2fV.How do i convert to handle 12V.I have 52 and 39 ohm resistors.?

Question

Installing LED in car.Using blue w/3.2fV.How do i convert to handle 12V.I have 52 and 39 ohm resistors.?

Answer 1

OK, blue LED with 3.2Vfd. You are not telling me how much current it will stand so I will guess 20mA (0.02 amps).

I will also assume that you are only hooking up one LED.

Next, you are putting this in a car. With the engine running, you must assume that you are going to have something close to 14.5 volts that you are starting with.

Subtract LED Vfd from supply voltage;  14.5-3.2=11.3 volts that you need to drop across the resistor at 20mA.

Divide voltage by current to get resistance value; 11.3/0.02=565 ohms. That's what you need, 565 ohms.

You can put resistors in series and the values will add together. if you put 11 of the 52 ohm resistors in series, this will net 572 ohms which is just fine. A real waste to need 11 of them but it will work.

If you just go buy a resistor somewhere (Radio shack is probably out but may you could get one from a TV or radio service shop), a 560 ohm resistor is a standard value.

At 20mA current thru a 560 ohm resistor, it will have to dissipate 0.224 watts, I would suggest a 1/2 watt resistor, expect it to get warm to the touch. Maybe a couple of 270 ohm resistors in series, they wont get as warm since the power being dissipated is divided between them.

Be careful of the polarity of the LED, if you have it backwards it could instantly destroy it. Usually the longer wire (assuming it has wires...) will be the positive connection.

It will not matter if you feed the 12v power to the led, then thru the resistor and then to ground or if it goes thru the resistor and then thru the led to ground. It seems more conventional to feed the +12v thru the resistor first.