David said:

Hi, I have 9 leds connected in parallel to one resistor on 12 V in my car,

the resistor is 47 ohms, and the resistor started to get hot and smoke!

I take it that I will need a higher wattage resistor, any suggestions?

The leds are bright blue and they will be running at approx 25ma, and the

total current draw is 9*25 = approx 225ma.

Thankyou!

Scott

Scott:

Ohms Law; Current squared multiplied by the resistance = watts.

(I x I x R = W)

i.e. (0.225 x 0.225) x 47 = 0.051 x 47 = 2.4 watts.

So you should use 'at least' a 3 watt resistor. (And it should

mounted in a manner suitable to its type so that cooling can

occur). Three watts being dissipated by small resistor can get

hot enough to take skin off your fingertips. As you have

discovered!

Another way; suppose that 10.5 volts is being dropped or 'wasted'

across the resistor.

(In order to have, say, 1.5 volts across the LEDs).

Ohms Law again; Voltage squared divided by the resistance =

watts. (V x V)/R = W

i.e. (10.5 x 10.5)/47 = 111/47 = 2.4 watts.

Same result.

There are better ways of 'Dropping the voltage'. However a

current draw, with most of the electricity being wasted as heat,

in this case, of less than one quarter amp, is probably not very

significant in a typical motor vehicle (car).

By the way, if those blue LEDs are outside the vehicle or can be

seen from outside the vehicle, you might want to check what light

displays are legal in your police jurisdictions. Here, blue

lights signify some kind of emergency or road maintenance vehicle

such as snow plough etc.

At one time (back in the 1950s!) there was a custom here, I'm

showing my age by this I know!, to have little blue lamps in the

corners of ones front windshield. They were declared illegal and

then disappeared. But there can be fines for 'illegal' light

displays, especially if they are on while vehicle is driving. The

fewer things you can give the authorities to 'pick on' the

better! have fun.

Couple of thoughts anyway. Good luck.

PS. All the LEDs together are using approximately;

(Using another version of Ohms Law!); Current multiplied by

voltage = Watts (I x V = W)

0.225 x 1.5 = 0.34 watts. However LEDs are much more efficient at

turning electricity into light than lamps.

At the same time 2.4 watts is being dissipated by the resistor!