1 ohm resistor

[seanyiya]

I'm designing a array....
When I used the calculator.... it came out to use 1ohm resistor...
In this case would I really need to use the resistor??

[justDIY]

if the 'left over' voltage is within the 'recommended' range for your leds, then I dont see why you'd need the resistor...

for example, you have 12.8 volts (for some reason) and only 6 * 2v = 12 volts of leds ... so that .8 volts, to a single led would be too much, but across 6 of them, its within the recommend range for a 2v led (usually 1.8 to 2.5v)

i think that makes sense ... the 1ohm resistor wouldn't hurt anything, but if you dont have any, I wouldn't worry about getting some.

[seanyiya]

I'm making array of white flux in 4 series for the car...

LED is 3.5 volt and I'm just assuming 13.6v for the car's power...  with 50mA
with calculator it recommand 3 series with resistor.... 68 ohm
But when I tried 14volt.... it comes out with 4 series with 1 ohm resistor...
Would this be OK without the resistor?? any benefit of using the resisotr.. maybe heat dissapation..??

[justDIY]

at 50 mA, those LEDs will need some sort of thermal management, either a printed circuit board with heavy traces or something else to take away the heat

if you figure

3.5v * 0.05a = 0.175w ... or 175mW each, sealed inside their little plastic shells

you shouldn't have any troubles running four of them without a resistor, in series across an automotive supply.

[Rob]

The resistor is really cheap insurance.

The whole process of finding a current is based on assumptions. One of those is that the numbers you provide in the calculator inputs describe an actual point along the operating curve of the LED, and that is your target LED current.

For example, entering 3.3V and 20 mA implies that you know that the forward voltage is 3.3V at 20 mA drive current. Not just that those are two numbers you found on the spec sheet. (Maybe you already understand this, so sorry for a bit of review if that's the case.)

Now the kicker is that this single point on the operating curve of the LED is only valid at some reference temperature, most often the junction temp at about 25C (room temperature). Your data may or may not include the specifics, but trust me, V vs. I in a diode is quite temperature dependent.

Now what happens when you light the LED up? It dissipates power. How? As heat. Immediately, the junction temperature changes. And what happens next is that the V vs I curve shifts.

Okay, you've stayed with me this far, let's look at what happens next with and without a series resistor (did I mention resistors cost a couple cents?).

Without a resistor, the current in the LED increases as temp increases. In turn, power dissipated increases, warming the LED up even more. This leads to a condition called thermal runaway. Even if your voltage source is precision regulated to 0.01%, it doesn't matter. There's no mechanism to limit the current.

With a resistor (the smart choice), as the current increases, the voltage drop across the resistor increases proportionately (V = I*R, R is the constant of proportionality). This effectively (and cheaply) damps the thermal runaway, as long as that resistor is chosen appropriately.

This leaves us with one more possibility, which is the one of seriously underdriving the LED. The only time I think it's okay to omit the resistor is if your drive current target is far beneath typical, and thermal management is adequate to solve the risk of thermal runaway. In this case, the power dissipated in the LED is conducted away and not causing a threat to the junction. Most people like to drive their LEDs pretty hard though (brighter is better, right?). So just use the resistor. It's two cents well spent.

[cpemma]

tbh, with car electrics I'd run 2 parallel strings each of two whites with their own resistor in series. The wasted power is negligible and the LEDs will still be well-lit at 12V (engine off).

If the 3.5V @ 50mA is correct, then a 150R resistor gives around 47mA at 14V, 33mA at 12V. Any risk of thermal runaway is massively reduced - if the LED forward voltage drops to 3V the current can only rise to 53mA at 14V.

[seanyiya]

Any good and cheap place to order resistor??