Is it a guessing game when trying to match resistors to LED's

[scubaspookf]

Is it a guessing game when trying to match resistors to LED's. So far I have gone to about 7 websites that have resistor calculators as I am trying to see what I need to drive a LED and I am coming up with 7 different answers.

[David_AVD]

No guessing. It's a simple maths equation. Maybe post what you are trying to do and we'll go from there.

[budude]

They should all be referring to the same law - - Ohm's Law!

R = V / I (it's really "E" for "V" but that's just confusing...). You have to know what voltage drop you will have left on your resistors after allowing for the full forward voltage (Vf) of the LEDs. Say Vf is 3.1v as an example and you're using a 12v supply. To get the voltage drop it's 12v - 3.1v or 8.9v. You then need to determine what current you want to run through the LEDs. Let's say it's 19mA. The resistor value required is R = V / I or R = 8.9V / .019A which would be 468 ohms. You will want to pick a standard value and go UP in value in most cases so a 470 ohm resistor would work. You then recalculate the current based on the standard value so I = V / R or 8.9v / 470 ohm = 18.9mA. Note that unless you are using precision resistors that "standard" value can be a +/- % (typically 5 or 10% for cheaper resistors) so you should actually measure if you need the exact value. For planning purposes you can use this standard value however.

The next step is to determine how much power the resistor will need to dissipate. Again Ohm's law - P = V * I so 8.9V x 18.9mA = .169W. Usually you at least double that value to determine what size to get - in this case a 1/4W resistor (.25W) would work but probably run fairly warm while a 1/2W would probably be better.

Using these formulas allows for any mix of LED voltages and currents - you just need to do the math in the same way.

[David_AVD]

Here's a wiki page I wrote a while ago on another forum. Hope that helps.

[scubaspookf]

Does it matter which side the resistor is connected to on the LED?
So I have 12v supply, a led at 3v, so R=9v/20ma=450ohm resistor correct?
A 1/4 watt resistor would be okay but a 1/2 would be better?

[David_AVD]

Yes, you have it correct on all counts.

By the way, 450 Ohms is not a standard value so you'd go to 470 Ohms.

[Andy.wpg]

Does it matter which side the resistor is connected to on the LED?

Not at all.

[mrpackethead]

@scubapoofk, no need to run away to another forum. All your answers can be found here, with this *Friendly* bunch.

No, it does not matter which side it is. Its just there to limit the current, and it can be done on either the anode or cathode side.

You can calculate the power that will be dropped across the resistor, ( Power = Voltage * Current ) in your case 9v * 20mA = 0.18W Your at about 75% of a 1/4W resistor so your probably ok with it.

Just remember that the forward voltage of the LED is nominal. If its stated as 3.0V, you'll find that it could be as low as 2.7V and as high as 3.3V.. ( have a look at the data sheets, they will also show that the voltage changes with current )..

Using a resistor like this is ok for small leds, where accurate control of the output is not so critical.. When you get higher powered leds, and need more control, then you need to look at using a current source power supply..

[mrpackethead]

Yes, you have it correct on all counts.

By the way, 450 Ohms is not a standard value so you'd go to 470 Ohms.

If you are using 1% resistors then you can get 453 ohms. 1% is pretty common these days..

[David_AVD]

The closest standard E24 value to 450 Ohms is 430 Ohms. E96 range etc are certainly available.

I went for the slightly higher value (safer lower current) side and chose 470 Ohms as it's a standard E12 range value that he can buy anywhere. :-)