blue led resistor

why do you need a resistor when you use an led?
also, what resistor should i use for this 3mm blue led?
led specs:
forward voltage(i dont know what this is and an explanation would be appreciated): 3.1-3.6V
forward current(i dont know what this is and an explanation would be appreciated): 5mA-17.5mA

why do you need a resistor when you use an led?

To prevent damage to the LED and or the driver/power supply.

also, what resistor should i use for this 3mm blue led?
led specs:
forward voltage(i dont know what this is and an explanation would be appreciated): 3.1-3.6V
forward current(i dont know what this is and an explanation would be appreciated): 5mA-17.5mA

What supply voltage do you have?
if VCC = 5
(5-3.6)/.0175 = ~80 ohms, use 82 or 100

See:
http://led.linear1.org/1led.wiz
.

I'd pick the worst-case forward voltage, so (5 - 3.1) / 0.0175 = 108 ohm

LarryD:
why do you need a resistor when you use an led?

To prevent damage to the LED and or the driver/power supply.

also, what resistor should i use for this 3mm blue led?
led specs:
forward voltage(i dont know what this is and an explanation would be appreciated): 3.1-3.6V
forward current(i dont know what this is and an explanation would be appreciated): 5mA-17.5mA

What supply voltage do you have?
if VCC = 5
(5-3.6)/.0175 = ~80 ohms, use 82 or 100

See:
http://led.linear1.org/1led.wiz
.

i have an arduino due so i think its 3.3V

will a transformer work?
to change the voltage from 3.3V to 5V?

No, transformers only work on AC.
Also, the Due has lower current I/O, so source 3 to 15mA MAXIMUM, sink 6mA to 9mA MAXIMUM.
You need to check which pin you intend to use.

coolnumber0129:
led specs:
forward voltage(i dont know what this is and an explanation would be appreciated): 3.1-3.6V
forward current(i dont know what this is and an explanation would be appreciated): 5mA-17.5mA

The forward current controls the brightness. Higher current - brighter LED. If you take it above the maximum current it won’t last very long! Below the minimum it will still work but be rather dim.

The forward voltage is just the voltage that appears across the LED when you feed it with a defined current.

Russell.

AWOL:
No, transformers only work on AC.
Also, the Due has lower current I/O, so source 3 to 15mA MAXIMUM, sink 6mA to 9mA MAXIMUM.
You need to check which pin you intend to use.

so i need to lower the current to 9mA on arrival somehow?(sorry for the nooby words)
is that where the resistor comes in?

russellz:
The forward current controls the brightness. Higher current - brighter LED. If you take it above the maximum current it won't last very long! Below the minimum it will still work but be rather dim.

The forward voltage is just the voltage that appears across the LED when you feed it with a defined current.

Russell.

so maximum forward voltage doesnt affect compatibility?
as long as the power supply is in the range of the led?

thanks for the replies

Hi Coolnumber, diodes ("regular" or light emitting) work like this:

When you put voltage across them, current flows as long as the cathode is connected to negative (ground). A bit of that voltage gets used up, which is just the way they work. Different kinds of diodes use up different amounts of voltage - this is the forward voltage.

Any voltage in excess of the forward voltage is passed through the diode. LEDs run on current, so you need to use Ohm's law, as LarryD did in the first reply. Ohm's law sez that voltage(V), resistance(R) and current(R) are related by the formula voltage = current times resistance. Write this algebraicly as V=IR, then 'algebra' it into the form: V/I=R

The LED's datasheet (or an educated guess) tells you what current the LED needs to work. To much & it burns out, too little and it's too dim to see. So calculate the available voltage (power supply voltage - forward voltage). Divide this by the current you want to run the LED at, to get the resistance. They give you a range of current, from dimmest to brightest.

Now you can go back and follow Larry's math to see how a resistor of 80 ohms will give you the maximum brightness from your LED.

ChrisTenone:
Ohm's law sez that voltage(V), resistance(R) and current(R) are related by the formula voltage = current times resistance. Write this algebraicly as V=IR, then 'algebra' it into the form: V/I=R

It should be remembered that Ohm's Law only really applies to linear devices such as resistors and conductors. It doesn't apply to non-linear devices such as semiconductor diodes except for very small changes.

Russell.

russellz:
It should be remembered that Ohm's Law only really applies to linear devices such as resistors and conductors. It doesn't apply to non-linear devices such as semiconductor diodes except for very small changes.

Russell.

This is true. In this case we are applying it to the voltage after the voltage has been dropped by the semiconductor junction, so the resistor is the only thing affecting the current - after the diode. At least I have always done LED calculations using that assumption.

You assumed correct... but to be honest... the difference in the calculation either way with RED led's is so slight that it is almost a non-issue. The higher voltage drop with a blue LED makes it start to matter.

LED efficiency is much better than in the old days. Back then, you'd always calculate for maximum current. But today's LEDS are often so bright that you don't need or even want the full rated current. Often I find that 1 or 2 mA is enough to make them plenty bright. So my seat of the pants choice is usually a 1k resistor, just to see what it looks like.

aarg:
LED efficiency is much better than in the old days. Back then, you'd always calculate for maximum current. But today's LEDS are often so bright that you don't need or even want the full rated current. Often I find that 1 or 2 mA is enough to make them plenty bright. So my seat of the pants choice is usually a 1k resistor, just to see what it looks like.

For indicators I always use a 1K, but when I want to illuminate something with reflected light from a LED, I use the maximum current (often 20ma) with a transistor to alleviate the current draw on the Arduino's output pins. Like this:

Then I need to calculate to get the current right. Different colored LEDs have different forward voltages, so you need to get it right.

I typically do 1k for LEDs I'm powering from 5v (giving me 2-3mA) or 680 or 470 if I'm using 3.3v (usually works out to around 1mA). I do 2.2k if I've got garishly bright LEDs that I just want acting as an indicator.

For equal perceived brightness, you often need a smaller resistor for red than other colors - which is surprising, since they also have a lower forward voltage, so you're putting more current through them in two ways, just to get the same apparent brightness.

so the brightness is controled not only by the PWM of the arduino but by the current as well?
if i were to use the same resistor on all leds will the PWM be reliable on controlling the relative brightness?
also, whats the units on the V=IR?
is it Voltage=AmpOhm?
or Voltage=mA
Ohm?

  1. Yes.
  2. Yes. Assuming you mean the same value resistor on all LEDs, not the same actual resistor.
  3. Amp.
  1. Brightness is (roughly speaking) a function of the average current going through the LED. If you PWM it, you're turning it off for part of the time, thus changing the average current going through the LED.

  2. Each LED needs it's own resistor (if you share resistors between LEDs, when two are on, they'll each be half the brightness). To make two LEDs have different brightness with the same PWM duty cycle, if the LEDs are different colors or from different manufacturers, you may need to use different resistor values on the different types of LEDs. Regardless, PWM will still adjust their brightness.

so the brightness is affected not only by the pwm but by the current as well?
if i use the same resistor on every leds will the relative brightness from the pwm be reliable?
what are the units in the V=IC?
is it voltage=mA*Ohm?
the current doesnt "go anywhere" right? like it doesnt shrink after passing an led?
if i connect 15 leds in parallel to one pin, will the amperage be enough to power them all?

thanks for the awesome comments!

Have some blue LEDs from a bag of 100 that are extremely bright, Vf is about 3.4 volts, right now I'm using one with a 1k resistor and its plenty bright enough for an indicator, current is only 1.6 mA.