If the source voltage is the same as LED voltage, is a resistor necessary?

[Harald Kapp]

Yes it is - unless you need to control the current to the LED for variable brigthness. In this case you'd use either a controllable constant current source or PWM (pulse width modulation). But that's another story.

If you need to light up more than one LED at the same current from a voltage source that is high enough to deliver the required voltage for more than one LED, you can also use a serirs connection of LEDs, also with a current limiting resistor as shown here for 2*3 diodes (as an example):

The voltage of the voltage source needs to be at least 3*Vdiode+Idiode*R or you calculate R respectively via R=(V-n*Vdiode)/Idiode where n=number of LEDs=3 in this case.

This setup reduces power loss in R in the case where V >> Vdiode.

 

[Gryd3]

I read the entire tutorial about LEDs that was linked in this thread.

Ok, so the Red LED has a lower forward voltage and thus draws all the current and that's why the Blue LED wasn't lit? Correct?

Additionally, I think I read one of the best ways to set it up is the first way I mentioned in the previous post. Using a resistor with every LED in parallel (as shown in the first "schematic" in the previous post) to achieve the appropriate current for each LED. Is this the preferred and reliable way to power multiple LEDs using the same power source?

That depends on your application.
putting a resistor in series with each led, then connecting the result in parallel will consume more total current than if you connect 2 leds and a resistor all in series. Everything in series will have the same current. The catch is that both LED's must use the same current, and the power supply must be of a slightly higher voltage if the additional LED in series requires it.

 

[geratheg]

That depends on your application.
putting a resistor in series with each led, then connecting the result in parallel will consume more total current than if you connect 2 leds and a resistor all in series. Everything in series will have the same current. The catch is that both LED's must use the same current, and the power supply must be of a slightly higher voltage if the additional LED in series requires it.

I think I asked this question before, but what if one of the LEDs blows while in series with say two other LEDs like so:

+ resistor - LED1 - LED2 - LED3 --
and if LED1 blew, would that most likely cause it to close and eventually cause LED2 and LED3 to also blow? Just curious though may not likely happen.

 

[Harald Kapp]

If an LED "blows", it will most likely open, not short-circuit. therefore no harm will be done to the other LEDs in the string. The whole string will turn off.

Using LEDs in series like this is common usage.

 

[geratheg]

Okay, thank you for all the help.

 

[(*steve*)]

If you can't put the LEDs in series, then the best alternative is to place them in parallel, each with their own resistor.

 

[kpatz]

Using a resistor with every LED in parallel (as shown in the first "schematic" in the previous post) to achieve the appropriate current for each LED. Is this the preferred and reliable way to power multiple LEDs using the same power source?

Correct. In parallel giving each LED its own resistor, or in series with a single resistor. Series wiring only works if the supply voltage is greater than the sum of the forward voltage drops of the LEDs. So a red and blue in series will likely need at least a 6 volt supply to drive them. Two blues in series will need closer to 7-8 volts.

When wiring blue and red LEDs in parallel, the red ones will need a higher value resistor than the blue ones, because the red one drops less voltage, meaning the voltage across the resistor will be higher, and per Ohm's Law the currrent will be higher as well. So you'll need to calculate the resistor values separately for the red and the blue LEDs.

 

[BobK]

Depends on the voltage source. If it is greater the twice the forward voltage + a couple of volts, you should have two LEDS in series with a single resistor. Higher still and you can put 3 or more LEDs in series.

For example, you have a 12V supply and red LEDs with 2.0V forward voltage. You could put up to 5 of them in series with a single resistor.

By why? Well, with 5 in series, they would draw only 20mA. With 5 in parallel they would draw 100mA, 5 times the power with no additional light output.

Bob