# White LED doesn't light up when a red LED is parallel to it

When I connect a white LED to 5v (Arduino) with a 220ohms resistor, it lights up perfectly fine. But, when I connect a red LED parallel to it, only the red one lights up. Note that when I connect the red light to a yellow or green one, both turn on.

Video and image of the circuit:

http://imgur.com/a/18mNPbq

• Schematic, and hopefully part numbers? Or at least a better description -- when you say you connect a RED led parallel to it, I assume you mean that the diodes themselves are in parallel, not that you've connected another string of a 220-ohm resistor and a red LED. Nov 10 '21 at 22:05
• Sorry, I'm very new to circuits. This LEDs are the ones that the Elegoo Most Complete Arduino Pack brings (sorry if that's not enough info). Also, there is only one 220ohms resistor which is connected to both LEDs. imgur.com/a/gW6JkCT Nov 10 '21 at 22:07
• You haven't actually asked a question, but I'm guessing that the questions that you mean to ask are, "Why isn't the white LED lighting up, and how can I get both LEDs to light up?" Nov 10 '21 at 22:17
• The Elegoo kit should have included some discs; one of these will have a folder containing datasheets for all of the components in the kit (mine did). Nov 27 '21 at 2:56

Welcome to EE.SE.

The red LED has an operating voltage that is considerably lower than the white LED. When you connect the red LED in parallel the voltage across the white LED drops to a value that is too low to allow the white LED to illuminate. (All devices connected in parallel receive the same voltage.) The operating voltages of your orange and green LEDs are only moderately higher than the red one so both can operate at the same time. It's likely though that the red LED will be brighter.

If you have a multimeter you can try measuring the voltage across the LEDs, both individually and in combination. Do your measurements match my explanation?

• Thank you very much for the clear explanation! As you may guess, this is my first time with electricity. I bought an Arduino pack (Elegoo) because I like programming and I thought that I should give real-life programming a go. Unfortunately, I only have that pack, I don't have appropriate measuring tools, but I'll try to learn more from tutorials and guides. Nov 10 '21 at 22:24
• @jgames07 Experiment away. You may come to feel you are working blind without a digital multimeter to make measurements on your Arduino circuits. Product recommendations are off-topic here but you can find beginners guides to buying a multimeter online. My national branch of an online shop has suitable multimeters for under £20 when you feel ready for that. Do just use one for your Arduino circuits and stay away from making measurements on mains circuits until you have considerably more experience. Nov 10 '21 at 23:10
• @jgames07 A digital multimeter will cost you less than a McDonalds meal or a large Starbucks - certainly much less than an Arduino kit. There is no reason not to get one. You don't need all the extras - you just need to be able to measure voltage, resistance, and currents between 1mA and 100mA. Those meters are very cheap. Nov 11 '21 at 7:51
• To make both LEDs light up, you can put a separate resistor in series with each LED, and then put the resistor+LED combinations in parallel. Nov 11 '21 at 10:07
• @ChrisH There are plenty of multimeters that claim to do mains current that cost around $1 USD. I have one though my current multimeter cost me around$5 USD. How cheap do you need multimeters to be? 20 cents? Nov 12 '21 at 7:34

All diodes, LEDs included, have a forward voltage threshold, Vf, where they begin to conduct. Below the LED Vf threshold they won't light up (much, anyway.)

Different LED colors have different Vf forward threshold voltages. In general, the shorter the wavelength, the higher the forward voltage.

Here’s a sampling of some LED colors vs. forward voltages (Vf):

• Infrared: 1.1~1.2V
• Red: 1.8~2.2V
• Yellow: 2.1V
• Green 2.2V (orange or yellow with a filter)
• ‘True’ Green 3.0 ~ 3.2V
• Blue: 3.2V
• White: 3.2V (blue with phosphor)

As you can see, a wide variation of forward voltage. (When in doubt, consult the LED datasheet.)

What happens with LEDs of two different colors are connected in parallel with a shared load resistor? The lower-Vf one conducts and the higher-Vf one doesn’t. This is because the lower-Vf LED limits the voltage to something below the higher-Vf LED threshold.

So a red LED (Vf=1.8V) in parallel with a white one (Vf=3.2V) will see the red one light and white one stay dark. Disconnect the red one and the white one lights up. This is what's happening on your setup.

Try adding a separate resistor for the red LED. Then both will light up.

• There's different ways of achieving a white LED, but the most common method uses a blue LED as the light emitting element, shining through a material that fluoresces yellow in blue light. The combination of blue and yellow light is balanced to look white. Hence, 3.2V for a white LED. Other white LEDs may be build differently. Nov 11 '21 at 0:54
• Re, "Green 2.2V." Nobody would be fooled into calling those "green" if they weren't potted in green epoxy! Nov 11 '21 at 4:42
• @SolomonSlow are they actually orange-with-a-filter? Nov 11 '21 at 18:33
• This is a rabbit-hole (albeit an interesting semiconductor physics and optics one) that I was hoping to avoid, given the nature of OP’s question. Nov 11 '21 at 19:48
• Your Vf description is useful in context but may mislead the OP in the longer term. Vf is the forward voltage at some stated current. Vf will indeed fall little as Iref is reduced substantially, so your comment about not lighting much is correct and useful. A footnote ma be useful. Nov 11 '21 at 23:16

All diodes have an exponential forward conduction current with voltage that differs with chemistry, colour, power and series resistance. This answer and others teach you how to use them easily.

This demonstrates it

But when a greater fixed resistance is added to the low diode resistance which tends to be less than < 20 ohms the diode voltage flattens out to a more constant value which driven with triangle linear ramp.

It is like adding a pillow top to beds of different height. The more powerful the diode, the harder the pillow. The lower power , the softer it is and higher R resistance.