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looking for help on setting up a large number of LED's

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masimunes

New Member
I admit i'm fairly clueless when it comes to basic electronics, i'm trying to learn. I've read about using resistors on the negative side or daisychaining the led's on both it seems that the number of LED's is limited by the amount of dc voltage that you use. I'm looking to use 50+ LED's for an art project i'm working on, i was hoping to use a 12v dc adapter. I'm looking to have all the LED's on at the same time.

Any help that could be offered would be greatly appreciated.
 

ljcox

Well-Known Member
LEDs require around 2 to 3 Volt, depending on the colour.

So with a 12 Volt supply, you could light 4 LEDs if they drop 2 Volt or 3 if 3 Volt. Use a resistor in series to limit the current. The value of it depends on how bright you want the LEDs to glow and the LED limits.

5 to 10 mA is usually sufficient.

So 820 Ohm would give you about 5 mA, and 390 Ohm about 10 mA assuming 2 Volt per LED. 560 Ohm would give you about 5 mA, and 270 Ohm about 10 mA assuming 3 Volt per LED.

To connect extra LEDs, each string will all need there own resistor. Don't be tempted to connect the LEDs in series parallel with only one resistor.

Len
 

masimunes

New Member
the one thing i'm not sure i have right, i'm planning on using 3v leds, so i can only use 4 per 12v power supply? or i need to make sure i have one resistor per 4 leds?
 

ljcox

Well-Known Member
You cannot connect four 3 Volt LEDs across a 12 Volt supply because the current cannot be limited to the correct value.

3 LEDs is the best you can do with 3 Volt LEDs as that leaves 3 Volt across the resistor and thus the current can be predicted with some degree of accuracy.

However, if your 12 Volt supply varies, then the LED brightness will also vary. If it is regulated, then there is no problem.

Len
 

cchalmers

New Member
Can you not put the LEDs in parallel with the supply. That way you can have as many LEDs as you require (eg. 50+) as long as your supply has enough current to drive all of them. each LED will need a protection resistor capable of dropping 12V - LED forward voltage (3.6V for white) = 8.4V. The current is set for each LED by dividing 8.4V/ 5mA = 1.68K. Resistor must be capable of 48mW (P= I2 R). For 50 LEDs you need a power supply that can drive 250mA. Get the idea?

Chris
 

Nigel Goodwin

Super Moderator
Most Helpful Member
cchalmers said:
Can you not put the LEDs in parallel with the supply. That way you can have as many LEDs as you require (eg. 50+) as long as your supply has enough current to drive all of them. each LED will need a protection resistor capable of dropping 12V - LED forward voltage (3.6V for white) = 8.4V. The current is set for each LED by dividing 8.4V/ 5mA = 1.68K. Resistor must be capable of 48mW (P= I2 R). For 50 LEDs you need a power supply that can drive 250mA. Get the idea?

You could, but it've very wasteful on both resistors and power, you are wasting 8.4V in the resistor, and only using 3.6V (from your figures). If you put the LED's in pairs in series, with a single resistor feeding them you are then wasting 4.8V and using 7.2V. Likewise, by using three LED's in series, you are wasting only 1.2V and using 10.8V.

This provides exactly the same brightness as individual resistors, but only uses 1/3rd of the resistors, and only takes 1/3rd of the power to drive them.

Three would appear to be the maximum you could use off 12V (of LED's with 3.6V drop).
 
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