# Using LED's

Discussion in 'The Projects Forum' started by LesSymonds, Sep 28, 2008.

1. ### LesSymonds Thread Starter New Member

Sep 28, 2008
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Hi...I'm a complete newcomer to this, so treat me as an idiot and I might just get the hang of it! Over the next year I'll be building a 10meter long display panel for a waterways festival and I want to include a load of LED's to help 'jazz-up' the display. I understand that I need to add resistors to the circuit, but do I need one for each LED? This would seem to be a lot of work, especially as I could well have as many as 20 to 30 LED's in each panel, all needing to be lit at the same time.

2. ### hgmjr Moderator

Jan 28, 2005
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What DC power supply voltage do you plan to use in your powering the LEDs?

hgmjr

Apr 5, 2008
15,806
2,390
Hello,

The number of resistors depends on the powersupply.
If you use a 5 Volt powersupply you will be able to light one led per resistor.
I assume the led get 20 mA.
If the led takes 3.5 volt to burn (white led), then the resistor will be (5 - 3.5) / 20 mA.
The resistor = 1.5 / 0.02 = 75 Ohm, the closest E12 value = 82 Ohm.
If you use a 12 Volt powersupply you will be able to light 3 leds per resistor.
The resistor will be (12 - ( 3 X 3.5 ) ) / 0.02 = 1.5 / 0.02 = 75 Ohms => 82 Ohms.

http://ledcalc.com/

Greetings,
Bertus

4. ### Wendy Moderator

Mar 24, 2008
20,772
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You can put the LEDs in series, but there still needs to be one resistor or current regulator chip per chain. Each LED drops a set voltage, but it is somewhat variable. LEDs are basically current operated devices, so it is the current, not the voltage they drop, that is important.

It has been asked, but the power supply you will use is important, and you need to decide on it now. Given you are going to have a LOT of LEDs, you probably need something on the high side (but safe), say about 24VDC or so.

May 28, 2008
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6. ### LesSymonds Thread Starter New Member

Sep 28, 2008
2
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...the power supply is a bank of 12v Dc batteries, holding approx 330ah. I also have a 240v supply, fed from the batteries through a 1500 watt capacity inverer. My guess is that directly through the 12v Dc system will be best.
Anyway, thanks for all the replies.I'll work my way through them and try to undertsand what you've all written.