wiring 4 led’s in a computer

Here are the specs for the LED's:

Power Dissipation: 120 mW
Peak Foward Current: 100 mA
Continuous Foward Current: 30 mA
Reverse Voltage: 5 V
Operating Temperature Range: -20°C to 80°C
Storage Temperature Range: -30°C to 100°C
Lead Soldering Temperature: 260°C for 3 sec.
Luminous Intensity: 2500 mcd
Viewing Angle: 15°
Forward Voltage: 3.50 V
Reverse Current: 100 ìA


This is in a computer, so I will have access to both a 12volt source and a 5volt source. All I want to know is:

1. How to wire them? Series or Parallel?
2. Should I use the 12 volt or 5 volt wire?
3. Do I need a resistor(s)?

You can wire them in either series or parallel, and you can also use either 12 or 5 volts.

If you wire the 4 LED's in Parallel:
and use the:
-5 Volt supply, then you should use a 15 ohm resistor (exact value required is 12.5 ohm but 15 is more common) rated at 1/4th watt (exact value needed is 0.18 watts)
-12 Volt supply, use a 75ohm (exact = 70.8333 watts) resistor rated at 2 watts (exact = 1.02 watts)

If you wire the 4 LED's in Series:
and use the:
-5 Volt supply, then you will not need a resistor - in fact, you won't quite have enough juice to fully power all 4 LEDs (they'll just be slightly dimmer than 2500mcd)
-12 Volt supply, then you will not need a resistor. same results as with the 5 volt wire, but brighter.

If you wire them in parallel i would recommend the 5 volt supply, wiring the resistor in series with the group of (paralleled) LEDs. If you wire them in series, i would recommend the 12 volt supply.

But personally i'd say save yourself the hassle/money of finding/buying a resistor and wire the 4 LEDs in series to the 12 Volt supply. That will work out nicely.

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-Santiago-
MECP certified (Bronze)