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I'm setting up my led cube and wanted to test with a few extra leds before hooking up my time intensive cube. FYI I am a computer scientist not an electrical engineer. I would like to make sure my numbers look correct.

LED SPECS: Forward Voltage (V) : 3.2 ~ 3.8 Reverse Current (uA) : <=30 Forward Current : 30mA Max Peak Forward Current : 75mA Reverse Voltage : 5~6V

POWER SUPPLY SPECS: 5v 3.8A

I connected one led to the supply with with two 100ohm resisters all in series. When I measured the current I got 9.4mA and for voltage across the resistor I got 3.14v

Do I need less or more resistance or is it ok? The spec sheet says 30mA forward voltage so I would assume I could stick with 100hm per resister.

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In order to determine the resistor value, you subtract the LED forward voltage (Vf), in volts, from the supply voltage, in volts, then divide the difference by the LED forward current (If), in amperes. The quotient will be the desired resistance, in ohms.

In order to make sure you don't overcurent the LED use the minimum value of Vf from the data sheet.

In order to determine the required power rating for the resistor, multiply the difference between the supply voltage and Vf by If. The product will be the power the resistor will dissipate, in watts, and good practice dictates that to minimize the resistor's temperature rise a wattage higher than that be used. I like to go 2:1 unless, for some extraordinary reason, I can't.

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  • \$\begingroup\$ So I would do 5v-3.2v = 1.8v. Then 1.8v/.03A = 60ohm. So do I need a 60 ohm resister? \$\endgroup\$ – Sam Munroe Mar 11 '16 at 3:26
  • \$\begingroup\$ Yes, and it would dissipate 1.8V X 0.03A = 0.054 watt. \$\endgroup\$ – EM Fields Mar 11 '16 at 3:51
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    \$\begingroup\$ You said that 30 mA is the Max Peak forward current - you should operate the LED at a somewhat lower current - perhaps 20 mA - to maximize LED life. A LED datasheet (and most other semiconductor datasheets) will have an "Absolute Maximum Rating" section - you should always operate at somewhat less than shown there. There should be another section showing typical values, which are safe to use. Note that LEDs are not at all fussy about current - lower current will give less light, but the LED will still operate. \$\endgroup\$ – Peter Bennett Mar 11 '16 at 4:04
  • \$\begingroup\$ Thanks @PeterBennett I am going to use a 100 ohm resister just to be safe \$\endgroup\$ – Sam Munroe Mar 11 '16 at 5:17

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