I'm very new on this for this has never been taught in my school.

I have ten 5mm blue LEDs. Each led has 3.4V and 30mA max current. The only thing I know is that I have 34V for my LEDs. The power supply I would be using has 14V (a 5V usb cable plus a 9V battery).

So, my question is: how many volts should I really have as a power supply and how many ohms as well as watts should the resistor have? (I currently have 220ohms 1/4 watt for the resistors)

P.S. these will all be attached to a breadboard. This will be made into a game. 10 LEDs 20 switches. It's called the nim number game. so the mechanics of the game is each player can turn on a maximum of 2 LEDs per turn. However, the last player to turn on the remaining LED that's not turned on wins. The project is done, honestly. We've attached the wires, Leds, switches, and ICs.

However, the initial power supply for the project is a 9V battery. But when the 6th LED is turned-on, the light it emits is very low already. Then, emission of light of all 10 Led lights becomes very low if all leds are turned on. So I added a 5V usb cable but I got the feeling that the resistors aren't compatible with the power supply anymore.

Thanks in advance for answering!

  • \$\begingroup\$ Can you make three paralell strings consisting of three series connected LEDs? If you have to use all ten without 3x3+1 or go to 2x5, 9 total would be a neat compromise. You will need three resistors of ~180 ohm for each string. \$\endgroup\$ – winny Nov 30 '16 at 15:31
  • \$\begingroup\$ All LED should be in parallel because we will be making a logic game for our discrete mathematics class, and the game will be a two-player game. That's why all LEDs should be in parallel (that means I have to use 10 resistors, right?) \$\endgroup\$ – Lara P. Nov 30 '16 at 15:35
  • \$\begingroup\$ Correct. Choose ~530 ohm, >0.5W resistors for each. \$\endgroup\$ – winny Nov 30 '16 at 15:45

While it would be possible to attach 9V batteries to a 5V USB bus to get 14V, its not really a viable or healthy solution.

I'd recommend one of the following:

Use the 5V from USB:


simulate this circuit – Schematic created using CircuitLab

Keep in mind the maximum current that your USB bus can supply. Some are 200mA, others 500mA, and USB power supplies can go up to 1A and more.

Use 9V and replace them as they get used up:


simulate this circuit

Typically you want to stay lower than 200~300mA total for a 9V battery, otherwise its lifetime gets really hindered.

The value for the current (in Amps) can be found using the following calculation:

Vsupply - Vdrop = Resistance * Current


9V - 3.4V*2 = 220/2 * Current

Current = 2.2V/110 = 0.020A = 20mA

You can extrapolate this info for other resistor values/combinations.


Your resistor should be placed in series with the LEDs. At 30mA, a 220 ohm resistor will drop 6.6 volts so, the approximate total power supply voltage you will need for ten LEDs is (10x 3.4 + 6.6) volts = 40.6 volts.

Be careful to ensure that the 30 mA figure you gave is not the absolute maximum current but a typical running current for those LEDs.

Fewer LEDs means a lower voltage is required so choose the number of series LEDs (with also the 220 ohm resistor) that can give you a voltage more suited to what you have. You can always parallel strings of series LEDs but don't forget the series resistor for each parallel string.

  • \$\begingroup\$ I can't make the LEDs and resistors in series because we will be making a game that consists of 2 players. \$\endgroup\$ – Lara P. Nov 30 '16 at 15:39
  • 1
    \$\begingroup\$ Resistors need to be in series with LEDs irrespective of what you are designing and, for the life of me, I cannot follow your reasoning that because you are making a game for two players you can't have series LEDs. If you can only have one LED then choose a resistor to drop the right voltage at the right current (10 mA or 30 mA or whatever) to suit the supply you have available. \$\endgroup\$ – Andy aka Nov 30 '16 at 15:43
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    \$\begingroup\$ @LaraP. -- no game and players were mentioned in your question. If that translates into specific needs of your circuit (i.e. all the LEDs not being in series), you should add that to your question. \$\endgroup\$ – Wesley Lee Nov 30 '16 at 15:43

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