How to wire LEDs more efficiently?

[TR4G-]

Hey guys,

I've been throwing LEDs in my psp's for a while now, but that was when I was much younger and basically asked people how to do it. Now, I have a massive assorted pack of resistors (500+) and a bunch of LEDs. I have found both a 5v as well as a 3.3v source on my psp. Here's the issue: Each LEDs has a forward voltage of 3.3 volts with 25 mA of current. Using LEDCalc it says that regardless I have to have a resistor on each LED as more than one would be higher than the supply voltage and thus not work. Back in highschool though I used to string together 10 or so LEDs and have only one resistor. How did this work?

Here's a question I have. Is there such a circuit that is NOT parallel OR series? For example: For one 3.3v 25 mA LED from a 5v source, I would use 68ohm resistor. However could I just put that resistor directly after the 5v source and then "branch out" each led from this resistor and still power as many as I would like? Or does that not work. If so, then hell yeah, I'll do that. I just am not too familiar with how exactly that would behave.

[Gizmo613]

It would depend on the source voltage and how many leds you want on it.
oh man, i think i forgot basic formulas on how to calculate this. LOL

Anyway, to answer your question, no. Its either parallel or serial.

[TR4G-]

So people that have a ton of leds in their psps... how are they accomplishing this? Mad resistors?

Example: 5v source. LED is 3.3 forward voltage with 25 mA. Resistor would be 68 ohms. I have calculated this. If I need to connect 10 leds, then I need ten 68 ohm resistors? That seems pretty outrageous...

Heres my question: if you can put as many leds with their own resistors onto this 5v source point, why would the same not apply for jsut he LEDs themselves directly after a resistor?

[Gizmo613]

So people that have a ton of leds in their psps... how are they accomplishing this? Mad resistors?

Example: 5v source. LED is 3.3 forward voltage with 25 mA. Resistor would be 68 ohms. I have calculated this. If I need to connect 10 leds, then I need ten 68 ohm resistors? That seems pretty outrageous...

Heres my question: if you can put as many leds with their own resistors onto this 5v source point, why would the same not apply for jsut he LEDs themselves directly after a resistor?

because resistance is futile, ohm's law.


If wiring in series then you have to add up all the led current and times it by the resistor which then can't exceed the source voltage since it is shared in a series circuit. Example 5V source 25mA led 3.3 vdrop 68 ohm resistor in a series circuit will max out at 2 leds. Why? well add the current up, since its a series circuit, 25ma*2 = 0.05ma then multiply by the resistor, since ohm laws is V=R*I, you get 3.4 volts. Now if you had 3 leds and do the math the voltage comes out to 5.1v, which exceeds the source but not by much. But you will see that the leds will have different brightness level to them cause of the voltage drop.

 
If you connect it in parallel then each led has its own completed circuit.

Dang, i suck at explaining things, my bad on that. And now that i did, yeah i guess you could use 1 resistor and power on as many led as you want, providing you have enough current. The only reason why i wouldn't use one resistor is that if the resistor is blown then all the led goes out as well. Plus using one resistor would require a 1 watt resistor which could be a problem if installing in a tight space(psp?). If each had their own resistor then you would know which is blown and resistor dont cost much anyway. also with individual resistor you can go down to as low as 1/8 watt which is tiny enough to sit across the bottom of the led.

[Modded Matt]

Wire your leds in parallel, tie all the positive leads together and place one resistor between that lead and the power source. Tie all grounds together and terminate at ground. when doing this, you need to separate the different colored circuits. so if you have two colors, you have two circuits each with their own calculated resistor.

This is how most people wire led mods to the psp.

The safer way to do this is to this would place a resistor on each led, because as any difference in voltage will be shared with all the leds in the circuit. This can be important when leds age.

http://www.hebeiltd.com.cn/?p=zz.led.resistor.calculator

[TR4G-]

awesome, exactly the information i was looking for. Much thanks!

"Wire your leds in parallel, tie all the positive leads together and place one resistor between that lead and the power source. Tie all grounds together and terminate at ground."

also, "because resistance is futile, ohm's law." lol I like you

[hyper999]

Make sure you don't exceed the power rating of your resistors if you are going to use more than one LED per resistor

Power rating is given in Watts or MilliWatts

The power dissipated in the resistor can be calculated by Power = Current * Current * Resistance (P=I^2R)

As long as you aren't dissipating more power in the resistor than it is rated for you will be fine

[TR4G-]

thanks for the heads up. Can you give me an example, or make sure im calculating this correctly?

well....whats "power" exactly?

[hyper999]

Power is calculated either as,

Power = Voltage * Current

or,

Power = Current * Current * Resistance.

So say you want to power 2 LEDs at 3 Volts and 25 mA each, from a 5 Volt source. That gives us 50 mA total current or 0.05 Amps.

Method 1
{
     5 Volts - 3 Volts = 2 Volts (Voltage drop across the resistor)
     2 Volts * 0.05 Amps = 0.1 Watts
}

Method 2
{
     We need a 2 Volt drop across the resistor still so using R = V/I we calculate we need a 40 Ohm resistor
     0.05 Amps * 0.05 Amps * 40 Ohms = 0.1 Watts
}

As you can see both methods give the same answer so just use whichever is easier for you. In this example we get an answer of 0.1 Watts this means our resistor will be dissipating 0.1 Watts of power as heat so if we were to wire this circuit up we need to make sure our 40 Ohm resistor has a power rating of at least 0.1 Watts (or 100 milliWatts).

I would imagine your pack of resistors are all likely to be 0.25 Watt resistors, but that's just a guess

[TR4G-]

thanks for the explanation! I actually have assorted .125 (1/8th) watt resistors :\ Guess I'll have to be careful about chaining LEDs up...

[TR4G-]

Ok, so finally getting around to wiring up my leds.


Here's what I have: 6 LEDs with 20mA and 3.2 Forwared voltage running off of a 3.7v source. I would need 25ohm resistor (closest is 27). this gives a total current of 111.1 mA or .1111A


--> 3.7 - 3.2 volts = .5 volts
-->.5v * .1111A = .05555 volts.

Thus I can run all 6 through a 1/8th (.125) watt resistor and have tons of room to spare!