I'm building an array of LEDs on parallel circuits.

I'm new to electronics and am trying to figure out how many 100mA LEDs I can place in each row, and perhaps what kind of resistor I might need at the beginning of each row. You see one row in the diagram. Several rows will be connected together (above and below the diagram, from the same power source).

Here's a diagram, with the |+| representing a positive wire connection to an LED and a |-| representing a negative wire connected to the LED. The 4.0V power source is on the far left. The // marks where I'd put a resistor if I need it.

LED characteristics:
* 3.4V forward voltage
* 100mA

+//++++++++++++++++++++++++++++++++ cathode
+ . . . . |+| . . . . . . . |+| . . . . . |+|. . . . . . .|+|. . . . . . .
[4.0V] (LED 100mA)(LED 100mA)(LED 100mA)(LED 100mA) etc...
- . . . . |-| . . . . . . . |-| . . . . . .|-| . . . . . . .|-|. . . . . . .
- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - anode

 

First, the online LED wizard... http://led.linear1.org/led.wiz This is one of my favorites, makes arrays easy.

Second, yes a resistor is always a good idea. LEDs live and die on current, the resistor helps protect them from getting too much. Dark LEDs aren't very interesting. You'll need a resistor for each LED in parallel, and each will want 100 mA for itself. 10 LEDs will need at least 1 amp availiable.

Now, if you had a higher voltage supply, you can wire several LEDs in series. for a 12 volt/ 1 amp supply, you could run 30 LEDs, only 10 resistors (3 LEDs in series + one resistor).

Play with the wizard, you get the idea...

 

or, you can just do a simple calculation.. (Vsupply-Vled)/(Iled) = Rlim

In your case, 4V-3.4V/0.1A=6 ohms

Using I^2*R to get power dissipation, you're looking at 0.1*0.1*6= 60mW, so a 1/4 resistor should do just fine, or 1/8W if you want to push it a bit.

Each branch will consume 4V@100mA, so, your total supply current divided by 100mA rounded down to the next integer gives you the number of LEDs you can power.

Steve

 

Each branch will consume 4V@100mA, so, your total supply current divided by 100mA rounded down to the next integer gives you the number of LEDs you can power.

Steve

Thanks, that's very helpful.

So if I'm running the parallel circuit I described, each LED will get exactly 3.4V and the only thing that reduces down the row is the current, right?

I suppose for maximum efficiency I'd want a power supply that dishes out as close to 3.4V as possible (if =3.4V, no resistor), and a relatively high amperage (say, 1.5A?), so I can have more LEDs. Ideally, a 10x10 configuration would be nice, because I will have 100 LEDs.

 

No.

LED's vary significantly as to the actual voltage they will drop at a given current, even in the same batch.

I recently tested a batch of 80 blue LEDs that were on a tape; it's fairly safe to assume they were from either the same batch, or batches close together. I saw up to 12% variation in the Vf.

Ideally, you should measure each LED, and match them up as best as you can. A spreadsheet will help this process a great deal.

You can't simply give a LED rated for 3.4v a supply of 3.4v, as the current through the LED will vary wildly. You have to limit the current either via a precision current limiter circuit, or a resistor.

There is another thread on here where I described an LED matching procedure, which should be helpful to you. That thread is here:
http://forum.allaboutcircuits.com/showthread.php?t=9819
Vf matching procedure starts at the 3rd post, but read through the thread so you can see what was being discussed.

 

LED's vary significantly as to the actual voltage they will drop at a given current, even in the same batch.

Is that why my blue LEDs say:

DC forward voltage
Typical: 3.4V
Max: 3.8V

?

 

Is that why my blue LEDs say:

DC forward voltage
Typical: 3.4V
Max: 3.8V?

Precisely!

While the median value will tend to be around 3.4v, you will find some of them to be as high as 3.8V, and some to be perhaps as low as 3V.

If you follow my matching procedure in the other thread, you will have a much more balanced current flow, longer life and more even brightness level from your LEDs. It will take some testing, but that's what's needed.

 

I am shocked that these may vary down to 3V from a 3.4V nominal. It seems that they can accurately set a minimum of 3.4V and a maximum of 3.8V, I think this would have a lot to do with knowing the energy bandgap of the device accurately. They can dope with pretty good precision(relative anyways), I would be surprised if you found a lower than 3.3V forward voltage.

As Wookie said, you definitely need to have some sort of resistor limiting current.

Steve