I need to make a signboard with hundreds of 3mm led and was wondering if there is any alternate method to wire them up rather than placing 4 in series with a resistor and then paralleling them with other strings. Will wiring them in parallel and running them on a limited voltage like 1.6v or pwm work ? Or anything which makes putting them together easier. Thank you.

 

What power supplies do you have available?
What LED current do you want?
What is the LED forward voltage? (Not many LEDs will give much light with only 1.6V).

 

If you want the LEDs to be controlled individually then wiring them in series won't work. There are controller chips that will do Charlieplexing for you which may save you a bit.

https://en.wikipedia.org/wiki/Charlieplexing

 

Thank you for the replies.
The power supply is a 12v lead acid battery, so a max of ~13.5v. The 1.6v was just an example, just so I can try to convey the question. Basically, I want to light up 100leds or so. Individual control is not required since its a sign board, wiring can be parallel or series as long as they light up with a safe current, but what I want is to simplify the build process, so was wondering if there is any other way to wire them up without having 3 or 4 leds in series with a resistor. Hope I explained my question correctly. Thank you!

 

I'd arrange them in a matrix. 10x10 will give you 100 individually addressable LEDs that will require 20 wires.

 

He has said he doesn't want individual control. All LEDs should light at the same time.

Wiring them in series / parallel with a resistor on each series is the correct way to do it. There is not a better way.

Bob

 

In general you need to decide how low your battery will be drawn so you have a known "min" voltage to work with..
Then you put as many LEDs in series so that the sum of the Vf is not greater than that voltage..

Example.. 12V battery..
If you say I want it to work until the battery falls to 10V and each LED has a Vf (forward voltage) of 2V then 10V/2V = 5 LEDs in series is what you would do.. Each group of 5 will have its own resistor.. Throw as many of those in parallel that you want to achieve your light array..

After the battery voltage falls below 10V there won't be enough voltage to overcome the forward voltage rating of the string and the LEDs will be off...

And you do know that they are going to get dimmer and dimmer as the battery is used up right?

 

Each of your LED strings will require one resistor to limit current. The number of LEDs in a string (N) is related to the forward voltage (Vf) of the LEDs and the supply voltage (Vs). McGyvr's example uses a Vf of 2.0V and a Vs of 12. The resistor is determined by ohms law by using the LED current (If) and the remaining voltage (Vs -(N*Vf))/If. If and Vf can be found on the LED datasheet or other literature from the supplier.

If you use a boost converter like this one, you can increase the number of LEDs per string. This will also give you constant illumination as the battery voltage falls during discharge. If you set the booster to 60V, and use Mcgyvr's example 2 Volts as the Vf, you can put 29 LEDs on a string and use a resistor with 2/If ohms. With an If of 20 mA, you would need a 100 ohm (2/.02) resistor per string. Note, I don't endorse that supplier or specific boost converter. I'm just using it as an example. By the way, white LEDs often have a much higher Vf - 3 to 4 volts.

 

Here is another way

http://www.linear.com/product/LT3956

 

Here is another way

http://www.linear.com/product/LT3956

That's good if the LEDs are higher current devices though I get the sense that the TS doesn't want to build a bunch of circuitry. You can buy fairly inexpensive modules based on that or similar ICs.

I have been assuming the LEDs are relatively low current, 20-30, mA types.

 

That's good if the LEDs are higher current devices though I get the sense that the TS doesn't want to build a bunch of circuitry. You can buy fairly inexpensive modules based on that or similar ICs.

I have been assuming the LEDs are relatively low current, 20-30, mA types.

You can program the current by changing the sense resistor?

 

Yeah. looks like 12.5 Ohms for a sense resistor for 20mA LEDs though that was with a quick read of the DS. I'm not sure they've actually tested that configuration as all their examples are for much higher current. I like the LT stuff but that's a pricey chip.

 

My ICON (Neds place) is a LED sign I created. It has a number of LED's in series, each series group has a resistor and the whole thing works on 12VDC and a a control to PWM the intensity of the LED's I did another one that is an open sign. The documents are posted. The PCB layout shows the position of the LED's. The schematic also shows the PWM dimmer and a SCR crowbar circuit to blow a fuse if the AC supply happens to exceed about 15VDC.

 

Here is a picture of my latest LED project. It has about 360 LED's all in strings and each string has a fixed resistor.

 

He has said he doesn't want individual control. All LEDs should light at the same time.

You can multiplex LEDs and have them appear to be on at the same time.

Here's a scrolling display using a 5x7 matrix:

 

You can multiplex LEDs and have them appear to be on at the same time.

You can also use a $1000 100W high end audio amp to drive a pair of cheap 2 inch speakers.

Bob