Question about LED project

[j611]

I have a 4 volt 4.6 amp hour lead acid battery that I want to power 3 or 4 super bright white Leds. Each of the Leds is listed at 20 ma. The reverse voltage is 5v and the forward voltage is 4 v max. I thought about putting them in series, but need help calculating the current limiting resistors. Thanks for any help.

[sundancer87]

At four volts, you barely have enough voltage to light one white LED, let alone two or three more.
To light more than one LED you are going to need more voltage, then you can work out a formula for your resistance. The smallest resister you have in your tool box will work for one white LED at 4 volts.

[russlk]

You have to go to a 3 cell battery (6 volts), then the current limiting resistor is: R=E/I = (6-4)/.02 = 100 ohms.

[sundancer87]

A formula for your head.
voltage-led fwd voltage x 50 =100 ohms
6volts-4volts=2volts times 50= 100 ohms.
(6-4)*50 =100 ohms
You get the same answer as Russ's formula but you don't have to mess with the decimal point and you can do this one in your head.
You are still going to need more voltage for your series application of LEDs. Remember, for series connection you have to ADD up the fwd voltages of your LEDs. In your case if you wanted 3 LEDs in series, that's 12 volts needed for optimum performance. In this case a 50 ohm resister would suffice for current limiting purposes.
Would you not agree Russ Kincaid? :p

[josmith]

A 4 volt lead acid battery is actually 3.6 volts noninal. You can run the leds directly off of it but you will need a small resistor like around 20 ohms to on each one if you run more than one in parallel.
The main reason for the resistor is that the leds vary in there caracteristics enough so that they will not light equally. Also it gives you insurance against overcurrent when the battery is fully cahrged or if you run the circuit on the charger.

[happy]

There are all kinds of white LED drivers available. The simplest was a BEAM robotics single transistor circuit. If you hook the LED's in series you will have higher efficiency than in parallel. Do a Google search for WHITE LED BATTERY TRANSISTOR. The EDN article is informative or hit "I'm feeling lucky" for the BEAM Joule Thief, single transistor schematic.

[L. Daniel Rosa]

Try a 33 ohm resistor in series with _one_ LED. This should get you in the 15~25 mA range. Increase or decrease as necessary for the desired current. Repeat for as many LEDs you wish to use, and use them in parallel.