Hello,

I have five 3 v LEDs running off a LI-poly Bat. The battery can run anywhere between 4.2v fully charged to about 3v. Is it necessary to run a resistor on each of the LEDs , if so at what voltage should I spec the resistor for since the voltage drops as the battery is used.

 

Is it necessary to run a resistor on each of the LEDs

Yes.

at what voltage should I spec the resistor for

4.2V (otherwise you risk burning out the LEDs).

 

ok thanks for the help

 

1. Each LED needs current regulation.

2. The current regulator and the LED(s) form a series circuit, the sum of the voltage across the regulator and the voltage across the LED(s) equals the battery voltage. To say this another way, the voltage across the current regulator is the battery voltage minus the voltage across the LED(s).

3. If you have enough headroom in the battery voltage, you can run multiple LEDs in series, and share one current regulator for the series string.

4. The current regulator can be aproximated by a single resistor, in which case it is not a very good current regulator, or the current regulator can be an active electronic circuit.

So now you know the basics, do you have more questions?

 

You'd really be much better off to use a boost circuit to regulate the current. Simply using fixed resistors will give you a widely varying LED currents over the battery charge/discharge cycle. A CMOS 555 timer, a transistor, an inductor, a couple resistors & capacitors can be used to make a fairly simple boost circuit. You can also make a "Joule Thief" type circuit that's quite simple.

 

"I have five 3 v LEDs running off a LI-poly Bat."



Don't you need more voltage than 4.2?

 

"I have five 3 v LEDs running off a LI-poly Bat."

Don't you need more voltage than 4.2?

Not if the LED/resistor strings are in parallel.