Another LED question

[mdp]

Hey guys...firstly please bare with me for asking this as im sure its been asked a million times here but i log on this site with my phone and i dont have the time or airtime to browse through all the old posts to see if it has been asked...another thing is im a complete noob when it comes to electronics so please bare with me i need all the help i can get...im lookin at puttin a normal bicycle brake light in my rc car. The make is toplight made by magnum.what i want to know is it has 3 Leds already but i would like to add more and it runs off 2 1.5volt AAA batteries.would i need to add more batteries and resistors for more leds.if so how many and what size resistors...is there anythin else i would need to add besides those 2.all help will be gladly appreciated..thanks

[AleG]

Hey guys...firstly please bare with me for asking this as im sure its been asked a million times here but i log on this site with my phone and i dont have the time or airtime to browse through all the old posts to see if it has been asked...another thing is im a complete noob when it comes to electronics so please bare with me i need all the help i can get...im lookin at puttin a normal bicycle brake light in my rc car. The make is toplight made by magnum.what i want to know is it has 3 Leds already but i would like to add more and it runs off 2 1.5volt AAA batteries.would i need to add more batteries and resistors for more leds.if so how many and what size resistors...is there anythin else i would need to add besides those 2.all help will be gladly appreciated..thanks

Just add more leds in parallel (each lead connects directly to the + or - battery ends), no need for resistors at 3V. Of course the more you add the shorter the battery life will be.

[Brandano]

Oh, and some LED's just won't work at all at 3 volts. Several white and blue LED's only work at 4 or more volts.

[AleG]

Oh, and some LED's just won't work at all at 3 volts. Several white and blue LED's only work at 4 or more volts.

Correct, I assumed that they will be red LEDs as used on all the bike brake lights I've seen.

[jeffs555]

Most red led's are 2 volts or less. Running them from a 3V battery with no resistor might draw excessive current and damage them.

[Ron van Sommeren]

Forum, resistance calculator, voltages per colour, array calculator etc.

http://led.linear1.org/
-> LED series resistance calculator

Vriendelijke groeten ;) Ron

[Brandano]

Most red led's are 2 volts or less. Running them from a 3V battery with no resistor might draw excessive current and damage them.
This might be a problem with rechargeable cells, that have a fairly small internal resistance. In practice with the common AAA alcaline batteries the internal resistance is high enough, and you will see LED's often driven without a current limiting resistor.
Incidentally, my approach on choosing a resistor for LED's with "unknown" base voltage is this: Choose a power supply with a sufficiently high voltage (red, yellow and some green LED's will turn on at 2 to 3 volts, white and blue usually need a higher voltage). Find a trimmer that can handle a little wattage, with a value around 270-300 ohm. These are cheap, and we are going to use them for only a small time so it's unlikely the trimmer will be damage and not a big loss if it is. Set the trimmer in line with the LED at the maximum resistance value, and connect an amperometer in line as well. Turn the trimmer until it reads between 20 and 30 milliamps, this should give you a bright enough light for most LED's and still be within a safe range for it. 30 ma is a bit on the upper end of the scale, but I haven't really seen many LED's fail even if a bit overloaded. Disconnect the trimmer leaving it set in the current position, and measure its resistance. Choose a resistor close to the value measured rounded up by excess. It's actually less machinous than it seems, and can be handy if you need to visually match LED's that don't have the same characteristics. This also allows you to choose a current limiting resistor without knowing the internal resistance value of the battery

[mdp]

Thanks everyone i really appreciate all your help