## im new. questions about led and a 9v.

circuit boardim trying to create a circuit board with leds on it that spells out my handle. i was thinking of using it as a throwie or like a calling card. im totally new to this so i didnt want to try and use a printed circuit board since i have never done anything like this before. i used a circuit board i bought at radio shack. im building it with standard super bright 5mm leds that are about 2.1 v and im not sure how many Ma. i assume 25 Ma. no matter how i plug these numbers into my formula (source volts - led volts / Ma) the leds always seem to run hot. i cant figure out what resistor to use. the power supply is a 9v battery. i also would like to know how to string about 30 of them together off one 9v. i know its alot but can anyone give me some advice or help me out?

active| newest | oldestAssuming the forward current is 25ma (0.025Amps) then here is the calculation for a single LED:

voltage across the resistor is 9v -2.1v (lets call it 7volt to keep it simple)

then the resistance from Ohm's law: Volts divided by current = resistance

7divided by 0.025 = 280 ohms (you need a resistor equal to or greater than this value)

The nearest resistor size >= 280 in the E12 series is 330 ohms.

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Alternatively, you could wire four LEDs in series giving you a total voltage drop of 8.4 volts (2.1 x 4) then calculate a series resistor to drop the remaining 0.6 volts.

or if you like to live dangerously just connect 4 in series directly across the 9 volts without any series limiting resistor this will give 9 volt divide by 4 = 2.25 volts across each diode. The maximum voltage(Vmax) for a typical diode would be about 2.5 volt so you should be OK with 2.25 volt.

*n.b. This isn't strictly correct as the Ah rating is based on a 20 hour discharge at 5% discharge rate (which is more efficient). However, it gives you a rough idea on the expected life.

http://metku.net/index.html?sect=view&n=1&path=mods/ledcalc/index_eng