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

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?


Patrick Pending

11 years ago

To calculate the series resistor you need to know 3 things, the forward voltage drop of the diode, the forward current of the diode, and the voltage of the supply you are connecting the diode across.

Assuming 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.
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.

rsfazePatrick Pending

Reply 11 years ago

any idea what kind of battery life one should expect if running 4 led with a limiting resistor ?

Patrick Pendingrsfaze

Reply 11 years ago

Well it very much depends on the Ampere-hour rating of the battery. The PP3 rechargeable 9V in front of me has a rating of 120mAh - 8.4V. So dividing this value by the given forward current of 25mA = 4.8 hours*

*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.