I had a 9V battery hooked up to a tiny yellow LED light and it went dead after two days of being on. There was a 250Ohm resister hooked up after the negative end of the battery (as per the instructions of the guy at Radio Shack.)
I really thought that setup would remain on for months on end with no problem. I thought a LED light takes up barely any electricity.
2006-09-15 18:25:18 · 5 answers · asked by rgbsj 2 in Science & Mathematics ➔ Other - Science
You could measure the volotage drop across the resistor. Then, using ohm's law, you could calculate the current being the LED is drawing. Might be significant.
Was it a fresh battery?
2006-09-15 18:30:37 · answer #1 · answered by David S 5 · 1⤊ 0⤋
A typical LED draws around 20 mA. From your data, assuming a 2.5 volt drop across the LED you were using 26 mA - a reasonable current draw. An alkaline 9-volt has about 400 mA-hr available. This means it should go dead in about 16 hours.
There are a couple ways to fix this. First, most of your battery energy went into your resistor not into the LED. If 2.5 volts was across the LED then 6.5 volts (or over 2/3 the total power) went to the resistor. You need to select a battery with a voltage that more closely matches your LED. A couple alkaline AA cells would give 3 volts only requiring a small resistor and around 3000 mA-hr. This gives you about 120 hours (5-days) of life.
A second, and perhaps more important issue is the duty cycle. A flashing LED can go a very long time because even a 5% duty cycle (on 5% off 95% of the time) will give bright flashes. Using an efficient flashing circuit will extend your life by another factor of 20 giving you a 100 day life. Such flashing circuits are common in bicycle taillights and such.
2006-09-15 18:42:29 · answer #2 · answered by Pretzels 5 · 1⤊ 0⤋
Very undesirable theory or layout to skill unmarried LED with 9v cellular, 70% of cellular capacity develop into wasted into ineffective warmth. replace it with 2 AAA cells to make a 3v battery %.. placed one 30 ohm 1/2 watt resistor in sequence with LED shall gentle it up and extremely last 10 circumstances longer than 9V cellular.
2016-11-27 01:46:39 · answer #3 · answered by crooker 4 · 0⤊ 0⤋
Either the 9V battery was old and/or the LED was shorted.
2006-09-15 18:35:56 · answer #4 · answered by Chug-a-Lug 7 · 1⤊ 0⤋
Most LED's need 12vdc to operate. A 9vdc battery doesn't have enough power.
2006-09-15 18:35:10 · answer #5 · answered by David S 3 · 0⤊ 1⤋