Post by David Murray on Feb 9, 2007 12:35:29 GMT -5
Okay.. I decided to start experimenting with LEDs for room lighting (and eventually interior of my shuttlecraft) and I had bought some 10mm super-bright white LEDs a few months ago. They are 20ma and run at typical 3.4V.
I decided to build a strip with 25 of these things in it. I wanted to run them off of regular 120V power. So I wired them in series and ran one of those LED calculator things on the internet and it said I needed 1,800 ohms of resistance, at .8 Watts. So I had some 1 watt resisters that were 1K each, so I put them in series. That should mean I have 2,000 ohms of resistence at 1 watt, right? (I figure I'll get some different ones later to try to get closer to 1,800)
Okay. I wired it into the full-wave rectifier I bought and they lit up. I took my volt meter and placed the probes on either side of an individual LED and found a reading of 3.0 volts. I was expecting more than that, but I knew I had slightly too much resistance. So i recalculated the resistance for 24 LEDS instead. So I took one LED out of the loop and it looks like 2,000 ohms is right where I need. Well, to my surprise the LEDs were not any brighter than before, and the voltage was still reading 3.0V. So I kept removing LEDs from the circuit and watching the voltage. I didn't get to 3.2V until I had only about 15 LEDs in the circuit (still with the 2,000 ohms resistance) But even then I didn't notice much of an increase in individual brightness. And, of course, the LEDs should run at 3.4V anyway.
Now.. just to clear a few things up. the LEDs are quite bright, but I'm not sure if they are supposed to be even brighter. I sort of expected 25 of these things to light a room up really well. But I don't think I'm getting any more light in the room than a single 40-Watt incandescent light bulb.
Also I noticed the resistors get warm. I have never left the unit on more than about 1 minute so I don't know how hot they will get. Should I use several smaller-ohm resistors in series to spread the heat out more?
Another issue I had was diffusing. The light shines like a flashlight and puts a bright strip of light on whatever the strip is pointed at. I wanted more of an ambient light. The LEDs I bought were clear, not diffused. I read one website that suggests sawing the round end off of the LED to diffuse it. I tried that one on of them and it certainly did work. I took off about 25% of the total material of the LED. Question is, does this affect the LED's heat dissapation any? I've only done it to one but before I do it to the other 24, I want to hear the answer to this.
So.. why am I not reading the voltages I think I should be reading? Should there be any significant brightness difference between running an LED at 3.0V and 3.4V?
What resistance should I really be using?
I know there are a lot of questions here.. If anyone can answer, I'd appriciate it!
I decided to build a strip with 25 of these things in it. I wanted to run them off of regular 120V power. So I wired them in series and ran one of those LED calculator things on the internet and it said I needed 1,800 ohms of resistance, at .8 Watts. So I had some 1 watt resisters that were 1K each, so I put them in series. That should mean I have 2,000 ohms of resistence at 1 watt, right? (I figure I'll get some different ones later to try to get closer to 1,800)
Okay. I wired it into the full-wave rectifier I bought and they lit up. I took my volt meter and placed the probes on either side of an individual LED and found a reading of 3.0 volts. I was expecting more than that, but I knew I had slightly too much resistance. So i recalculated the resistance for 24 LEDS instead. So I took one LED out of the loop and it looks like 2,000 ohms is right where I need. Well, to my surprise the LEDs were not any brighter than before, and the voltage was still reading 3.0V. So I kept removing LEDs from the circuit and watching the voltage. I didn't get to 3.2V until I had only about 15 LEDs in the circuit (still with the 2,000 ohms resistance) But even then I didn't notice much of an increase in individual brightness. And, of course, the LEDs should run at 3.4V anyway.
Now.. just to clear a few things up. the LEDs are quite bright, but I'm not sure if they are supposed to be even brighter. I sort of expected 25 of these things to light a room up really well. But I don't think I'm getting any more light in the room than a single 40-Watt incandescent light bulb.
Also I noticed the resistors get warm. I have never left the unit on more than about 1 minute so I don't know how hot they will get. Should I use several smaller-ohm resistors in series to spread the heat out more?
Another issue I had was diffusing. The light shines like a flashlight and puts a bright strip of light on whatever the strip is pointed at. I wanted more of an ambient light. The LEDs I bought were clear, not diffused. I read one website that suggests sawing the round end off of the LED to diffuse it. I tried that one on of them and it certainly did work. I took off about 25% of the total material of the LED. Question is, does this affect the LED's heat dissapation any? I've only done it to one but before I do it to the other 24, I want to hear the answer to this.
So.. why am I not reading the voltages I think I should be reading? Should there be any significant brightness difference between running an LED at 3.0V and 3.4V?
What resistance should I really be using?
I know there are a lot of questions here.. If anyone can answer, I'd appriciate it!