Any electricians?

[sucram]

Upon seeing so many posts about making your own moonlights, I made some myself. I'm using a 4.5V, 400mA power source and have 4 blue LED's rated at 3.7-4.5V, 20-30mA in a parallel circuit. Question is: do I need to use resistors? I've tried it out and it works great, I'm just worried about running it for any long period of time.

 

[sucram]

Ya, I read those, that's where I got the instructions. I just want to make sure. Hopefully gator will see this and let me know.
Thanks.

 

[sucram]

I still need a conformation that my moon lighting is wired correctly before I have it run for an extendend period of time. Here's a picture of it in effect though. Someone please let me know if the wiring is correct.
Thanks.

 

[lonestar]

It seems to me that you have a parallel circuit with 4.5 volts as the supply with a maximum current draw of 400ma. In a parallel circuit, the voltage remains constant across all resistors (in this case, the LEDs). You say your LEDs are 4.5v 30 ma. Resistance = voltage/amperage. Resistance = 4.5v/.03amps. Resistance = 150 ohms. You have 4 LEDs at 150 ohms. The formula for finding total resistance is 1/ (1/r1+1/r2+1/r3+1/r4) or if all all resistors are the same, you can simply take the resistance total and devide by resistors. 150 ohms / 4 = 37.5. Your resistance total = 37.5. If your voltage total = 4.5 volts and your resistance total = 37.5 ohms then your amperage total must be 120 ma. Your power supply is rated for 400 ma so you should be ok.
It has been a long time since I did this so I may be wrong, but id say you are safe.
Jeremy

 

[dustball]

If you do the math, theoretically you will have 100 mA going to each of the led's. This is not good. I'm surprised they have not burned up yet. You will have to put a current divider into the circuit. That is, a parallel resistor combination to achieve the 20-30 mA. You just have to find the right values of resistors to use to get the right current. I hope you have some test equipment.

 

[lonestar]

Just because a power supply is rated at 400 ma doesnt mean thats what it is putting out. It means that the maximum load will be 400 ma. The lower the load, the lower the amperage from the power supply. The use of a resistor in series before the paralleled LEDs would be to reduce voltage, not amperage.
Jeremy

 

[sucram]

Thank you all. I will take LoneStar's advice at heart. I thought that resistors only reduced voltage, and that the Amps were more a measure of how many resistors (or LED's) could be used in the circuit. The way I figure it, all the LED's are getting 4.5 volts, and I don't need a resistor. If anyone can argue with this, I would like to hear it. So far, it's working fine and I don't think 4.5 volts can cause a fire or anything anyway. Thanks for your help and I would appreciate any additional input.
Thanks.