Wiring LEDS - Tilted Forum Project Discussion Community

happymaan · 09-20-2004, 04:21 PM

I got the following LEDS:

* Emission color : White
* View Angle : ±23°
* Forward Voltage : 3.2V-3.8V
* Luminous Intensity : 16000mcd
* Lifetime : 100000 hours
* Reverse Current : Max 30mA
* Soldering Temperature : 240 C for 5 seconds

I plan on using 15 of them hooked up to a 3V DC transformer I got. I know I would be under powering them a bit but I'm fine with that.

But then I was thinking how cool it would be to make it USB powered, so I read how I can get 5V DC from a USB cable. If I wanted to drop the 5V into the acceptable range can I put a resistor into the USB cable rather then have to hook a resistor separately onto each identical LED?

Thanks....

Happymaan

kazoo · 09-20-2004, 10:48 PM

Years ago I was screwing around with a bunch of neon bulbs and couldn't get them to operate without individual resistors. If you can, borrow a decade box and play with the circuit.

crazycousinjim · 09-21-2004, 04:51 AM

You are not going to want to run you LEDs serially because of your voltage drop over your led's. And, generally you do not want to run multiple LEDs in parallel off one resistor, but in this I don't see too big of a problem, depending on our power requirements. With a 5V power supply you will want a voltage drop of 1.5V (Somewhere safely in the middle), so using the the Equation If = Vd / R, where If is you forward current, Vd is our voltage drop (1.5V) and R is our Resistor value. The only thing we are missing is the If, your forward current. We need that range so that we can complete the equation so if you can find that or just the part number we should be able to get you fixed up real quick. Now for the bad news. Your USB host cannot give you unlimited current. In fact, without using a USB controller chip it will only source 100mA. That is no that much when you consider that a white LED usually wants about 20mA. So you will have to use strings of 5 LEDs. However you If might be a bit lower and we might get lucky. So find that If for you LEDs and we can figure out where to go from there.

09-20-2004, 04:21 PM	#1 (permalink)
happymaan Insane Location: los angeles	Wiring LEDS I got the following LEDS: * Emission color : White * View Angle : ±23° * Forward Voltage : 3.2V-3.8V * Luminous Intensity : 16000mcd * Lifetime : 100000 hours * Reverse Current : Max 30mA * Soldering Temperature : 240 C for 5 seconds I plan on using 15 of them hooked up to a 3V DC transformer I got. I know I would be under powering them a bit but I'm fine with that. But then I was thinking how cool it would be to make it USB powered, so I read how I can get 5V DC from a USB cable. If I wanted to drop the 5V into the acceptable range can I put a resistor into the USB cable rather then have to hook a resistor separately onto each identical LED? Thanks.... Happymaan

09-20-2004, 10:48 PM	#2 (permalink)
kazoo Psycho Location: Where the night things are	Years ago I was screwing around with a bunch of neon bulbs and couldn't get them to operate without individual resistors. If you can, borrow a decade box and play with the circuit. __________________ There ain't nothin' more powerful than the odor of mendacity -Big Daddy

09-21-2004, 04:51 AM	#3 (permalink)
crazycousinjim Crazy Location: Copenhagen, Denmark	You are not going to want to run you LEDs serially because of your voltage drop over your led's. And, generally you do not want to run multiple LEDs in parallel off one resistor, but in this I don't see too big of a problem, depending on our power requirements. With a 5V power supply you will want a voltage drop of 1.5V (Somewhere safely in the middle), so using the the Equation If = Vd / R, where If is you forward current, Vd is our voltage drop (1.5V) and R is our Resistor value. The only thing we are missing is the If, your forward current. We need that range so that we can complete the equation so if you can find that or just the part number we should be able to get you fixed up real quick. Now for the bad news. Your USB host cannot give you unlimited current. In fact, without using a USB controller chip it will only source 100mA. That is no that much when you consider that a white LED usually wants about 20mA. So you will have to use strings of 5 LEDs. However you If might be a bit lower and we might get lucky. So find that If for you LEDs and we can figure out where to go from there.