I am currently working on creating a usb powered led circuit for my desk as back lighting.

here is the rig in theory

usb port supplies power at up to 500 mA 5v
i have two options

1st option is an ac adapter powered usb hub that rates each port at the full 500mA output (obviously it would be software regulated to standard 100mA if powered through the optional usb plug) but it should supply a full 500mA if only powered by ac adapter

2nd Option: i could use my free usb ports on my pc, this is possibly problematic because power output is usually regulated by driver software in each usb peripheral .

i would have several individual strings of 4 or 8 White 5mm flat top leds rated at 20mA a piece. all of the individual strips of 4 or 8 leds would all be connected together with the usb cords positive and negative. they would be spliced when necessary.

all in all a total of 20 leds would be on my first test circuit. 20 leds at 20mA per led for a total need of 400mA

in a normal led circuit with excess voltage/amperage you would need resistors.

my questions are these:
1. do i need resistors or can i regulate the usb power through software or other means (like a volt regulator or other means)
2. if i need resistors what would be appropriate
3. am i missing a more simple option
4. in theory what if i had a full 25 leds on a single circuit that would of course pull the full 500mA would i still need resistors?
5. does anyone know how a usb outputs power if there is no device to ask it what amperage it needs? (example cut off a mouse and plug in the cord) does it output 100mA at default?

Thank you so much, im not new to electronics nor am i experienced i am open to all criticism and helpful ideas.

 

hi dr,
Your White LED's, if they are the standard type will have a forward voltage drop of ~ 3.2V.
This means at 5V, only one with its own series resistor can be connected across +5V.

Look at this d/s for information on USB power and options for converters circuits for USB.
E.

 

Or 2 LEDs in paralleled then a single resistor in series,
or 3 LEDs in paralleled then a smaller single resistor in series,
or ...

Fewer parts if you put the LEDs in paralleled then use a single limiting series resistor. But, in parallel, if one LED fails the others in the gang will tend to fail as well.

 

Fewer parts if you put the LEDs in paralleled then use a single limiting series resistor.

Unless the LEDs are closely matched for Vf that will result in very uneven lighting; or perhaps not all the LEDs would light.

 

Unless the LEDs are closely matched for Vf that will result in very uneven lighting; or perhaps not all the LEDs would light.

so in theory you could match the output voltage and exactly match the amount of leds in a series to "take" all the voltage but they would likely be underlit?

 

thank you guys so much for your help. that pdf was very useful. last question would be: i have looked at resistor to led calculators but would like your oppinions as to what would be the best resistors in a straight series with what i described.


from what im seeing a series of 20 standard white led on assuming voltage drop of 3.2 at 20mA would mean i could just use a 100ohm 1/4watt resistor on each in the series.

would the fact that the series would have breaks and reconnects affect this? total length of connecting power wire would be around 6 feet with about 10 break/splices

basically im making 6 or 7 led strips and soldering the usb wire in at leads and out the other end to connect 6-8 of them.

 

so in theory you could match the output voltage and exactly match the amount of leds in a series to "take" all the voltage but they would likely be underlit?

No. You would fry at least one of the LEDs. You MUST control the current through LEDs, e.g. by using a suitable series resistor. The manufacturing tolerances of LEDs are considerable, so the actual voltage drop (not the quoted 'typical' value) will vary from one LED to another, even if they are all of the same type and from the same supplier.

 

would the fact that the series would have breaks and reconnects affect this? total length of connecting power wire would be around 6 feet with about 10 break/splices

basically im making 6 or 7 led strips and soldering the usb wire in at leads and out the other end to connect 6-8 of them.

The break and splices shouldn't affect it too much but its always a possibility. You are adding resistance with soldering and with length of wire but it should be negligible.

 

And stop using the word series. You cannot put even two of them in series as was already pointed out. They must be in parallel.

Bob

 

thanks everybody i think its going to work. its going to make a really cool desk back lighting system. if it runs smooth with no hiccups ill make an instructable and do it all over with green and blue leds as well. going to use a usb hub rated for 500 mA per port that has lit switches so each color would be on a different switch. you could even go so far as to have different areas of your desk attached to different switches. then mount the usb hub under your keyboard platform so you could reach under and "flip on" the different backlight effects like a bond villian. all for less than 20 bucks.

you guys have been great

oh and bob i said in my first post that im not familiar with this form of electronics, and that i wasn't an expert. so if you want to correct me kindly provide an explanation with your correction so that i can learn and you don't sound like your being a jerk.

 

on another note, my dad taught me all about soldering and various electronic repair when i was a kid. he's not around anymore so getting some instruction from you veterans is very nostalgic for me i really enjoy this site. thank you all so much for your help and patience with me. its been a great experience and has conviced me to take up more of these kinds of projects in the future.