LED ring on USB 5V

[edy]

Hey Folks,

I just want to confirm something. I bought an LED ring lamp that hooks up to a laptop USB, took it apart and here's a photo of the switch and part of the ring. The LED's are all hooked up in parallel obviously.

So I see they used 3 SMD resistors all labeled "330" which is 33 ohm, also placed in parallel. Total resistance of this combination is: 1/33 + 1/33 + 1/33 = 3/33 = 1/11. .....   11 ohm.

Some dumb questions:

1. Why use 3 resistors instead of 1?
   - does it improve "accuracy" or tolerance, or worsen it?
   - is this a safety issue?

2. What is the characteristic voltage-amp curve on these unknown white LEDs?
   - If I want to drive them using 3 AAA batteries (4.5 V?)
   - If I want to drive them using 2 AAA batteries (3.0 V?) - if even possible
   - What resistor would you recommend, or do I even have to change anything?

On another note, I have a whole bunch of LED's bought in China by a friend who brought them back. No spec sheets or anything. How do I go about figuring out the right resistors based on various input voltages (battery multiples like 3, 4.5, 6V) and also 3.3V and 5V (for use with a micro-controller).

How would you recommend I start testing them so I don't burn them out?

For example, if I hook up an amp-meter or volt-meter, what do I look for?

Thanks!

[SeanB]

Being 5mm units i would not drive them at over 20mA, and even then they might fail due to poor manufacture.

As to the LED ring, the 3 resistors are used for power dissipation, it will work fine as is off 3 AAA cells, but may only run a short time on 2 or may only light dinly.