Curious about white LED circuit

[amdx]

I bought a few surplus pc boards that contain 29 LEDs and some
resistors.

13" long x 2" wide.

The wiring seems odd, there are two circuits repeated.

The first: two series LEDs in series with a 560 ohm resistor.

The second: one LED in series with a 820 ohm resistor.

These two circuit are in parallel with each other.

This is repeated all across the board for 29 LEDs.

I'm running it on 7 Volts and it's bright enough for my purpose.

Each 560 ohm has 1.4 volts across across it. 2.5 ma

Each 820 ohm resistor has 4.1 volts across it. 5 ma

It only has one power connection.


Any thoughts about why this was made like this?





I have the board mounted under the top of my computer desk. My
keyboard is on a slide out shelf the gets lit by the LED board.
It lights up the keyboard for me in the morning before I've
turned on other lights.

Mikke

 

[Dan Coby]

On 5/15/2012 3:41 PM, amdx wrote:

I bought a few surplus pc boards that contain 29 LEDs and some resistors.

13" long x 2" wide.

The wiring seems odd, there are two circuits repeated.

The first: two series LEDs in series with a 560 ohm resistor.

The second: one LED in series with a 820 ohm resistor.

These two circuit are in parallel with each other.

This is repeated all across the board for 29 LEDs.

I'm running it on 7 Volts and it's bright enough for my purpose.

Each 560 ohm has 1.4 volts across across it. 2.5 ma

Each 820 ohm resistor has 4.1 volts across it. 5 ma

It only has one power connection.


Any thoughts about why this was made like this?

I have no idea about why there are two separate circuits repeated
unless it is related to wanting 29 LEDs and not 30. (I am assuming
from your description that there are ten 2 LED circuits, and nine 1
LED circuits. Which is not the best from an efficiency stand point.)

From the resistor values, I am guessing that the circuit was
designed to operate from 12 volts. With an LED voltage drop of 3 volts,
you would have about 10.8 mA going through the LEDs in both circuits.


Dan

 

[amdx]

On 5/15/2012 6:17 PM, Dan Coby wrote:

On 5/15/2012 3:41 PM, amdx wrote:
I bought a few surplus pc boards that contain 29 LEDs and some resistors.

13" long x 2" wide.

The wiring seems odd, there are two circuits repeated.

The first: two series LEDs in series with a 560 ohm resistor.

The second: one LED in series with a 820 ohm resistor.

These two circuit are in parallel with each other.

This is repeated all across the board for 29 LEDs.

I'm running it on 7 Volts and it's bright enough for my purpose.

Each 560 ohm has 1.4 volts across across it. 2.5 ma

Each 820 ohm resistor has 4.1 volts across it. 5 ma

It only has one power connection.


Any thoughts about why this was made like this?

I have no idea about why there are two separate circuits repeated
unless it is related to wanting 29 LEDs and not 30. (I am assuming
from your description that there are ten 2 LED circuits, and nine 1
LED circuits. Which is not the best from an efficiency stand point.)

From the resistor values, I am guessing that the circuit was
designed to operate from 12 volts. With an LED voltage drop of 3 volts,
you would have about 10.8 mA going through the LEDs in both circuits.


Dan

Ya, it all seems a bit strange, but I paid $1 and it does it's job
fine. When I was testing it, I did run it up to 12 volts and it is very
bright. I found a 6.2V wall wart and used that for power.
Mikek

 

[petrus bitbyter]

"amdx" <amdx@knologynotthis.net> schreef in bericht
news:4e347$4fb2dceb$18ec6dd7$16325@KNOLOGY.NET...

I bought a few surplus pc boards that contain 29 LEDs and some
resistors.

13" long x 2" wide.

The wiring seems odd, there are two circuits repeated.

The first: two series LEDs in series with a 560 ohm resistor.

The second: one LED in series with a 820 ohm resistor.

These two circuit are in parallel with each other.

This is repeated all across the board for 29 LEDs.

I'm running it on 7 Volts and it's bright enough for my purpose.

Each 560 ohm has 1.4 volts across across it. 2.5 ma

Each 820 ohm resistor has 4.1 volts across it. 5 ma

It only has one power connection.


Any thoughts about why this was made like this?





I have the board mounted under the top of my computer desk. My keyboard
is on a slide out shelf the gets lit by the LED board.
It lights up the keyboard for me in the morning before I've
turned on other lights.

Mikke

White and white are not always the same as white. The taste of white that a
white LED produces depends on its physical properties and the current that
flowa through it. So this particular combination might have been designed to
produce a special white for some purpose. I ever saw a bunch of white LEDs
with some lonely red and green ones between them just to produce the white
that was required.

petrus bitbyter