Re: Brightness revisited
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Andrew Stone
Feb 2, 2012, 4:47:32 PM2/2/12
to Phil Spitler, toasted-circu...@googlegroups.com
Great! I'm glad they are really bright. Please hook a multimeter up in series with the LED on the "Amps" setting to measure the current going through the LED. And adjust it to less then 20mA via the on-board screw pots. Otherwise you might dramatically reduce your LED life.
The reason a coin cell was dim is because a LED's brightness depends on the current passing through it, not the voltage. And a coin cell battery cannot provide a lot of current (the speed of chemical reaction in the battery limits it).
For something like a resistor, the current is proportional to the voltage so they are co-dependent. But an "ideal" LED actually drops X volts regardless of the current. In other words, if you put X+1 volts across it it'll draw as much current as your power source can supply (until it burns out)! If you put a 4A wall wart transformer across a LED you'll see it burn out basically instantly.
In practice a LED's voltage will vary from 3 to 3.5 volts (a factor of 1.16), while the current moves from 1 to 100mA (a factor of 100).
Luckily for you you didn't blow a LED because 3v is lower then the likely "nominal" voltage of your LED (3.3v), and also as I previously said the coin cell's voltage will actually drop because it cannot provide more than 10s of mA of current.
Cheers!
Andrew
The reason a coin cell was dim is because a LED's brightness depends on the current passing through it, not the voltage. And a coin cell battery cannot provide a lot of current (the speed of chemical reaction in the battery limits it).
For something like a resistor, the current is proportional to the voltage so they are co-dependent. But an "ideal" LED actually drops X volts regardless of the current. In other words, if you put X+1 volts across it it'll draw as much current as your power source can supply (until it burns out)! If you put a 4A wall wart transformer across a LED you'll see it burn out basically instantly.
In practice a LED's voltage will vary from 3 to 3.5 volts (a factor of 1.16), while the current moves from 1 to 100mA (a factor of 100).
Luckily for you you didn't blow a LED because 3v is lower then the likely "nominal" voltage of your LED (3.3v), and also as I previously said the coin cell's voltage will actually drop because it cannot provide more than 10s of mA of current.
Cheers!
Andrew
On Thu, Feb 2, 2012 at 2:04 AM, Phil Spitler <ph...@bonfirelabs.com> wrote:
Hi Andrew, sorry to be bombarding you with emails again.I am revisiting the LED brightness conversation and am curious about something.Currently with the 12V PSU plugged into the board and 20 LEDs in the breakout board, they look amazing. Really bright.I measured the voltage across one of the LEDs and it was about 3v.I have a bunch of coin cell batteries that are also 3v (CR2032) but if I connect my LED to that battery it is really dim. I measured the voltage on the battery and it says it is 3V.Of course, I am happy that my Lightuino is super bright but curious why a battery with the same voltage produces a dim LED....Thanks.PhilPhil Spitler | Associate Creative Director | Bonfire | t : 415.394.8200 | c : 415.571.3139 | Bonfirelabs.com
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