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# What is this SMT Resistor?

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#### Mr Red

##### New Member
I've got an SMT Resistor here from a cell phone that says 681 on it. Is there anyway someone can tell me exactly what this is and where I'd be able to purchase them?

So far I've gotten the concept of it controlling the amount of power going to the LED it's connected to. I guess what I'm curious to know is what resistors would I use to brighten or dim the LED. Any explantion of how I figure this out would be great.

To increase the brightness of an LED, you need to increase the current through it which requires lowering the value of resistor connected in series with the LED. And if you want to decrease the brightness, current should be decreased which means the resistor value should be increased.

One thing that should be noted over here is that you can't increase the brightness beyond a certain limit. Doing so will fry your LED.

Most LED's operate at near about 10mA current so choose your resistor value accordingly to get optimum light.

The formula for calculating resistor value is
R= (Vs - Vf)/If
Where
Vf = Forward Voltage Drop of LED
Vs = Supply Voltage
If = Forward Current of LED

the numberin 681 would mean - for a resistor (!):
6 - 1st number
8 - 2nd number
1 - # of zeroes behind the numbers

6 8 0 - 680 ohms.
----^ one zero

other Examples:
110 - 11 ohm
271 - 270 ohm
683 - 68k ohm
125 - 1.2M ohm

Brighten an LED it by making the resistance smaller.
Then the current increases - until you smoke the LED.

try putting a potentiometer , e.g 2 k ohm in parallel and decrease slowly
to see the effect.

charteristics of an LED that is about to fry?

Hi,

I got a few LEDs via ebay cheap, so I knew that a 4.5 volt button battery was too much current, but wanted to try anyway. (the forward voltage is rated 3.3)

it didn't fry the LED in this case, but I didn't seem to get more light and the LED was heating up. so that is a clue that you're putting too much volts thru, since it runs almost without any heat within voltage.

Any other signs? I noticed that some LEDs also have a brighter MCD #, but you need to pulse it, not run continously.

John

Applying more voltage than rated will decrease LED's life.

Pulsing the LED gives more light that rated mcd is true since in this techniqe around 4 times the rated current is passed though the LED for a short period for thousands of times a second. This does not destroy the LED since it does not get enough time to heat up and destroy the semiconductor junction.

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