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  #1  
Old 03-30-2009, 07:38 PM
rfhelp rfhelp is offline
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Default Resistors to pos or neg side of led? two answers??

Here is what I read on another tutorial page:

"I looked through all the resistors, found the one that was brown, green, brown, gold, and wired it in line on the positive electrode of the LED. (Whenever using a resistor on an LED it should get placed before the LED on the positive electrode").


On here I am told to connect to the neg side? please advise.
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Old 03-30-2009, 07:47 PM
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A LED has no positive nor negative. Has anode and cathode. That tuturial is not very good to start.

The resistor --/\/\/\/-- goes at ANY side of the led --A|>|K-- ; but the cathode (K) of the led should path to the negative (gnd) of the supply, anode (A) towards the positive (+)

+--------------/\/\/\/---------A|>|K-----------------gnd

or

+--------------A|>|K----------/\/\/\/----------------gnd

Miguel
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Old 03-31-2009, 08:04 PM
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depends on what you want the resistor to do.

for example: you have two leds and connect the resistor to voltage and the other side to both leds. no problem here, but if you connect both leds to voltage and both cathodes (negative side) to the resistor then it won't work as nicely.

One of the leds may be dimmer or not on at all, because the current through first is causing a voltage in the resistor, making the second led "see" a smaller voltage or non at all (remeber voltage is a difference between two points)
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Old 03-31-2009, 09:38 PM
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It doesn't matter if place the resistor before or after the LED because is the amount of current the resistor limits as not to burn the LED. In a series circuit the current is the same for all components and because the LED and the resistor are in series you can connect them as you want.
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Old 03-31-2009, 11:19 PM
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<smartass>
It depends if current is moving from negative to positive or positive to negative. (Sorry, it was another thread...)
</smartass>

In reality, no, it doesn't matter which side the resistor is on. The two components together make up a Series Circuit, and current is the same at all points, but voltage changes.

To get the correct value of resistor, Use this: LED series parallel array wizard

Rule O' Thumb:
LEDs with clear lenses (super-bright) are usually 20mA
The colored lens LEDs are usually 10mA

If you have the spec sheet, currents may be higher, but that rule of thumb gets most lit very brightly.
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Old 04-01-2009, 12:40 AM
rfhelp rfhelp is offline
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They are clear and very bright. I picked up some 1k, and 3k, resistors today to dull them down


Emitted Colour : Green
Size: 5mm
Lens Color : Water Clear
Forward Voltage (V) : 3.0~3.6
Forward Current (mA):20
View Angle: About 25 degree.
Static Sense:Yes
Luminous Intensity: 15000mcd
Life Rating : 100,000 Hours
Free Resistors:
Supplied with free resistors for DC 12V
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Old 04-01-2009, 12:50 AM
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Quote:
Originally Posted by thatoneguy View Post
<smartass>

To get the correct value of resistor, Use this: LED series parallel array wizard

Yes, found that very usefull
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Old 04-01-2009, 01:18 AM
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Super-bright LEDs can cause permanent vision damage if you look directly at them for very long. They'll burn blind spots in your retina without you even knowing - until you look at something else. Better to err on the side of caution, and run them at low current.

As has already been said, it doesn't matter which "side" of the LEDs the resistor is on.

I've developed a preference of having the resistors on the anode side of the LED where possible, as accidentally shorting the LED's anode or cathode to ground won't hurt either the LED or the resistor. If the LEDs are on the more positive side of the resistor, accidentally shorting the cathode of the LED to ground will result in a fried LED.

Sometimes it's not possible to have the resistors nearest +V, as when wiring up a common anode 7-segment LED display. But, with something like dashboard trouble lights, that's not much of a concern.
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Old 04-01-2009, 01:28 AM
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I was told that you can see which is the anode and cathode by the size of the metal in the glass. the larger is the anode(+) and the small is the cathode(-)

I only say (-) and (+) in respect to which leads they are connected to. I realize the led itself is not (-) or (+)
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Old 04-01-2009, 01:43 AM
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Quote:
Originally Posted by rfhelp View Post
I was told that you can see which is the anode and cathode by the size of the metal in the glass. the larger is the anode(+) and the small is the cathode(-)
Actually, it varies - but usually it's the other way around.

For lack of a better illustration offhand, have a look at this page:
http://www.kpsec.freeuk.com/components/led.htm

Note this line:
"If you can see inside the LED the cathode is the larger electrode (but this is not an official identification method). "
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