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Genral help regarding electrics

simong93

Dec 20, 2011
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Hi all im hopeing i have put this in the right section but i need some help.

I feel its about time i learnt a bit more about electrics does anyone no of any step by step guides on how to build stuff that explains what the components are for or just a list of components and what they do.

At the moment i am attempting to add some leds to my Xbox controllers and ive been hit with this Ohm resistor what the hell is one of them i no what they look like but what do they do it says

Lighting the Dome- To get the dome illuminated we tried several variations, one LED, two LED's, but ended up going with four for full bling. With blue LED's, 4 in parallel a 22 Ohm resistor is connected to all positive legs and the resistor is connected to the positive source. All negative legs (short ones) are connected to ground. When using 4 red LED's a 47 Ohm resistor was used (How to calculate the proper resistor for your LED's).

I know what parrel circuits are but how did they get 22 ohm im at a lose please help me :D.
 

(*steve*)

¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
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Jan 21, 2010
25,510
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25,510
Check this thread out. It has pointers to sited which allow you to calculate the resistance required.

Note that connecting the LEDs in parallel like that is not recommended.
 

simong93

Dec 20, 2011
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Check this thread out. It has pointers to sited which allow you to calculate the resistance required.

Note that connecting the LEDs in parallel like that is not recommended.

How would your recommend wiring them up because at the moment im getting confused.

Hes just wired up 4x 2.7v led to a controller that has 3v is that possible.
 

(*steve*)

¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
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Jan 21, 2010
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each string of LEDs should have a resistor. In your case the strings of LEDs are just one LED, so each one should have a resistor.

3V for a 2.7V LED is marginally OK as long as the 3V rail is pretty stable.

The resistor for each LED should be (3 - 2.7) / 0.02 ohms (that comes out to approximately 15 ohms.

Beware though, if your voltage is 3.3 volts, the LEDs will receive twice their likely rated current, thus my caveat that the 3V rail is stable.
 

Raven Luni

Oct 15, 2011
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I'm sure there are 1 or 2 threads around here about LED resistors, but basically to calculate them you need to know 2 things: the voltage of the power source and the voltage and current specs for the specific LEDs you are using.

You take the voltage of your power source, subtract the forward voltage for the LED, then choose your resistor so that the current for the remaining voltage does not exceed the LEDs rating.

If you do not know these values, a good estimate is about 2V and 15mA, so if you had a 5V source, 5-2=3, using ohms law, divide the voltage by the current in amps = 3 / 0.015 = 200 ohms. So thats 50 ohms (or 47 which is the nearest available value) for 4 standard red LEDs

BUT DONT DO IT THAT WAY! Use a separate 220 (nearest available for 200) resistor for each LED. If you use a single resistor for all of them and one of them dies, more current will flow through the remaining ones and will probably kill those too.
 

simong93

Dec 20, 2011
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Now im getting what they are for thanks for that that cleared up alot.

Also just to understand here Four 2.7V leds can run off 1 3v controller ?
 

(*steve*)

¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
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Jan 21, 2010
25,510
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yes, but each led has a resistor and they are connected in parallel. So get one working, then wire up another, then another, then another. All with their own connections to the 3V supply (that's what parallel means.

You really should look at that thread I pointed you to... I didn't do it for my health you know.
 
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