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I wanted to run LED bulbs with a mobile phone charger/adapter ( input: 220v AC, output: 5v 350mA, nokia). but the LEDs become very hot & I'm afraid they may burn. i don't know the ratings of these LEDs bcoz the shopkeeper from whom i bought these also doesn't know. but they r the common type in market. i think I should reduce the power. but which one? (ampire or volt, which harms LED?) & how can i do that? i tried to add a resistor in 'series connection' but it make the LEDs very dim. i heard resistors in series don't reduce voltage, they reduce ampire. is it true? should i add resistor in parallel to reduce voltage? please pardon my poor English. I'm not a native English speaker & also newbie to electronics.

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    \$\begingroup\$ What value of a resister did you use? LED's primarily operate via current. Resisters in series do drop some voltage, but it's primarily acting as a current limiter for an LED. We can guess, but it would be helpful to have some data on the LED's in question. Your resistor choice should be based on how much voltage you want to drop across the resistor to provide the LED with it's forward voltage (which in turn feeds it the proper current). \$\endgroup\$ – Jarrod Christman Jan 16 '15 at 21:29
  • \$\begingroup\$ Are your LEDs in series or parallel? How many are there? By common type do you mean 5mm? What color are the LEDs?(I've found that forward voltages can vary quite a bit depending on LED color). Without knowing the LED specs (forward voltage and current rating) it's really just a guessing game. Choose a resistor that would work for a same color/size LED that you have specs for, and then adjust the value to get the right brighness/current. \$\endgroup\$ – I. Wolfe Jan 16 '15 at 22:27
  • \$\begingroup\$ Follow advice here BUT also do some reading on voltage, current, and resistors and "Ohms law". Once you understand the basics add LEDs. This learning will reward you many times over. If you just accept answers without understanding you will remain bemused, \$\endgroup\$ – Russell McMahon Jan 17 '15 at 10:18
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For most common red, yellow and green LEDs, you should use a resistor of 150 to 510 Ohms in series with each LED.

150 Ohms will limit the current to about 20 mA - the recommended maximum for common LEDs. Higher resistance will reduce the current, and LED brightness.

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You should a resistor in series with each LED in order to limit the current through them, unless the LEDs contain their own resistor.

If the LEDs become too dim, try a smaller value resistor.

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