problem about LED not getting 20mA current as per calculation

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rahol

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hello,


i am glowing an LED with 5V power supply. along with 250ohm resistor after that i m getting 20mA current
LED having Vf=3.0v
but according to the calculation , v=5v and Vf=3.0v and current should be 20mA. so

R=(V-Vf)/20mA.
R=100ohm

but practically i am not getting the actual 20mA current. and getting around 13.7mA.

my question is that why i m not geeting 20mA current according to the calculation??

thanxs in advance
 

Hello rahol,

your calculation is correct.

If you don't get the current, it can be, that Vf is not 3,0V. Look for the datasheet, there you see a minimum-, normal- and maximal forward voltage.
Make your calculation with the maximum Uf. So you are one safety side. If Uf is lower, the current will be a little bit higher, but this don't care.

Regards
Rainer
 
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    rahol

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@rahol something is wrong
250 Ohm * 0,020 A=5V
 

@js - you are forgetting the resistor only drops the 5V down to Vf of the diode so there will only be about 2V across it. 2/0.02 = 100.

Brian.
 

Use a voltmeter to measure the forward voltage of the LED. Connect the voltmeter across the LED. Also measure the resistance of the current limiting resistor. Ensure (by measuring) that the supply voltage is stable at 5v.
 

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