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### Topic: Calculating resistor size for Led`s (Read 639 times)previous topic - next topic

#### DmarkL

##### Mar 27, 2013, 06:23 pm
Hi everyone,

i am very very green when it comes to electrionics but i`m learning!
I have a very basic question,
when calculating the reistor size for led`s do you use the typical forward voltage value or the max one and
do you use the typical forward current value or the max one?

D

#1
##### Mar 27, 2013, 06:30 pm
I use the min voltage, as that will cause the highest current draw:

(Vs - Vmin)/Resistor = current

ex.
(5V - 3V)/100 = 20mA
(5V-3.4V)/100 = 16mA

Plan for no more than 20mA (generally) continuous current.  Using the MAX for continuous will really shorten the LED life.
Many LEDs can be pulsed with higher currents, but only for very short times.  The data sheet will have details of the on time and the frequency.
Designing & building electrical circuits for over 25 years.  Screw Shield for Mega/Due/Uno,  Bobuino with ATMega1284P, & other '328P & '1284P creations & offerings at  my website.

#### afremont

#2
##### Mar 27, 2013, 06:33 pm
I'd just go with the typical Vf, but I'd stay far away from the max current.
Experience, it's what you get when you were expecting something else.

#### DmarkL

#3
##### Mar 27, 2013, 07:33 pm
Thanks for the responses guys

#### fungus

#4
##### Mar 27, 2013, 07:47 pmLast Edit: Mar 27, 2013, 07:49 pm by fungus Reason: 1

when calculating the reistor size for led`s do you use the typical forward voltage value or the max one and
do you use the typical forward current value or the max one?

The minimum one. And round the resistor upwards.

If you want more you need to use a current controller circuit, a resistor isn't good enough.
No, I don't answer questions sent in private messages (but I do accept thank-you notes...)

#### A4kash

#5
##### Mar 28, 2013, 10:16 am
just know the forward voltage drop and subtract it from the voltage you are supplging and given the max amount of current for your leds use ohms's law
V=IR.

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