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Help with LED.

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electricity86

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I'd like my UC's GPIO output pin to drive a LED which its VF is 3V min and 3.8V max, and its IF is 20mA.
The GPIO outputs 3.3V (at high level), and can output maximum of 20mA.

Would it be ok to connect the LED to the pin without using a current limiting resistor?
I mean, how can I be sure that the 20mA is not 20mA @ 3V, and that at 3.3V the LED will consume more than 20mA?
Would it damage the GPIO pin?

Thank you in advance.
 
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ericgibbs

Well-Known Member
Most Helpful Member
I'd like my UC's GPIO output pin to drive a LED which its VF is 3V min and 3.8V max, and its IF is 20mA.
The GPIO outputs 3.3V (at high level), and can output maximum of 20mA.

Would it be ok to connect the LED to the pin without using a current limiting resistor?
I mean, how can I be sure that the 20mA is not 20mA @ 3V, and that at 3.3V the LED will consume more than 20mA?
Would it damage the GPIO pin?

Thank you in advance.
hi,
If the project is for driving just one led, I would suggest you determine the actual LED working Vfwd voltage. If its a max of 3.8V the LED will not work.
[lets know what you measure]

Its always advisable to fit a series resistor between a MCU pin and an LED.
 
Last edited:

Willbe

New Member
I'd like my UC's GPIO output pin to drive a LED which its VF is 3V min and 3.8V max, and its IF is 20mA.
The GPIO outputs 3.3V (at high level), and can output maximum of 20mA.
Try a resistor of value = (3.3-3)v/.02A for those 50% of the cases where you need this resistor. The other half of the time the LED may hardly light at all.

For this low-production-yield circuit it sounds like you should use two or three resistors and a xsistor to drive the LED.
 
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