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user LED circuit on EP3C120F780 development kit

Altera_Forum
Honored Contributor II
950 Views

Hi, 

I checked the schematics of user LED ciruity on EP3C120F780 development board, the ciruit seems weird to me.  

 

The LED anode is connected to 3.3 through 56 ohm resistor and  

the cathode connected directly to FPGA pin: 

 

 

 

(led) 3.3 [V] 

(pin) o---|<]----www--` 

 

56 [ohm] 

 

 

What the voltage drop on the LED when it is on and when it is off?  

 

LED on:  

When FPGA drives '0' (LED is on) and, say the voltage drop on LED is 0.7 , the resistor current will be (3.3 - 0.7)/ 56 ~ 50 mA which seems to much  

... 

 

LED off:  

Now the pin is connected to bank 4 which has 1.8 I\O voltage so how the LED is turned off ?
0 Kudos
3 Replies
Altera_Forum
Honored Contributor II
105 Views

The attempt to draw the schematics didn't come off well 

 

I'll try again, (dots are just to fill space) 

 

 

.................(led)..........3.3 [V] 

....................................| 

....................................| 

...(pin) o----|<]---www--` 

..................................... 

........................56 [ohm]
Altera_Forum
Honored Contributor II
105 Views

a LED does not have a voltage drop of 0.7V. 

Depending on the color, you have a voltage drop of about 1.9 to 3.5V. 

see https://secure.wikimedia.org/wikipedia/en/wiki/light-emitting_diode
Altera_Forum
Honored Contributor II
105 Views

Ok, Thanks!

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