# Thread: Led current calculation

1. ## Led current calculation

Hello
What is current (milliamp) in Master Iocard output ?
I know the voltage is 5v and I want to calculate using differences voltage of leds.
Thanks

2. Take a look at this site. The electronic calculator has a resistance calculator for LEDs. http://electronics2000.co.uk/

Dave

3. Hi,

out of experience I found that the opencockpits outputs have an internal small resistance already. This means that the output voltage (5Volt) will drop a little the moment you start
to take current from them (lighting a led) If I remember it well there was some 10 ohm
internal resistance. If you only want to light one led with one input then you need to calculate the resistor by using the (5v - voltage over the led) / current for normal led operation
in case you like to run two leds in series then the internal resistance limits the current enough
so no need for a extra resistor.

Greetz Peter

4. ## Thank You

Originally Posted by DaveC3
Take a look at this site. The electronic calculator has a resistance calculator for LEDs. http://electronics2000.co.uk/

Dave
The "Electronics Assistant V4.1" (FREE), very useful, thanks for the link

5. Originally Posted by Matt Olieman
The "Electronics Assistant V4.1" (FREE), very useful, thanks for the link
... yes, but why does a "calculator" require access to the internet?!!

6. You can download it and run it on your computer. Just click on the "Download" Tab at the top of the page.

Dave

7. I have also a very nice online site to calculate resistors http://ledcalc.com/ but it needs de current in milliamp.
The IOcards manual says resistor 330 ohms that's why I assumed it is 10 milliamp for one led 2.2 V
Even using 2 leds they calculate for 2 resister of 330 ohms.
I am not electronic expert bat I agree with Peter... 2 leds of 2.2 = 4.4... no resistor required.
I just need the brightest light with secure not to burn it.
Regards
Cesar

PS: I just order USB expansion card and one more master from Opencockpit.