# this bugs me / noob question`

Discussion in 'General Electronics Discussion' started by elioacm, Jul 7, 2013.

1. ### elioacm

25
0
Jul 7, 2013
so i have a 9 voltage battery...

how many LED that it would light.

is this correct?.. ea: the led is 1v each, then the 9v battery would run 9 led with each one is in the same brightness..?

25,491
2,833
Jan 21, 2010
3. ### elioacm

25
0
Jul 7, 2013

my prob to that vid is how did he know the mA of the LED hes using.,.
is it possible to use multimeter?

4. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

25,491
2,833
Jan 21, 2010
You read the specs for the device.

Typically a low power LED is OK up to 20mA. Lower is OK too.

Higher power LEDs may allow much higher currents and that is normally specified in their datasheet. Knowing Vf and the power of the LED you can usually calculate a reasonable value.

5. ### NuLED

294
0
Jan 7, 2012
I am not sure how practically you can measure specs for LEDs. I think it would be quite difficult and you need tools in addition to a multimeter. For example, probably an oscilloscope, so you observe how the voltages and currents behave over time across the LED.

Generally I think you need to have the specifications. I have a bunch of LEDs that have unknown specs so I just guess at them. Now I separate the LEDs that have specs (from the package) from LEDs that I know nothing about. But as Steve said, GENERALLY you can make a guess at the most common specs.

6. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

25,491
2,833
Jan 21, 2010
No, a scope isn't going to help.

7. ### BobK

7,682
1,688
Jan 5, 2010
You turn up the current slowly unitl it pops and then reduce it by a factor of 2.

Do this with each of your LEDs, then you will have to buy new ones, and you will know their specs.

Bob