LED current, bit of an issue

[kerick]

Is there a way to test or calculate the current of an LED? I bought some super bright 8 mm LEDs fom American Science and Surplus. The package stated 4.5 v but not the mA. I know 20 mA is the default # but I'm wondering if this larger size LED uses more than that. At 12V power supply and 20 mA I use a 150 Ohm resistor with two LEDs in series and at 30 mA a 100 Ohm. Or just go with the 150 Ohm and accept a bit dimmer light? Its a 50% difference in Ohms or am I just thinking to hard?

Also I may run a 3V motor on the same project, will I need a totally seperate power supply for it or can I "split off" 3V?

I may get the hang of this soon. Thanks in advance.

[Pat Amaral]

It looks like your number crunching is right but I'd suspect your LEDs may not last as long at the higher current level. It would be nice to find out what the current rating for the LEDs actually is. I would also suggest you give the motor in an independent power supply.

Hope that helps,

[kerick]

It looks like your number crunching is right but I'd suspect your LEDs may not last as long at the higher current level. It would be nice to find out what the current rating for the LEDs actually is. I would also suggest you give the motor in an independent power supply.

Hope that helps,

I think I will end up with a 12v lead acid gel cell and a 3v C cell power source. This will give me weeks of power for what i"m doing. Hate to double up but it looks like the best solution short of a step down tranformer. I stopped at a local Radio Shack to pick up the resistors and checked the LEDs on hand. Anything close to what I am using was 20 mA so that is what I'll use. Thanks for the help.

[tetsujin]

Is there a way to test or calculate the current of an LED? I bought some super bright 8 mm LEDs fom American Science and Surplus. The package stated 4.5 v but not the mA.

Get yourself three AA Alkaline batteries and a multimeter.
Set the multimeter to current reading mode.
Hook up, in series, the three AA batteries, the multimeter, and the LED.

The three AA batteries should provide a total of 4.5V. The voltage drop of a multimeter on current reading mode will be minimal, so the LED will be operating at 4.5V. So the level of current going through the LED will correspond to 4.5V across the LED.

If you had to take this measurement and didn't have a battery of the same voltage as the LED, the way to do that would be to add a variable resistor to the series. (A 1K variable resistor would probably be sufficient...) First turn the resistor up to full resistance, remove the meter from the series, switch it to voltage mode and put it in parallel with the LED. Turn the resistor down until the voltage across the LED corresponds to its recommended operating point. Then remove the meter from the circuit, switch it back to current mode, and put it back in series. The voltage across the LED should still be very close to what it was when you measured it, so the current through it will be your operating point.


Oh, and one way to get your 3V for a motor would be to have a voltage regulator to switch your 12V down to 3V...

[kerick]

I do have a multi meter (smack fore head) and I'll try your advice. I should have thought of that!
I'll also look into a voltage regulator. I may add a pot to adjust the motor speed.
Thanks!!!!