SparkFun Forums

[Woodsplinter]

Greetings everybody!
First post so I apologize if this is in the wrong forum or if it's been covered before. I did search but couldn't find what I was looking for.

I have a 3 watt green LED with a forward voltage of 3.2-3.4 volts. Max forward current is 750 mA. It is powered by a drone battery which puts out 16 volts @1.1A. The voltage is reduced by using a DC-DC buck converter with 1A capability.
(Voltage regulating but not constant current, yes,I know the proper way to control LED's is through current not voltage).

Checking the current draw, I found it is about 280 mA. I then ran the output of the converter to the input of a constant current regulator connected to the LED. The C-C regulator is adjustable from 80-800 mA. Input voltage 2.2-6 volts. The current draw of the LED remained the same, no matter how the C-C regulator is adjusted. I can turn the current down from 280 but no higher.

So, my question, is there any way to "pump" more current through the LED (to increase brightness) or does the LED decide how much it's going to draw?

Thanks!

[davep238]

Have you tried not using the CC regulator, and instead connecting a resistor in series with the LED to the output of the buck regulator? The resistor will turn the constant voltage into a constant current.
BTW, the battery only puts out 17.6 watts, so 3 watts is a lot of power.

[Woodsplinter]

Dave- thanks for your reply. I have not tried that as my original thinking was I could use the c-c regulator to adjust & "tune" the led to get an acceptable level of brightness with minimum current draw.

I suppose once I know the appropriate value I could replace the c-c with a resistor.

But if the led only pulls 280 mA when connected to the buck converter, using the resistor won't increase the current draw. Maybe my original post was confusing, but I'm trying to see how bright the led gets with more than 280 mA.

And, I believe your saying a 3 watt led can put a significant drain on the battery, correct?

[lyndon]

You don't say what voltage the DC-DC converter is outputting. Undervoltage is the most likely reason the C/C regulator is limiting.

[jremington]

To increase the current through the LED, you have to increase the voltage applied to it.

The voltage-current relationship is exponential and device temperature dependent (Ohm's law is not obeyed), which is why you need a current limiter.

[Woodsplinter]

To increase the current through the LED, you have to increase the voltage applied to it.

The voltage-current relationship is exponential and device temperature dependent (Ohm's law is not obeyed), which is why you need a current limiter.

I had the DC-DC converter set to deliver 3.2 volts. Increasing the voltage did the trick & raised the current and thus the brightness to a better level.

Thanks for all your help! Don't know why that little detail was so hard to find elsewhere.