I’m not an electronics guy so I’m assuming I’m missing something here.
I am trying to light a single 3.0-3.4v, 20mA LED and I keep burning out my LED’s, I have a 21VDC battery connected to a DC-DC step-down variable regulator set to output 10v to the Femtobuck. I thought I just had to supply 7-36v to the Femtobuck and it would put out the correct voltage and current.
I’m still trying to learn this voltage-current relationship but so far it’s stumping me.
Your help is greatly appreciated,
Bruce
Hi Bruce.
The Femtobuck is designed to drive [high power LEDs that need 300 or more mA of current. What it does it force 300 mA through whatever it’s connected too to and since your LED is only rated for 20mA, it burns out.
If you already have a 10 volt power source, you really only need a resistor to limit the current through your LED and you can use ohms law to calculate what value resistor is needed.
The formula you need is R = E / I where R is resistance, E is voltage and I is how much current in amps you want to flow through the LED.
We know E is 10 since you have a 10 volt power source and I is 20mA. If we convert mA to amps, that’s 0.020A
Dividing 10 by 0.020 gives us 500 ohms.
So what you’re going to need is a 500 ohm or larger resistor.
500 ohms isn’t a value we carry, but we do have a 680 ohm resistor in our [resistor kit. That would work but the LED would be slightly dimmer than if you used a 500 ohm resistor.
Another option would be to take two [1000 ohm resistors and connect those together in parallel, then connect the pair to your LED. Those in parallel give you 500 ohms and that would do the trick very nicely. :-)](Resistor 1K Ohm 1/4 Watt PTH - 20 pack (Thick Leads) - PRT-14492 - SparkFun Electronics)](https://www.sparkfun.com/products/10969)](https://www.sparkfun.com/products/13106)
The Femtobuck is designed for LEDs that need between 330mA and 660mA. Putting that kind of current into an LED that needs 20mA will kill it instantly.
I would use the buck converter as you have it to drop the 21V (though if you don’t need 10v elsewhere, I would crank it down to 5V to reduce heating in the resistor) and use a series resistor between the buck and LED. There are on-line calculators that will figure out the resistor value for you.
Given a 5V supply voltage and a 3.4V LED, you need to drop 5-3.4 = 1.6V. Using Ohm’s law, you need a 1.6V/0.02A = 80 ohm resistor. 82 ohms is a standard value. Power dissipation will be 1.6V * 0.02A = 0.032W so use a resistor with a power rating of at least 0.06W to give some margin of safety.
If you need to run at 10V, a 330 ohm resistor will work.
/mike
Thanks everyone,
I’m kinda getting the hang of this, I did get it to work with a resistor
