I need more power!

Before making a PCB, I suggest you learn more about LEDs. They are current devices, not voltage. You *did* put a resistor in series with the LED?

Yes and no, a resistor is in series with the red LED, though to calculate the resistor I looked at the voltage the data sheet required. Then using the expected current figured it out from there... I.E. for the red part it wants a voltage around 2.0V. So I put a 65 hm resitor in series which drops it 1.3v at 20mA. This seemed to work.

For the blue in green they want a voltage of around 4.0v for a 20mA current. As I only have 3.3 I just connected that straight. That probably explains the jump to 40mA. How should I calculate the resistances for those?

Thank you for the information on the fet's I will look into that!

Matt

I assume a ULN2003 will give you what you want, however in a similar situation I would probably find me a pile of SOT-23 logic level FETS instead. Go to Fairchild Semi and look around for SOT-23 logic level FETS. Finding good 3.3 ones is slightly hard. Best to find a 2.5v one and use that so you know it is fully on.

Hawker

On 3/20/2007 10:38 AM, The digits of snipped-for-privacy@gmail.com's hands composed the following:

Alright, Thank you very much! I will go back and do some more tests. One last general question about Tri colored LED's. Is it normal for RED to be barely visible in comparison? To make white I have to basically not Pulse RED, and then BLUE and GREEN have to be nearly off. It is also possible that I damaged the RED by applying to much voltage at first.

Thanks Matt

You need to look at the part data sheet, which will show you an I-V (current - voltage) curve. The curve will show you what the forward voltage drop for the device at a given current level. So if for example, you have a 3.3V supply and want 10mA to flow through the diode, you find the 10mA line on the chart and the corresponding part on the curve, which will then tell you the forward voltage at that current level. To continue the example, say at this point the forward voltage is 1.3V. Then the resistance you need is (3.3V - 1.3V) / 10mA = 200 Ohms.

Similary, a look at the datasheet should provide some insight as to what happened when you put 3.3V across the diode, assuming you didn't run into a physically limiting feature.

With the diodes, you need to be sure you don't exceed the maximum forward current, least their life will be very short.

Since all you want to do is light LEDs at ~ 20 mA, you can use a small boost DC-DC converter chip, like a TPS61070. The datasheet shows you how to hook it up.

Ed