limiting and measuring?

[justDIY]

greetings

I'm working on a little lighting project with RGB leds.

Here's a bit of backgroun on my setup:

I understand according to the specs, the RED leg requires much less power than the blue and green legs.

Due to the nature of my circuit, I'm using common anode leds, which causes me to put an additional current limiting resistor on the low side of the red leg, as well as a regular limiting resistor on the high side.

I'm working with a 5 volt supply, using a 68 ohm resistor on the high side, and an additional 100 ohms on the low side of the red leg. 

My Questions:

I used your calc with VS= 5, VF = 2.5, I = 20 and it calculated 150 ohms... I have on my circuit a 68 ohm 5% on the high side (anode) and another 150 ohm on the low side (red cathode), yet across the LED i'm measureing 2.8 volts

Now the funky part, when i hook the ammeter between + and the anode, I'm reading 13.7 mA draw

Which measurement should I be concerned about; voltage across the LED or current draw?

[sanjed]

cool question.

i've got a feeling that the man's gonna like this one.

[Rob]

Current is the critical thing.

I understand according to the specs, the RED leg requires much less power than the blue and green legs.

If you're saying a lower forward voltage, that sounds okay to me.  At any given drive current, lower voltage will result in lower power.

I can't comment too much on the voltage measurement, but if you have a typical multimeter, you're probably measuring at the very bottom of the 20V range. I'd expect the accuracy to suffer as a result.

The current measurement would be in the middle of the 20 mA range and I'd trust the meter more there. That may account for it. If you want a voltage sanity check you may try measuring your source voltage, and deducting the resistor drops to see how well they agree.

And welcome to the boards.

PS Where'd you get a common anode RGB LED?

[justDIY]

I bought the rgb's from superbrightleds.com, orginally they were for a Dallas-Maxim LED driver that i was experimenting with.

Now they fit my current circuit well, I'm using some low side mosfets to drive several banks of leds, so the more readily available common-cathode leds wont work.

I think your idea about accuracy is probably the case, the difference between 2.X and 2.8 on the dmm's 20v scale might be a bit foggy!

I'll work to keep the mA's in the 'safety' range and I think that should be good enough, dont need these to last more than a few years anyhow   Right now i'm just working with two lamps, but I want to be able to scale up, so need to plan out the pcb accordingly!

[Rob]

Sounds interesting--I'd like to see what you're working on, if you don't mind sharing.