Using 20mA current limiter with 3 color common cathode LED's

I have been using 3 color 5050 breakouts from ebay, and they usually come with smd limit resistors on the boards but occasionally I end up with a batch of keyes cnt1 3_clor modules, which lack the limiting resistors. The three R G & B leds need different resistors, and I recently came across a device that would seem to be a relatively cheap solution that avoids extra soldering:

http://lighthouseleds.com/20ma-led-current-limiter-driver.html

So could I just put this on the ground line, and it would automatically adjust for the different forward voltages needed by each LED?

what happens if I accidentally tried to power two different leds at the same time, with this current limiter on the ground line only?

20ma going through a blue LED will give a different intensity than a red one, etc. If this is what you are suggesting.

EKMallon:
I have been using 3 color 5050 breakouts from ebay, and they usually come with smd limit resistors on the boards but occasionally I end up with a batch of keyes cnt1 3_clor modules, which lack the limiting resistors. The three R G & B leds need different resistors, and I recently came across a device that would seem to be a relatively cheap solution that avoids extra soldering:

http://lighthouseleds.com/20ma-led-current-limiter-driver.html

So could I just put this on the ground line, and it would automatically adjust for the different forward voltages needed by each LED?

Looks like it.

(They’re a bit expensive though… )

EKMallon:
what happens if I accidentally tried to power two different leds at the same time, with this current limiter on the ground line only?

You need one per LED, ie. three of those for each RGB LED (RGB LEDs have three LEDs inside them)

A KA2284 will limit the current for 5 LEDs and they cost about $0.20 each on eBay (just tie the input pin HIGH and you get 5 current sinks)

@Fungus: Thanks for the tip on that linear IC! I have to check its quiescent draw before I put it my data-loggers, but they will come in very handy for another project I am working on. My power supplies will be dropping below the 3.5 volts they need though. http://www.elecrow.com/download/VU%20meter%20a%20led,%20KA2284.pdf

And I found a cheaper source for the limiters: http://www.hansenhobbies.com/products/lighting/electronics/led_cl_20/ so they come in at about 50 cents each for >10 pc. I will simply order some and see if they will work with only one unit on the ground line, as they are rated for operation to 2v and my project goes down to 2.7v

I am wondering if there might be response time issues with the limiters: ie will they allow a short period of excess current before they regulate properly? Also curious about noise etc.

I think this is the data sheet for these little guys:

http://www.onsemi.com/pub_link/Collateral/NSI45020-D.PDF

will they allow a short period of excess current before they regulate properly

No.

Also curious about noise

What about noise? It is not a critical factor in circuits like this.

@GM

I know the LED is not affected much by noise, but I did not know if the operation of the current limiter (if it wobbles for a while before settling?) would some how introduce noise into the ground line that the other devices also use. Some of my I2C sensors (like accelerometers) are pretty noisy already when they are set to their maximum sensitivity, so I just wondered if this would add to that problem.

If noise is a concern make sure you wire it up in a star ground configuration. That is all the ground wires come from one point instead of them being chained.

Those things are analog. The noise will come from the PWM, not the device.

It concerns me that the description for those constant current chips specify a "drop-out" voltage of 2V - reminiscent of the LM317 circuit - so you are restricting yourself if you propose to use a 5V supply.

The ever-faithful (and dirt cheap - about ten cents) two transistor, two resistor circuit which includes the switching transistor - so you can happily use it for more than 20 mA, or 20 mA from multiple chip pins - has a drop-out of less than one volt.

And clearly you cannot use just one current limiter for all three elements in an RGB LED. If the red LED is lit, the others will never be.

Thanks for that circuit Paul! I am sure I will be using that later.

For now, I thought I would pass on another idea that was posted in a different thread, as it gets me closer to my ultimate goal, which lowering the power used by all my components, including the LED's

--- user teckel writes----

I typically attach an LED that blinks every minute so you know it's still working. The trick with the LED to use the least amount of power is to use a very bright LED, use a larger resistor to limit the current to maybe 1mA, and use the built-in sleep of the ATmega8 for the time the LED is on. In other words, turn on the LED (only using 1mA) put the ATmega8 to sleep for 15ms, wake back up, turn off the LED, then return to long sleep. This uses less power because it's only using 1mA for 15ms instead of leaving the ATmega8 awake for 5ms during a shorter blink and consuming more than 10mA.


Now I have two approaches to try.