I have 4 constant current led drivers with PWM. Here's 300-1500mA LED Driver datasheet.

I want to use 3 of these drivers to power each color of a 90w rgb led. I was under assumption that I could connect the positive output of the driver to the individual colors of the led and the negative output to the shared negative connector of the led.

What happened when I tried that was a puff of smoke and one destroyed driver (not output voltage). If I connect negative output wire of the driver to individual negative led connectors everything works as expected.

What am I missing here? The main reason I need shared ground is because PWM flickers if arduino that I use for PWM is not on same ground as the led driver.

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    \$\begingroup\$ Adding a schematic and LED part number would be helpful... Are you referring to a constant power supply ground, or a constant negative LED return, because with that type of driver, both leads of the LED can must go to their individual returns. \$\endgroup\$ – Kurt E. Clothier Mar 26 '14 at 23:41
  • \$\begingroup\$ I am referring to constant driver led return as negative (not input side negative). There is no spec provided for led and it should not matter as like I said led works when ground is not shared. \$\endgroup\$ – DominicM Mar 26 '14 at 23:52
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    \$\begingroup\$ your question is very unclear and horribly written, consider revising. \$\endgroup\$ – hassan789 Mar 27 '14 at 1:12
  • \$\begingroup\$ Question edited, not sure what else I could change specifically to make it clearer. \$\endgroup\$ – DominicM Mar 27 '14 at 1:19
  • \$\begingroup\$ Again, when you say "if arduino that I use for PWM is not on same ground as LED driver" it sounds like you mean the negative power terminal "ground" which is true- it must be shared for this to work correctly. With this type of driver, you CANNOT share the negative LED return (which is not "ground" anyway). If you can't make a schematic, open MS Paint and draw a picture there and post it. I (and I'd imagine others) really don't understand what you are struggling with. \$\endgroup\$ – Kurt E. Clothier Mar 27 '14 at 2:00

As expected, the answer is right there in your datasheet. Have a look at the schematic on page 8 (document page 5):

LED Driver Schematic

Do you see the "LED+" and "LED-" labels on the right hand side? Those correspond to the LED '+' and '-' terminals on the driver board. A constant current driver works by monitoring the feedback across a resistor, in this case "RSENSE," and changing the output voltage accordingly. You CAN connect multiple LEDs in series or parallel in between these leads, they would either share or split the output current. However, you CANNOT connect the negative terminal of one LED to one driver and its positive terminal to another driver. If you are using three drivers for an RGB LED, it CANNOT be common anode OR common cathode unless you want to put all three LEDs in parallel on the same driver, which would be pointless. You have to have three independent LED anodes and cathodes, one pair for each driver.

You CANNOT have a "shared ground" between your LEDS with this driver. You are confusing the LED ground (common cathode) with that of the power rail ground. They are NOT the same thing unless the LED is being driven some other way and its cathode is tied to the negative power rail.

Also, you mention in your question:

connect the positive output of the driver to the individual colors of the led and the negative output to the shared negative connector of the led

which would imply common cathode (shared negative), not common anode (shared positive) as you mentioned in a later comment. It seems as though you have absolutely no idea what you are doing or talking about... which is OK, it just means you need to learn to research these things on your own. It's hard to help if you don't even know how to ask an appropriate question.

  • \$\begingroup\$ The anode comment was an error, it is actually a common cathode. You're right I did confuse led and driver ground. I wanted this driver so I wouldn't have to deal with too much electronics but this tripped me up still. Seemed to me like like ground is ground, didn't think about how cc driver works internally :) That answers my question, thanks! \$\endgroup\$ – DominicM Mar 27 '14 at 3:09
  • \$\begingroup\$ I was actually connecting arduino ground to the cc driver ground (not led negative) and flickering was gone, not sure why I convinced myself I needed the led to be a cathode (contacts can be cut making it essentially 3 leds on one heat sink). I guess I saw it done like that too many times before... \$\endgroup\$ – DominicM Mar 27 '14 at 3:19
  • \$\begingroup\$ No problem, just try to clarify what you are doing better next time so it's easier to help. \$\endgroup\$ – Kurt E. Clothier Mar 27 '14 at 5:07

The led driver you are using requires a current limit resistor at the ground end of the led. This means your led cannot be grounded. Read the data sheet. There's a circuit that shows the current limit / sense resistor. See figure 5.1 in the data sheet.

If your RGB LED has a common anode or common cathode connection you can't use this driver without jumping thru hoops.

  • \$\begingroup\$ If the datasheet answered my question I wouldn't have asked it here :) Can you elaborate a little more regarding this design? My led is common anode but I can cut the contacts and use the colors as separate leds. Issue with this is that arduino needs to share ground or the led will flicker, any ideas on how to get around this? \$\endgroup\$ – DominicM Mar 27 '14 at 0:09
  • \$\begingroup\$ Again, a schematic and LED part number would be beneficial. \$\endgroup\$ – Kurt E. Clothier Mar 27 '14 at 0:32
  • \$\begingroup\$ Like I said there is no led part number, it's a cheap ebay item with basic info only. A schematic would take me far too long to do as I am not very familiar with electronic design. \$\endgroup\$ – DominicM Mar 27 '14 at 1:00

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