# Driving constant current LEDs with constant voltage?

I have about 6 pieces of bar leds in different colors (all of them are 12V, but different power 18W, 12W, 9W ...) I need to connect them in parallel by regulating 24V, but the problem is that the LED with the highest current(18W-Red) is the brightest, but the other LEDs(12W-Blue and 9W White) draw less current, so the brightness is lower.Waiting for your suggestions.

• (1) How are you going to regulate the voltages required? (2) What is your specific question (rather than "waiting for your suggestions")? Commented Jun 30, 2019 at 15:31
• A simple but very wasteful way to power from 24V. Treat each module as a single (12V LED) and calculates a series resistor for each module. The resistor will dissipate about the same as its associated module. Unless you use a switching CC regulator for each module all answers will be linear (power inefficient). Commented Jun 30, 2019 at 15:41
• (1) I want to provide regulation with LM2596. (2) I want all LEDs to illuminate at maximum brightness, but only the red LED (18W) is illuminated at maximum brightness. Other LEDs (blue-white) give very little light. Commented Jun 30, 2019 at 15:44

I have done this several times with about 60W of LED arrays even different voltages and currents using universal 4pin:barrel adapter 19V Laptop chargers.

The trick is to define the voltage AND current that you want in each string then add LED or power Rs to RED to match current with others, at desired brightness level with applied constant Voltage. Balance is the key with good thermal design!

## Other

I had White, Yellow, Orange, Red using 1W LEDs for each used on metal clad boards for most, but I wanted light to be dynamically changing from Sunrise to White light brighter than sunlight then dim to Sunset colours Red lower wall, Orange to Bluish black ceiling.

By knowing the voltage thresholds of each colour string or array xSyP, and effective series resistance at rated current Rs~1/2Pd +/- xx% I chose how to do this with string voltages at threshold and full brightness and adjusted LED string n*Vt + IRs to get net string Vf @ If for each. (Vt= threshold)

• it worked and my wife was impressed.

• the trick to dimming the charger lied in the 4 pin circuit using some 1.25V constant voltage feedback to compare with end of cable voltage to regulate constant voltage with thin cable loss at 4.5A

• my solution used a pot, cap resistors and a blue LED as power indicator somewhat like regulating an LM317 except it was a Buck- Boost regulator.
• it could be boosted to ~50V with no feedback but I worried about cap ratings, so I used pot plus resistors to tune from 12V to 24V to get the full range of sunrise to sunset. No photos handy now, but here’s another example using 4x1W strings all in parallel ( and well matched by accident and good Cree parts , then AWG 18 Speaker wire to allow for voltage drop from front 200W rectified 12V transformer (Schlumberger) formerly used for front porch 16x8W tungsten wood staircase lamps (replaced by LEDs) to,give 16V DC at source and 12.5V at end of fence on 4S // 8P string 1Wx4 LEDs about 20m away to back yard fence. Luminaries were simply green fence painted Cedar open box cut, glued and nailed together for no glare results.

My dim level before OFF looked like sunset using ceiling projection with chrome egg-crating painted black to block glare and cast down on a sun shaded by huge tree livingroom rear bay window.

• You've got my snow, you bum. Give it back! We used to have good snow here every year -- often three or four times with one of them at almost a meter depth each year. Silver thaws perhaps every two years. But since 2003 perhaps only twice have I seen a meter. Global warming has also reduced the 11 glaciers on Mt Hood by more than 50% mass-balance in the last 30 years, as well. Cripes, now we actually can grow palm trees here!! Can you believe it? There's a business that specializes in palm trees just walking distance from my home. Geez, I miss the snow.
– jonk
Commented Jul 1, 2019 at 5:37
• @jonk if you like I can bring some snow in a couple weeks as we drive out towards the “right Coast”. I get it anytime I need a kiddie pool full of beer and snow. ( actually from Zambonie machine near skating rink), let me know and I can drive it over there. My palm tree is struggling. Commented Jul 1, 2019 at 16:24
• TOday is “canada is always great” day Commented Jul 1, 2019 at 19:54