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I'm putting together a project where I'll be driving 4 x 10W RGB LEDs (these ones) with one PicoBuck LED driver. I'll be using all three channels on the LEDs at the same time and controlling dimming with an Arduino Uno using PWM. I'll be using heatsinks on each LED.

My questions:

  • What power supply should I be looking at using for the LED driver and therefore the LEDs? I'll be honest that I'm a bit confused about the voltage requirements of LEDs - is it enough to use a DC power supply that supplies in this case at least around 40W and at least 350 mA current, regardless of its voltage? For example could I use a 19.5V 65W laptop DC power supply, of which I have several to hand? (for reference, I derived 40W from the maximum forward voltage of the three RGB channels added together (7 + 11 + 11 = 29) multiplied by the number of LED chips (4) multiplied by DC forward current of each channel (350mA) = 29 x 4 x 0.350 = 40.6)

  • Do you think I can achieve this project with just one of these PicoBuck LED drivers, if the four LED chips are connected in series (diagram attached)? The PicoBuck uses the Al8805 chip, a high efficiency 36V 1A buck LED driver. It says that "the device can drive up to 8 LEDs, depending on the forward voltage of the LEDs, in series from a voltage source of 6V to 36V.". Does this mean that if I am using 4 RGB LEDS, this is considered as 4 LEDS, or would it be 12 LEDS, considering each LED has 3 channels with separate power? If the latter, there is the possibility of dropping the number of LEDs to 3 for this project.

  • I was looking at using a heatsink for each LED with a thermal resistance of 5.8°C/W - would this be suitable?

LED specs:

  • Forward voltage at 350mA:
    • red 6.0 min 7.0V max,
    • green and blue 9.0 min 11.0V max

DC forward current: 350mA each channel

EDIT: the setup will be outside in winter, so we're looking at average maximum 9.0 °C | 50 °F and minimum - 1.0 °C | 30.0°F (at least round these parts)

Circuit diagram

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  • \$\begingroup\$ These aren't LEDs, they're assemblies with a number of LED chips inside. If you look at the photos of the "LEDs", you'll notice they have three LED chips in series for each color, therefore they count as 3 LEDs. This is coherent with the stated Vf for each color, red being around 2V per LED, so this assembly gets 6V Vf on the red. Blue is 3.3-3.6V, that's pretty close to their 9-11V for the blue part of the assembly. \$\endgroup\$
    – bobflux
    Oct 8, 2021 at 17:50

2 Answers 2

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You're trying to use a Buck (down) converter to drive 4 series 11v LEDs at 350mA. That means your drive voltage is ~4*11 = 44v. Your driver is rated for a max of 36V, so this isn't going to work. You need to either put fewer diodes in series so that you stay below the maximum voltage of your driver or get a driver that runs at higher voltage.

As for your original question, a Buck converter can only decrease voltage. I'm not familiar with the PicoBuck, but it sounds like a Buck converter. That means if you use a 19.5v supply you'll be limited to less than 19.5v maximum output voltage.

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1. What power supply should I be looking at using for the LED driver and therefore the LEDs?

If you take a look at the datasheet to the AL8805, it shows the following input voltage versus efficiency chart:

AL8805 Input Voltage vs Efficiency

This chart assumes lower forward voltages than the LEDs you are using, but in general, it is more efficient with lower input voltages, as long as there is sufficient voltage for the number of LEDs being driven.

The datasheet for the LEDs shows the following tables for their forward voltage and current:

Forward Peak and Average Current

Forward Voltage

In series, this means that the maximum voltage for each color will be:

  • Red: 7 * 4 = 28 V
  • Green: 11 * 4 = 44 V (!)
  • Blue: 11 * 4 = 44 V (!)

The driver states that its maximum output voltage is 36V, so you'll only be able to connect three in series instead of four.

The min voltage of the green and blue channels is given as 9 V, which in theory means you'd be able to connect four in series for a total of 36 V, but LEDs vary in manufacturing, so if one or more of them happen to require more than 9 V, you'll likely see brightness differences between them and/or the driver will have trouble maintaining a steady output.

Usually you want to calculate the power required based on maximum values and add about 20% for safety and overhead.

If you connect three in series, the output power will be:

  • Red: 7V * 350 mA = 2.45 W (* 3 = 7.35 W)
  • Green: 11V * 350 mA = 3.85 W (* 3 = 11.55 W)
  • Blue: 11V * 350 mA = 3.85 W (* 3 = 11.55 W)
  • Total: 7.35 + 11.55 + 11.55 = 30.45 W

The efficiency of the driver is >90% so we'll add another 10% on top. Therefore I would look for a power supply that can provide 30.45 W + 30% = 39.585 (40 W). A 24V power supply is relatively common, so if it were me, I'd maybe consider a Meanwell RS-50-24 for <$20 USD. NOTE: This isn't an outdoor-rated supply, so you'll need to add your own weather protection solution or use an outdoor-rated supply. Since the driver is a buck-only regulator, the input voltage will need to be a minimum of the highest required LED channel (but don't exceed 36V which is the maximum for the AL8805). I'd consider the Meanwell OWA-60U-36 which is approximately $35 USD. It's a sealed plastic case and the datasheet indicates it is suitable for damp locations from -35°C to 75°C.


2. Do you think I can achieve this project with just one of these PicoBuck LED drivers, if the four LED chips are connected in series (diagram attached)?

In short, no, based on the max voltages stated above. You could try it, but if the sum of the forward voltage drops are >36V, the driver won't be able to provide the necessary compliance voltage.


3. I was looking at using a heatsink for each LED with a thermal resistance of 5.8°C/W - would this be suitable?

Using an LED heatsink calculator like this one, any heatsink with a 9.7°C/W or lower should work fine. I used the Thermal Resistance Junction to Board value of 10°C/W from the datasheet, an ambient temperature of 25°C (I realize you mentioned a max of about 9°C in winter, but standard "room" temperature gives a good margin), and the default/suggested values for the mounting base and heatsink attachment (6 and 4.5, respectively). The resulting calculation was 9.7°C/W, and your 5.8°C/W heatsink being below this value exceeds the requirement.

Disclaimer: I'm not an expert on heatsink calculations, thus why I used an online calculator. I recommend asking this as a separate question if you'd like more/better info.

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  • \$\begingroup\$ If the AL8805 that the PicoBuck uses is a step-down DC/DC converter, would I not need to use an input power supply with a voltage greater than the maximum combined drive voltage of the most demanding channel on the LED (the green and blue channels - 11v) - as @user1850479 mentioned? So, if I'm using three LEDs in series, it would be 3 x 11v = 33 v? And if this is the case, would I be better off using two PicoBucks, each driving 2 x LEDs, so that I can get away with using a 24v adapter on both PicoBucks? (Thanks so much for the reply by the way! Very clear and enlightening) \$\endgroup\$
    – EKB
    Oct 8, 2021 at 8:46
  • \$\begingroup\$ Oh that's absolutely right. Sorry, I was thinking of it as a buck/boost converter. So yes either you'd need to use a 36V supply, or as you said just use two LEDs per channel. I'll edit my answer accordingly when I am next able to. \$\endgroup\$
    – JYelton
    Oct 8, 2021 at 8:55
  • \$\begingroup\$ @gtb I've updated my answer and linked a different power supply. \$\endgroup\$
    – JYelton
    Oct 8, 2021 at 17:08

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