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Newbie here, I am working on a project with a panel of 12 - 15 LEDs(std 5mm through hole LEDs) in paralell. I am currently using a 5V 1A SMPS. I used 330 Ohm resistors for current limiting. It works great! (Schematic in first figure). [enter image description here]

I know using resistors for current limiting is not efficient, moreover I have no control over the brightness of the LEDs.

Now, I want to make upgrades to the panel:

  1. Use a driver IC to save power
  2. Use a POT to control intensity of LEDs1
  3. Do the above 2 points without a Micro controller or a timer IC (Tomake it as simple and cost effective as possible).

Is there any such driver IC(powered by a 5V 1A SMPS) that can take input resistance from a POT and drive 15 RED leds by controlling the intensity of brightness..

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  • \$\begingroup\$ To be clear, you want to control brightness of the whole bank but do not require individual brightness control(which would require extra circuitry)? \$\endgroup\$ Commented Sep 25, 2018 at 7:00
  • \$\begingroup\$ Current limiting resistors can be very efficient. With your setup if you reduce the power supply to 2.5 V, you'll get about 88% efficiency using LEDs with a Vf of 2.2V \$\endgroup\$ Commented Sep 25, 2018 at 7:41
  • \$\begingroup\$ @K H, I want to control the brightness of all the LEDs at once \$\endgroup\$ Commented Sep 25, 2018 at 8:08
  • \$\begingroup\$ Can we get a 2.5V SMPS readily in the market for cheap?? Also if I were to design an SMPS that outputs 2.5V how do we control the brightness of the entire led array \$\endgroup\$ Commented Sep 25, 2018 at 8:57

2 Answers 2

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How efficient a solution with resistors is depends mainly on the input voltage. So your solution with 5 V power supply and red LEDs is around 40 % efficient (2 V over the LED assumed).

Most of the LED drivers I know of, use a single string of series connected LEDs and regulate the current through the string. With 15 LEDs you are looking at around 30 V (look up the numbers in the datasheet for the forward voltage). So that is the required output voltage of the driver.

With a 5 V input, you need a step up LED driver to make that work. Depending on the driver, the low current you want might be a bit tricky to achieve (around 10 mA). You probably have to calculate the duty cycle of the driver and work out the correct value for the inductor, which might require some fiddling around with the numbers.

The formulas are given in the datasheet usually. Some drivers come with a spreadsheet which allows you to easily calculate the values.

A lot of the drivers come with a dimming input which works analog or with a PWM. You can use the analog dimming with your idea of the potentiometer.

For example the PAM2841 might fit your bill. But there are hundreds of drivers out there and some are probably even better suited for what you are doing.

With this you might be looking at around 80 % efficiency. Beware though, that switch mode power supplies can cause havoc if you do not follow the layout considerations closely. They can be very good at producing electromagnetic interference.

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  • \$\begingroup\$ Thanks Arsenal for your wonderful solution, Why not connect all in paralell, and use a buck converter to reduce the voltage output and use PWM to adjust the dimming (by controlling the frequency and duty cycle) This may be cheaper and easier to obtain... \$\endgroup\$ Commented Sep 25, 2018 at 10:17
  • \$\begingroup\$ @AkshayGs because you cannot control the current through each LED that way. The variance in forward voltage will cause some LEDs to draw more current and other to draw less current. That might cause some LEDs to fail prematurely. Might be okay if you don't need to run it very long or difference in brightness is no concern. \$\endgroup\$ Commented Sep 25, 2018 at 13:59
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The simplest way is to connect the LEDs in series.
Power them with a Mean Well HLG-40H-36B CC LED driver.
This driver has 88.5% efficiency. Cost ≈ $28.

Connect a 100K pot to the blue/white dimmer wires.
Use a smaller pot if the LEDs cannot handle 1.1 amp.
A 10K pot would allow a maximum of 110 mA.

enter image description here

I made a PCB with a pot and dip switches to set the current.

enter image description here

Cree XPE red

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Powered with a Mean Well HLG-40H @ 700 mA

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As seen from my lab window at night @ 300 mA.

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