I want to control 61 SMD 3528 RGB LEDs (KT-SMD3528-RGB) with 3 arduino Micro's 3 PWM pins. I learned from reading up that I'll have to use 3 NPN Transistors to control RGB LEDs and external power source.

So I decided to go with external 5V power supply to power up those LEDs and since I have 61 of them and I'm noob in calculating power requirements I would like to ask a question of how much amperage draw I'll need a power supply

I have 3528 RGB led which draws roughly 25mA on each channel, meaning I'll need 25*61 = 1.5 Amp to power up each channel. Does this mean I'll need 3*1.5=4.5 Amp 5V power supply?

Also regarding resistors as I understand some channels have more Vd than others (I haven't measured them yet as I'm still waiting for shipment to arrive) but according to sellers, specs are 1.8 to 3.5 Vd on those SMD LEDs. So for example to calculate resistor I need for 1.8 Vd I need to do math like this (5-1.8)/0.025 = 128 meaning I'll need at least 128 Ohm resistor for each led channel. Is my logic correct?

Also regarding NPN transistor for each channel I need a transistor that could sustain at least 1.5 Amp is this also correct?

  • Most 3528 leds require 20mA. The 25mA is the maximum (which you should not apply to them). Do you have data or a link for those leds ? Mosfets are easier for high currents. Most mosfets require 9V to 12V at the gate, but a "logic level" mosfet can be used with the low voltage of 5V or 3.3V of an Arduino board. sparkfun.com/products/10213 The resistor does depend on the forward voltage drop of the led. That is correct. You can buy neopixel or other programmable ledstrips that do not require to add resistors, only a library (FastLED or NeoPixel) and a digital pin and a power supply. – Jot Nov 1 '17 at 10:51
  • Unfortunately i don't have any data specs for those leds and the only thing I know is that they have Vd of 1.8 to 3.5 and are made by company called Kento and model is KT-SMD3528-RGB however googling this name didn't gave me anything. Now regarding mosfet from your link I saw that those have maximum of 60V 30A. As I have only 3 PWM pins available I want to control all 61 leds with those 3 pins so I'll have only 1.5 A on each channel so those mosfets will do right? – Troydm Nov 1 '17 at 12:03
  • Also I can't use led strips as I'll have those leds mounted on custom PCB (for USB keyboard actually) so each 61 key will have it's own led however I only need to control the color of all leds to be same for all 61 keys – Troydm Nov 1 '17 at 12:03
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    High levels of currents can cause heating tracks on the PCB. They should also be broad. It is better to use the serial connection of the LEDs. 6-pins LEDs, for example, KT-5050-RGB-X, are suitable for the serial connection. The same manufacturer. – AltAir Nov 1 '17 at 15:23

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