Hi guys,

Quick question about RGB LED Strips.

I'm trying to understand the Voltage input and the PWM signals being output to the transistors and into the LED for Red, Blue and Green.

Here's an image of the circuit;

Referring the LED strips, I see PWR(+12V) and R, G and B.

  1. Now, is the individual LEDs being supplied by the 12V? Or from the transistors?

In this diagram, it looks like the 12V is supplying the current to individual LEDs which makes sense. Now there is the transistor output where it is controlled by PWM from the micro controller. I'm aware the PWM adjusts the brightness of each R,G,B colour, but so does the 12V control it aswell? I am a bit confused.

  1. Where is the ground for the LED strip?

Thanks in advance.

The strip has 12V supplied to it at “PWR”.
Each transistor, when turned on, provides a path (a path for current) to Gnd.

  1. Now, is the individual LEDs being supplied by the 12V? Or from the transistors?

From the way you have drawn it the LED's are powered from the 12V (PWR) line.

  1. Where is the ground for the LED strip?
    Once again, from the way it is draw the R, G & B lines are the ground outputs from the strip. The transistors (they need a resistor between base and the arduino pins) are turned on/off by the Arduino. When a transistor is on it allows a route to ground for the R,G,B output from the strip and the LED's light up. When the transistor is off the route to ground is blocked so the R,G,B LED's turn off.

You need a resistor between each Arduino output pin and the base of your transistor.

Thanks for the reply. It makes total sense now.

I do have another question that I need clarifying. I’m aware that the BJT transistor are current controlled and the MOSFET are voltage controlled.

Regarding this LED strip application, say for example I am supplying the system with a 12V,5A power adapter. Say for example the LED strip is 1M long, so that sufficient.

My question is what controls the current flowing through each LED(R,G,B)?

Based on what I have read, the PWM from a microcontroller will produce a logic gate to the Gate of the MOSFET. By analogwrite(0), there will be a high resistance at the Gate/Source so there wont be any current flowing from Drain to source. However when analogwrite(255) is written, it’ll turn on at 100% of the time so theres current flowing from Drain to Source.

With the maximum output current say for example for the Arduino Uno is 40mA, does that have an effect on the current flowing through the LED?

Does MOSFET amplify any parameters?

Because there has to be something limiting the 5A from the supply through the LEDs. Is the voltage across the LED limiting it?

Sorry for it’s confusing.

The resistor in series with the LEDs in the strip are what limit the current. If you supply too much voltage, that will overcome the current limit anyway.
(Vs - Vf - Vf - Vf)/Rlimit = current.
If 12V, 2.5V Vf, and 150 ohm resistor, then:
(12V - 2.5V - 2.5V - 2.5V)/150 = 30mA.
As long 12V - (3 x Vf) >0, the LEDs will turn on with varying levels of brightness.

Using parameters you mentioned with Vcc = 12V and Vf = 2.5V, it'll draw a maximum of 30mA.

Since the MOSFET will be controlled by a PWM where it'll allow current from the Drain to Source, does that mean the maximum current that can flow from drain to source is 30mA (100% Duty Cycle)? And if the PWM is set to 50%, it'll allow 15mA through?

At 50% duty cycle, the current will be 30mA for 50% of the time and 0mA for 50% of the time, so 15mA on average. The leds are in fact always either full on or off. Its the human eye that perceives the dimming effect, because the switching is too fast to see.

Why post a circuit with transistors and then start talking about FETs? That is a great way to piss people off. It is like talking about driving a car and then much later on saying oh by the way it is a motorcycle I am driving. Similar but not the same.

As mentioned current is limited by the resistors in the strip, and a FET or correctly used transistor "insulates" the use of current out of the Arduino from the use of current in the strip.

Does the FET amplify anything, sure - the action of the output. It converts a voltage switch into low resistance. In the real world the leakage current on the gate is amplified to the switching current you use. In true theory it is a control not an amplification.

Thanks PaulRB. That cleared things up.

"Grumpy" Mike, you really do live up to your name. My intentions were not to piss off anyone but not living in the ideal world, it'll happen.

The circuit I posted were initially for a representation that I can refer to regarding the transistors. However throughout the post, I concentrated more on FET's and I can see why it can be confusing when people are reading and trying to help. I'll make sure I'll be clear as possible next time and thanks for input Mike.