Help in choosing correct power supply for LED matrix

Question

I'm quite new to electricity and electric components. I'm building an LED matrix from scratch on a small PCB. I will be using an IC for multiplexing, Transistor for controlling external power supply besides Arduino, resistors and of course the LEDs.

Now, my general question is - power supply of what type(constant-current vs constant-voltage) and what values(Voltage + Amps) should I buy?

My PCB with already soldered LEDs looks like following:

• 3 parallels of - 5 3mm blue LEDs in series. Forward Voltage 3.1V; Current: 20mA - each.
• 3 parallels of - 5 3mm white LEDs in series. Forward Voltage 3.5V; Current: 35mA - each.

Now, after reading tons of articles on Current, Voltage, power sources and so on, there are still questions to which I don't have definite answer. I want to understand the core of all these, so I have following questions:

1) Can I power all 6 parellels, meaning whole PCB with one power supply? If I have understood correctly, I can not. I will need a separate driver for each of the LED color. Correct?

2) How should I calculate the driver values. In my understanding - 5(LEDs in series) x 3(parallels of blue) x 3.1(V each LED) = 46.5V. For Amps, should sum up the values of all 15 LEDs also, or should I calculate by just one LED as the Amps are same across the circuit? To sum-up, I will need a driver of 46.5V / 20mA?

3) Should the same calculation be used for white LEDs, and then two separate power sources connected together somehow?

Please help me to understand not just the values of the power source, but also generally the idea behind the topic. Should I be considering something else into my calculations? Any advice for a better design? Thanks a lot.

^{simulate this circuit – Schematic created using CircuitLab}

Answer 1

Using the following info: three parallel branches of five 3mm blue LEDs in series with Forward Voltage 3.1V; Current: 20mA - each. And three parallel branches of five 3mm white LEDs in series with Forward Voltage 3.5V; Current: 35mA - each.

To determine voltage of your power supply would require looking at the max voltage for each parallel branch. For the blue LEDs, we have 5x3.1V=15.5V and for the white LEDs we have 5x3.5V=17.5V. So since 17.5 is the highest we’ll have to look at that. Since you’ll be needing current limiting resistors, you need some headroom voltage. I’d say 19V Would be your minimum voltage of your power supply.

As for your current specification you’d sum up all currents to determine minimum current. This being I_total=3x20mA + 3x35mA=165mA. You’ll need a power supply with this current plus any other current drawn from your circuit. If this is the only thing being powered I’d say a supply with 200mA current capability would be fairly sufficient.

Answer 2

Before building a matrix of LEDs I highly recommend you to build a circuit to drive one single led, then a circuit to drive leds in series, finally a circuit to drive leds in parallel. That way you will know how it works, it will help you a lot because it is an elementary subject in electronics.

Answering your questions.

1. It's possible with only one power supply.
2. 46.5v would be excessive, you're calculating it wrong. If all the rows of leds are going to be in parallel, just calculate the voltage of the row that needs more voltage, in this case the row with the white leds.
3. No need of two power supplies or two calculations.

Good luck!

Answer 3

Post-Edit Answer

My PCB with already soldered LEDs looks like following:

• 3 parallels of - 5 3mm blue LEDs in series. Forward Voltage 3.1V; Current: 20mA - each.
• 3 parallels of - 5 3mm white LEDs in series. Forward Voltage 3.5V; Current: 35mA - each.

No. What you have are 6 parallels of 5 parallel LEDs. Nothing is in series. This will not work well because there is no way to enforce the current through each individual LED. You should never solder anything until you have a completed design and know exactly what you are doing.

I'm going to keep and include my pre-edited question answer, because it describes how to drive what you actually said.

Pre-Edit Answer

You haven't included how you plan to control the LEDs, so I didn't include anything in the schematic. I would recommend you research an N-Channel MOSFET switch which can easily turn your strings off and on.

One power supply can handle it all; you will just use different value current limiting resistors. On that note, you don't have to drive and LED at the rated current; I only do for illumination related projects, and then I overdrive them.

• Blue LED String Voltage: 5 x 3.1V = 15.5V
• White LED String Voltage: 5 x 3.5V = 17.5V

So you need an 18V rail for those White LED strings. I would highly recommend you redesign so you can use a lower voltage power supply, like 12, which is much easier to come by. Use more parallel strings with fewer LEDs in series.

• Voltage across R1, R2, & R3 = 2.5V. For 20mA, use 125 ohm resistors.
• Voltage across R4, R5, & R6 = 0.5V. For 35mA, use 15 ohm resistors.

Total Current = 3 * 20 + 3 * 35 = 165mA. If you add enough for your microcontroller plus some overhead, you would need an 18V, 250mA power supply.

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All that said, it seems you need to look up the fundamentals of electricity, specifically

1. Ohm's Law: Electromotive Force (Volts) = Current (Amps) * Resistance (Ohms) -> V = I*R
2. Kirchhoff's Laws: how voltage and current are shared/split in series and parallel connections.
3. Electrical Power (Watts): P = I*V = I^2 * R = V^2 / R