What AC/DC adapter do I need to power 10 high-power (3W) LEDs?

Question

I'm attempting to build a near-infrared (850 nm) lamp using high-power LEDs and am struggling to design a suitable way to power them directly from the AC power plug (220V).

I have 10 high-power LEDs with the following specs:

VF = 1.4 - 1.8 V;

IF = 400-700 mA.

I was thinking about powering them with a simple USB phone charger (5V, 1A) as in the following diagram:

^{simulate this circuit – Schematic created using CircuitLab}

In this setup, each LED should receive 1V (slightly lower than the specified 1.4V in the datasheet) and 500 mA current (within the LED's current range). However, I'm uncertain if this is the most appropriate configuration. I wonder whether I should use a different adapter (e.g. I have a 12V-2A available but that is probably too much). I have two main doubts:

1. Is it a problem to supply less voltage to each LED (1V instead of 1.4V)? Should I consider reducing the number of LEDs?

2. I've seen that it's good practice to insert an additional current-limiting resistor in series with high-power LEDs (even though this will cause further voltage drop). What resistance should these resistors have?

I'm open to the possibility of regulating the brightness of LEDs, for example, with a potentiometer downstream of the current source, but I guess this would require changing the driver circuit. I'm just starting in the field of electronics.

Answer 1

It’s not hard to find constant current supplies (drivers) that are designed specifically for LEDs. Lighting LED arrays often run at 30-50V so you’d use all 10 in series (18V). Some supplies have a 1-10V dimmer input that can be controlled using an external potentiometer.

Answer 2

You would be better off with the 9 V @ 2 A supply. At 1.8 V each 5 LEDs will be 9 V, and you'd have 1 A available for each string, which should be enough current with a bit to spare assuming they take no more than 700 mA.

Figuring 1.4 V @ 400 mA as the low end that's 7 V for 5 LEDs, so a 5\$\Omega\$ resistor would drop the 2 V. That should be the most resistance you'd need.

I don't think you'll have enough compliance voltage for a current regulator, you'd either need a higher voltage, or change it to 5 strings of 2 LEDs which would require more current, and more losses.

If you can post a spec sheet or part number for the LEDs I could take a look at that and get a better idea of what current at what voltage you can expect from them, and maybe simulate it.