600 lm Color LED bulb

Other Hardware LEDs and Multiplexing
1

Hi - I'm building a ~600 lm color LED bulb prototype controlled by Arduino. Using Phillips Hue as a reference, I ordered a bunch of ~60 lm Cree LEDs in Red-Orange, Green and Blue.

LEDs: Red, Green, Blue

I'm very new to electronics/hardware world (I've been doing software most of my career) but thanks to the internet and the forums here, I'm learning. Using tutorials, I used a few resistors out of the set and created a dim LED circuit with LEDs out of a flashlight. I want to keep this experiment as cheap and easy as possible. It is a small part in a larger software prototype but it is fun to learn electronics.

I read a few posts on this forum that I would use transistors to control the power through the board and that Arduino won't be enough to power the bulb on its own. Can you give me few directions on how to go about building this? Maybe initially using 3 of the LEDs to try out first and then blowing it up to 600lm?

2

Hi and welcome.

Bad news. High current leds like those can't be used in the same way as "normal"/5mm leds. Transistors won't really help.

The two problems are (1) current limiting and (2) heat dissipation.

With ordinary 5mm leds, you have a series resistor designed to drop some of the supply voltage or output pin voltage down to what the led needs. Because the current involved is small, e.g. 20mA, this is no problem for the Arduino pin to supply, and does not generate any noticeable heat in the led. The series resistor will also be dissipating some heat, but again this will not be noticeable at these low currents, so 1/4 Watt resistors are normally fine.

With these high current leds, the heat problems become much more serious. The led itself must the soldered to a large heatsink, or it will very quickly die.

The second problem is the series resistor that would be needed for such a large current. A series resistor would need to drop 2.9V (assuming a 5V supply), so the resistor value would need to be around 5 Ohms for 700mA current. The resistor would need to dissipate around 2.5 Watts.

In theory, you might be able to find such resistors, but they would be large & expensive and still get quite hot. Your power supply would also need to be large and provide a lot of current, most of which would simply be wasted as heat by the series resistors.

Unsurprisingly then, series resistors are not used with high current leds. Instead, a circuit known as a "Constant Current Source" is used, which adjusts its output voltage to ensure just the right amount of current flows through the led, without needing a series resistor and wasting masses of power.

So you need 3 of these constant-current led drivers, one for each led, and you need a design that has a PWM input that you can connect to the Arduino pins to allow you to fade each colour.

Hope this helps.

Paul

3

PaulRB:
Unsurprisingly then, series resistors are not used with high current leds. Instead, a circuit known as a "Constant Current Source" is used, which adjusts its output voltage to ensure just the right amount of current flows through the led, without needing a series resistor and wasting masses of power.

Hang on now!

A "Constant Current Source" as such, wastes just as much power as a resistor. It may be practical then to source it from a lower supply voltage, nearer to that of the LED itself, if it is a decent design, and save some power.

The only thing wasting less power is a current-driving switchmode converter. Perhaps what you meant, but did not say.

4

Need part like this

This one is shown with full wave bridge rectifier to convert AC to DC, but will work from DC only as well to control 1A into high current LEDs.

See Figure 30 for a low component count application.

This one should be easy to use also
http://www.digikey.com/product-detail/en/ACT6360US-T/1488-1053-1-ND/4990878

5

Paul__B:
The only thing wasting less power is a current-driving switchmode converter. Perhaps what you meant, but did not say.

Paul, you are right. I was making the assumption that the led drivers I have seen on eBay etc. are all switch mode, but they might not be, especially the cheapest ones, and if so would be just as wasteful.

6

My impression is that in general, there are two sorts of LED drivers offered. Those that convert mains 100-240 V to a LED supply for 3, 10 40 W etc. would be switchmode, but those for a low voltage DC supply would generally be current controllers.

7

I guess I was remembering this kind of module. Problem is, it has no pwm control input, and I don't know if it would operate correctly if you attempted to switch the input or output power with a transistor.

8

PaulRB:
I don't know if it would operate correctly if you attempted to switch the input or output power with a transistor.

I can guarantee it will not.