Power LEDs.... HELP!

[skillz21]

STORYTIME!!

So I have this 1 watt LED that I power directly from the 3.3v output of my Arduino Uno. It draws.... let me check.... Oh, sh*t the 3.3v output is broken... (it drew around 300mA) let me explain. I got this 20 watt led in the mail today and I connected up one of these buck-boost converters to my Arduino, set the output to 32+ volts and hooked up the LED... It worked, but I was dismayed to find that it only drew around 80mA from the output.... WHAT???

It was also a bit brighter than the 1 watt led, and it didn't heat up at all (of course it didn't, it was only drawing 80mA for god's sake). I checked the input of the converter and found that it was drawing around 800mA. Then, just for testing, I shorted the converter and got around 1A+ on the output. Then the Arduino stopped providing power and then I found that it had heated up in the area of the voltage regulator... after a bit of fiddling it came back to life, but as I discovered earlier, the 3.3v rail was dead. Anyway, side question, why did that happen? Moving on.

I am pretty confused, the output of the boost converter shows that it is capable of delivering 1A+... So why doesn't the LED draw that? And how can that beast of a LED outdo the 1 watt LED with less power usage and less heat? I have two of those DC-DC converters, should I wire them up in series and put 15v on each, and connect them to different power sources? Will that make a difference? (I did some research on putting multiple DC-DC converters in parallel, and learned that it is a bad idea). What should i do? What is the best way to power the 20 watt LED?

[skillz21]

*sigh*

It was the power source... I connected it up to a 14v drill battery, now it draws around 500mA at the output......

[Zero999]

The LED probably needs a constant current power supply. Look at the data sheet for the recommended forward current and use a power supply which is current limited to that value and can provide a higher open circuit voltage, than the maximum LED forward voltage, specified on the data sheet.

20W of power is 6A at 3.3V (more as the DC:DC converter won't be 100% efficient), so no, your Arduino won't be able to supply that.

[skillz21]

I powered the Boost converter with the 5v rail but yeah.....

[skillz21]

Can anyone suggest a good way of cooling this... thing?? It gets really hot.

[Zero999]

The usual way to cool an LED is with a heat sink and fan.

If this is a cheap LED, then it should be de-rated to around half the power dissipation listed on the data sheet.

If you're connecting the LED up to a constant voltage supply, with no series resistor, then you're asking for magic smoke. I think you've missed the point about LEDs requiring a constant current source. The voltage doesn't need to be controlled, only the current.

[skillz21]

What do you mean series resistor? I don't need a resistor for this, do I? Also, I don't have a constant current source....

[grumpydoc]

This is easier with a diagram rather than words but I don't have access to any drawing tools at work.

If you have a resistive load the relationship between applied voltage and the current flowing is linear and given by Ohm's Law - if you apply 3V to a 3Ohm resistor 1A will flow. Increase the voltage to 6V and 2A will flow.

A LED is a semiconductor so the relationship between voltage and current is not linear. Nothing much happens until you reach a certain threshold voltage - known as the forward voltage V_F. Once you reach that voltage even if you increase the current the voltage across the LED will not vary (in practice it will rise very slightly, for the rest of the argument assume it stays the same). Typical power LEDs have a V_F of around 3V

So if you apply exactly 3V to a 1W LED with a 3V V_F it will light, not very brightly because not much current will be flowing - the current, assuming that the LED is just connected to a voltage source with no other components will be determined by the difference between the voltage source and the forward voltage of the LED and the resistance of the wiring, plus the internal resistance of the voltage source.

So, say you connect your 3V V_F LED to a 3.01V source via wires with 0.03ohms of resistance - the current will be (3.01-3.00)/0.03 = 333mA.

Your LED will be happy, it is rated at 1W and is disipating 1W - all is well assuming adequate cooling.

But increase the voltage to 3.1V and now 3A is flowing, your LED is trying to dissipate 10W and will shortly let loose the magic smoke.

As you can see controlling LEDs from a constant voltage source is tricky - especially when you consider that each LED in a batch will have a slightly different V_F.

So you don't do that - you either connect a series resistor to allow a bit of voltage drop and reduce the impact of small changes in the system.  Say you add in a three ohm resistor. At 333mA that will drop a volt so you need a 4V supply but now if the supply rises to 4.1V the current will only go up to - well, I'll leave you to work it out, or what the effect of about 0.1-0.2V spread in the V_F would do to the system.

OR, you connect the LED to a constant current source - this will adjust the voltage continuously to keep the current flowing at a set level. the power dissipated by the LED is now the product of the V_F and the current flowing.

HTH

[Zero999]

For small LEDs, a series resistor is normally used to limit the current. See the tutorial linked below:
https://www.sparkfun.com/tutorials/219

For larger LEDs, a switched mode constant current supply is used to deliver the correct current to the LED. If you post the data sheet or at least a link to the shop you bought it from. I might be able to  recommend a suitable power supply.

[skillz21]

Here you go, sorry I should have posted it earlier: https://www.ebay.com.au/itm/New-10W-20W-30W-50W-100W-900-9000LM-High-Power-LED-Lamp-SMD-Chips-light-bulb-DIY/311585025813?ssPageName=STRK%3AMEBIDX%3AIT&var=610549034742&_trksid=p2057872.m2749.l2649


Also would something like this work?: https://www.ebay.com.au/itm/LM2596S-Power-Supply-DC-DC-Constant-Current-Voltage-Adjustable-Module-LED-Driver/291145975033?hash=item43c9a704f9:g:LGsAAOSw5VFWFy5P

[Zero999]

Here you go, sorry I should have posted it earlier: https://www.ebay.com.au/itm/New-10W-20W-30W-50W-100W-900-9000LM-High-Power-LED-Lamp-SMD-Chips-light-bulb-DIY/311585025813?ssPageName=STRK%3AMEBIDX%3AIT&var=610549034742&_trksid=p2057872.m2749.l2649


Also would something like this work?: https://www.ebay.com.au/itm/LM2596S-Power-Supply-DC-DC-Constant-Current-Voltage-Adjustable-Module-LED-Driver/291145975033?hash=item43c9a704f9:g:LGsAAOSw5VFWFy5P

The open circuit voltage isn't high enough: only 30V. The forward voltage is between 32V & 34V.

You need the 20W model of the  PSU linked below:
https://www.ebay.com.au/itm/10W-20W-30W-50W-100W-Waterproof-High-Power-LED-Driver-Supply-LED-Chip-SMD-Lamp/172565156908?hash=item282daf542c:m:m88z3l-yso3zRBLLit4HnHA

[Audioguru]

Why do people buy cheap no-name-brand junk with no detailed datasheets from ebay??

[skillz21]

Why do people buy cheap no-name-brand junk with no detailed datasheets from ebay??

Ummm because it's cheap, and I don't need it for any proper project, just prototyping, I'm not the kind of person to pay extra money for a thing that I can get for much cheaper, if I don't have a specific plan for it... so yeah.