COB LEDs

I have 9 of these COB LEDs that I want to run in parallel from a 12v 2A power supply.

http://www.ebay.co.uk/itm/252291172289?_trksid=p2057872.m2749.l2649&ssPageName=STRK%3AMEBIDX%3AIT

I'm not sure if they have resistors built in or if I even need to use resistors?

This is the power supply:

http://www.ebay.co.uk/itm/181571289406?_trksid=p2057872.m2749.l2649&ssPageName=STRK%3AMEBIDX%3AIT

I also bought this inline dimmer in case they're too bright:

http://www.ebay.co.uk/itm/151620583763?_trksid=p2057872.m2749.l2649&ssPageName=STRK%3AMEBIDX%3AIT

Will it be ok to simply wire them up and plug them in?

I've not used COB LEDs before so I'm a bit unsure.

Thanks

Hello, are you aware you posted the Arduino forum? Was this an error? Your question seems to be unrelated to Arduino.

Paul

OK, they are 3 W at 260 mA, meaning that to light them fully, they require a nominal 11.5 V.

They require a current-limited supply or preferably, a constant-current supply.

You have the wrong power supply for them. You cannot operate them in parallel, they require a separate constant-current supply for each.

You have nine of them, eh?

You will need three "10 W" LED drivers rated at 260 mA, to drive them as three separate series chains of three. These supplies are readily available on eBay but you will need to check the specifications carefully.

Dimmable drivers are however, not so readily available.

They do not have built in resistors or other current limiting. You must provide such. The dimmer looks like it just pwm's the signal, and provides no current limiting.

You can probably put them in parallel (modern led process control is good enough that leds from the same batch can usually be put in parallel. It's better to give each one it's own current limiting, though.

You can use resistors for the current limiting - though it's less efficient, and you may have to play with the resistor value to get the right current, and the resistors might need to be beefier than you're used to. Using resistors would probably let you use the cheap dimmer though.

But how would you use resistors to control an 11.5 V LED from a 12 V supply?

I'm fully aware it's an Arduino forum but it's also the best place I know of to ask general electronic questions as the people tend to be very knowledgeable and helpful!

Is it possible that the numbers on the eBay ad have been rounded? It does state 9V-12V?

Might it actually be 3.12W (12V, 260mA) but they've rounded to 3W?

Could it maybe work using a ~30-33 Ohm resistor for each LED? (assuming voltage drop of 3.6V)

I have been told that you cannot use resistors to limit the current with high power leds. The problem is that they get quite warm in use, which affects their forward voltage. So the correct value resistor when they switch on from cold is no longer the correct resistor when they get warm. A constant current circuit is needed which will adapt as the led's temperature changes.

I have a whole collection of COB LEDs and, from experience, those LEDs have a Vf of ~10volt@330mA. One 2A dimmer and 6 COB LEDs in parallel is not a good idea. Current will be shared unevenly. Adding a 1ohm/0.5w resistor in series with each LED, and mounting them on one heatsink (same temp) could minimise this. Leo..

All the LEDs will be mounted to one common, large (300x250mm) heat sink so hopefully they would be roughly the same temperature.

If the resistance changes as they get hotter, presumably the resistance will go up with temperature. These LEDs will be on constantly for 12 hours at a time. I assume they will reach an operating temperature fairly quickly and maintain that temperature until they're turned off. Therefore if I use the correct resistor for when they're hot, I think they'll just be a bit dimmer as they're warming up but this isn't a problem.

Is there a safe/easy way I can measure the actual Vf or voltage drop of one of these LEDs without blowing it?

I guess the only way I'll know for sure if they share the current evenly is by wiring them up and trying it.

If the resistance changes as they get hotter, presumably the resistance will go up with temperature.

It ain't neccessarily so. Leds are semiconductors, and some semiconductor's resistance drops as temp increases, leading to "thermal runaway" and failure.

As said, add a small 1ohm resistor in series with each COB LED. That takes care of some of the imbalance.

Those LEDs have a negative temp coefficient. Lower Vf when hotter. Practically not a big issue when you have them all on the same heatsink and use 9 COB LEDs with a 2A CC driver. I think 260mA is conservative. Most of those COB LEDs are listed as 300-330mA. Use a small amount of thermal grease under each LED. Leo..

Thanks Leo.

Can I use the power supply I have?

ScrambledEggs: I'm fully aware it's an Arduino forum but it's also the best place I know of to ask general electronic questions as the people tend to be very knowledgeable and helpful!

Yet you seem to be determined to ignore this advice and think you know better?

Oops. Forget everything I said. Just saw that the dimmer is NOT a constant current dimmer but a PWM dimmer.
That changes everything.

You must use a current limiting resistor for every COB LED.
About 8.2ohms, but 10ohm if you want to be on the safe side.
They use ~0.5watt, but I would use 2 or 3watt cement resistors for easy mounting on the heatsink.
With these resistors values, LED imbalance is not relevant anymore.

With 9 LEDs and 8.2ohm resistors you are just over the current capability of the supply.
With 10ohm resistors, just under.
Leo…

Grumpy_Mike: Yet you seem to be determined to ignore this advice and think you know better?

Do I?

I'm going to do exactly what Leo has said so I have listened to the advice given.

If I'm asking further questions that seem to contradict the advice given it's only because I don't understand as I'm not well versed in electronics. For example I didn't know the power supply I've bought isn't constant current. Why wouldn't it be if its stated as 2A?

You sure know how to welcome new members!

The current capability of a power supply is only what it can supply not what it will supply. We welcome people who are willing to learn but you seem not to be one of those.

I suggest you look over your answers again and see how they come across. If you don’t understand something then say so. Do not argue.

Leo,

Could you post a link to a suitable resistor please?

I've had a look on RS components but I'm not 100% sure what I'm looking for.

I have trouble finding things on the RS website since they moved to that bloody smartphone view. Like looking at things through a peep-hole.

They don't seem to have small (3watt) cement resistors.

These ones are 10ohm/3watt. http://nz.rs-online.com/web/p/through-hole-fixed-resistors/2142493/ Maybe you have to search for "2142493" in your country.

You have to figure out a way to mount them. They get warm/hot. Maybe a piece of Vero-board, or two rows of joiner-blocks. Leo..

Wawa: You must use a current limiting resistor for every COB LED. About 8.2ohms, but 10ohm if you want to be on the safe side. They use ~0.5watt, but I would use 2 or 3watt cement resistors for easy mounting on the heatsink. With these resistors values, LED imbalance is not relevant anymore.

With 9 LEDs and 8.2ohm resistors you are just over the current capability of the supply. With 10ohm resistors, just under. Leo..

So I gave this a go using 10ohm 1/4W resistors (as it's what I had to hand) and it seemed to work well but I didn't have them on for too long. They didn't get hot at all even without a heatsinnk.

Since my first post I've been reading up on powering LEDs and now understand it a bit better. Therefore I'd like to use a proper constant current supply but I'd still like to make my own rather than buy one as it's more enjoyable and I like the learning process.

I came across this (link below) as an option.

http://www.onsemi.com/pub_link/Collateral/AND8109-D.PDF

With reference to figure 4...I understand each string of LEDs has its own transistor so that the string will turn off should an LED fail and it won't damage the LEDs in the other strings. But can the current draw be uneven across the LEDs in different strings or even in the same string?

As I have 9 11.5-12V LEDs and a 12V power supply would I be better using one linear voltage regulator for each LED? Basically replicating the circuit in figure 1 9 times but linking them all to the same power supply?

But how would you use a current limiter to control an 11.5 V LED from a 12 V supply?