How to: drive common cathode led with Arduino?

Where are you putting the red and black leads of your meter?

To be honest no I don't know with you are seeing a -ve voltage unless it is a meter fault or you are purring the probes in the wrong place.

Thank's for all your replies.

I've put my meter here:

As I said, when the lights are on, I measure approximately 15V as I should, but when they're of I measure -9V.. I am pretty sure it's not a meter error as it Would the first time I've had my Fluke fail on me :slight_smile:

Best regards
Joe

I measure approximately 15V as I should, but when they're of I measure -9V.

Do you actually mean -9V? How can you measure 15V when there is only 9V connected to it according to that diagram.
What you are saying makes no sense at all, therefore I suspect you are not measuring what you think you are.

Note when the transistor Q1 is off then effectively there is nothing to measure and so the inputs to the meter floats. You will not see minus nine volts but you might get a bit of pickup. This is true because the load is sevral LEDs so it is not a linear load.

Sorry for the confusion Grumpy_Mike
I've corrected the schematic in my former post to reflect the circuit I'm currently having :slight_smile:

  • When the Arduino I/O is high, I measure 15V (probing points marked in former post), when it's low I measured -9.7V and just before at the same conditions -15, The value drifts a little, I don't know why.

If I remove the led, and still measure at the two points marked, I measure 15V when Arduino I/O high, and 0V when low :slight_smile:

Best regards
Joe

if I remove the led, and still measure at the two points marked, I measure 15V when Arduino I/O high, and 0V when low

Which is as it should be.

I think we need to know more about the 15V LED. No LED actually takes 15V, that is just physics, so this must be something like a chain of LEDs with built in resistors or constant current drives.

There is clearly something going on with this load that we haven't been told yet. Perhaps something to do with capacitors in the 'LED' load.

Here comes information about my leds :slight_smile:

The load consists of two of these connected in parallel:

The leds on the pcb is connected in series, no extra components added to the pcb or anything. Each led-pcb draws 75mA when connected to a 15V power supply.

Regards
Joe :slight_smile:

Oh dear!

Mike is not going to like you using them in parallel and without current limiting resistors. :astonished:

Paul__B:
Oh dear!

Mike is not going to like you using them in parallel and without current limiting resistors. :astonished:

Quite right strike two.
Time to do some serious reading about LEDs and how to drive them. Time to look at your power supply and see if it can still output the full voltage when you have that load on. Time to ask some serious questions about your meter.

Have you got a link to those LEDs? Couldn't find anything under FS-0441 LED when I googled it.

Each led-pcb draws 75mA when connected to a 15V power supply.

Dream on sunshine.

Grumpy_Mike:
Dream on sunshine.

I think Mike may be trying to say you need to do a refresher on Ohm's Law.

P= I x E

I = P/E

I = 5/15

I = 1/3A

Quod erat demonstrandum, the current is greater than the 75mA you suggested by a factor of over 4.

Weedpharma

Hmm.. I know these should have a current limiting resistor, but I don't have any datasheet for the leds used on the pcb. To be honest all I did was connecting these to my power supply, and slowly cranked up the voltage until I saw light, then one pcb seemed to run great at 15v... But how do I calculate a resistor when all I really know about them is that they draw 75mA from my power supply when supplied with 15V? :slight_smile:

My power supply does output full voltage when the load is connected. I adjust my power supply to 15V, connect the load and it still says 15V. My meter is a Fluke 179.

I brought these led pcbs at Ebay, the seller doesn't really list any useful specifications, it says "Voltage 110-220V" which I find a little weird, but then again, I haven't even been able to calculate the resistor.. I clearly missed something here. I'm glad for all the help I receive :slight_smile:

The led pcbs:
http://www.ebay.com/itm/5W-Led-Spot-Light-Panel-SMD-5252-Integrated-Chip-Bead-Surface-Condenser-Lens-/221526157711?pt=US_Light_Bulbs&var=&hash=item3393fce98f

I appreciate your time.

Best regards
Joe

It looks to me as though each of those modules have 5 LEDs in series, so they shouldn't be described as common cathode!
I think that by calling them common cathode, you have complicated the circuit requirements.

Where did the description common cathode come from? It's not mentioned at all in the seller's description, as far as i can see.

I would imagine that each individual LED is in fact a 350mA 1W device, to make the complete module 5W .
Are they still quite dim at 75mA?

Sorry.. I should have stated this from the beginning. I call the setup common cathode because they're wired like this:

Yes, the LED specifications are fairly straightforward, they are five 1W LEDs in series (you can almost see the connections in the photograph), and their working voltage is about 3V so they require about 330 mA controlled current drive to show full rated brightness.

The beauty of LEDs is that you do not have to drive them to full brightness to get the right colour.

You need a current driver circuit to power them, and if you are going to use two, then since you need more than 15V anyway, it would be better to run both in series from some sort of 35V supply using a current driver:

For 330 mA, the "sense" value here is about (0.7V / 0.33A) 2.2 ohms. The base resistor could be about 470 ohms and the feedback transistor any general[purpose NPN. The power transistor needs to be rated for a few watts with a heatsink but actually does not need to be rated for the full 35V for two of those modules in series.

Thanks for your reply Paul__B :slight_smile:
I would like to know, why I need to use the current driver circuit? wouldn't i be enough to use a current limiting resistor. Wiring them in series is sadly not an option, they have to be connected like my diagram in the former post.

Best regards
Joe

know these should have a current limiting resistor,

No you should not!

With high power LEDs like this you should be using a constant current supply, resistors just do not cut it.

The other thing is why you think this is a common cathode LED? There is no common connection at all. You could have done exactly the same as you are doing now with a NPN transistor or N channel FET.

Anyway that page says the device has a SMD 5252 Integrated Chip, not sure where but that is what it says. From bits of a data sheet I have managed to find for a QX5252 it looks like it is a switching regulator. However that does not square up with the 110 to 220V input.

What do you want to do with these LEDs? Are they for serious lighting? If so you are going to have to push a hell of a lot more current down them than 75mA.

This is my guess. The eBay seller hasn't got a clue what he has got, he probable just picked up a batch of random junk and this was in it. That specification looks to me like it was the specification of the whole finished lighting unit before it was pulled to bits. It looks like you just have five bare 1W LEDs. These normally take 350mA, so look for a constant current circuit that can provide that. Get two of them, one for each LED plate.

Grumpy_Mike:
From my former picture showing the wiring of my leds, I guessed it would be correct to call them common cathodes since they all share the ground wire. Now I know they shouldn't be called that... :slight_smile:

I've installed these in my ceiling planning to use them as ceiling lights. They give the right amount of light, and I am happy by the result driving them from my 15v power supply.

I know nothing about constant current driver circuits, but would it be possible to find/build a current driver that could drive two led-pcbs at once, as that's how I already wired them, instead of supplying each led-pcb with a driver?

Are there one such standard led current driver circuit, or do they vary a lot for these kind of leds? - I am thinking of which circuit to use to drive the led-pcbs.

Whoa.. I've got much to learn, constant current drivers are a whole new world to me... :smiley:

Best regards
Joe

So I found this driver circuit which I guess would be appropriate:
LED Current Driver

and then I would be able to PWM it with this added:
PWM

Am I totally wrong here? :slight_smile:

Yes that is the sort of thing. Note you will need a voltage greater than 15v and you need one for each LED group. You can not wire them in parallel because they will not shair current equally.
At the moment disasister has been avoided because you are totally under running the LEDs but that is not a long term stratagy for any electronic design.

Yes you will be able to PWM that circuit.

I will supply each led-pcb with their own controller. What specifications should I go for?

  • 17v - because it have to be more than 15v (can it be much more than 17v)?
  • minimum 5W - because that's what each led-pcb is? (does it matter if it's larger than 5W, as long as it isn't under?)
  • 350mA or more, does it matter if it can deliver lets say 700mA? I guess the led "takes what it needs"

Thank you very much Grumpy_Mike :slight_smile:

Best regards
Joe