The brightness of LEDs are different, even with constant current setup

Other Hardware LEDs and Multiplexing
1

It is about an arduino uno driven binary clock.

The LEDs in question are 1W, 350ma, 3.2V forward-voltage high power LEDs. They are driven by arduino output pins using 2N2222 NPN transistors.

I am using a 5V external power supply to power those LEDs. The biggest number of LEDs which turn on at once is 9 LEDs. So, 9 x 350ma=3.15 amp. The power supply is capable of putting out 4 amps. I tested the power supply by putting load on it and it was putting out approximately 4 amps. So, it should be OK.

I realize that these LEDs require a constant current source. So, I have connected them according to the following diagram:

I am using 2 transistors to allow for constant current. The right transistor is the main transistor through which the main current passes and lights up the LED. When the current rises, some of the current is diverted to the base of the left transistor which in turn takes away some current from the base of the right transistor thus creating a feedback loop.

I am using 5V because I have had more success with it. In my previous project, I was using 3.45 V, but I was getting some flickering. So, I am using 5V this time to eliminate the possibility of thermal runaway. I think 0.7 V gets used up by the right transistor and the remaining 1.1 V is used for the current limiting circuit. Or the other way around.

Anyway, the issue is that, despite my efforts to level out the brightness of LEDs, some of the LEDs are brighter than others. The difference is NOT like 200% but it is quite noticeable. So, I was thinking may be, the power supply is not providing enough current, but the thing is that, even when I have 1 or 2 LEDs on, the brightness does not change for those weaker LEDs. I am not sure why it is the way it is.

Isn't the constant current setup supposed to level out the brightness? Additionally, how would this type of circuit behave if there was not enough current for the system? Would it divide the current equally between the LEDs or would it supply some of them at maximum output and leave the others running at half power?

Additionally, in the process of painting my project, I had masked one of the LEDs with a small piece of duct tape and when I was done with painting, I started to remove those duct tapes. But for one of the LEDs, the little transparent plastic cover came off with the duct tape. I pushed it back in. Would that cause a problem in the long run?

Any input will be highly appreciated.

Cheers.

2

You're driving the main transistor with about 1mA base current, while you should drive it with 5-10% of the collector current.
Try replacing the 3k3 base resistors with 220ohm resistors.
I assume the current sense resistor is ~2.2ohm.
Leo..

3

Where did you get the LEDs from, eBay or a reputable distributor? It is not uncommon for eBay components to be from the reject bin and one of the reasons they end up in there is the brightness is not within limits.

4

Wawa:
You're driving the main transistor with about 1mA base current, while you should drive it with 5-10% of the collector current.
Try replacing the 3k3 base resistors with 220ohm resistors.
I assume the current sense resistor is ~2.2ohm.
Leo..

The 3k3 resistor is actually 220 ohm. The diagram shows 3k3 because it is a generic picture which I found on the internet. Sorry about that. Oh and yes, the current sensing resistor is 2.2 ohm. I got the LEDs locally, from the electronics shop.

5

Some LEDs (blue) are very sensitive to ESD.
Soldering them in with a non-ESD safe poker could also reduce brightness significantly.
Leo..

6

Wawa:
Some LEDs (blue) are very sensitive to ESD.
Soldering them in with a non-ESD safe poker could also reduce brightness significantly.
Leo..

Hi. I have not soldered them. I have built my entire circuit on a high quality solderless breadboard :slight_smile:

By the way, I think I found out what the issue is. Those LEDs are just slightly different color. Upon closer inspection, I found out that some of those leds are white, while others are actually yellowish which appears like difference in brightness.

7

Have you measured the actual current that is flowing, for the brighter Vs dimmer leds.?
Using breadboard could also account for some of the difference. Even on high quality breadboards, the contact resistance is not zero. It does not take much to change a 2R2 into 2R3, 2R4...

8

PaulRB:
Have you measured the actual current that is flowing, for the brighter Vs dimmer leds.?

Yes, the lowest current amongst LEDs is about 300 ma. The highest is 400 ma. Despite my constant current setup. :slight_smile:

9

Despite my constant current setup

That means your constant current generator is not working. Almost certainly due to you not soldering it up.

I have built my entire circuit on a high quality solderless breadboard

Sorry their is no such thing, there is crap breadboard and less crap breadboard, but high quality simply does not exist.

10

Grumpy_Mike makes a good point, how did you verify that the constant current setup circuit is working as it is supposed to? Maybe it would make sense to get a high quality LED or get a couple of dummy loads and see if the current stays the same.

As for leveling out the brightness for LEDs, is that how it actually works? Maybe it does for higher quality LEDs but not so much for lower quality ones.