LED Driver (NCP3066)

[nyquist]

So I have a "working" setup using a constant current driver NCP3066 in the circuit as follows:

When I toggle the on/off switch, the LEDs will toggle, but when I replace the two LEDs in series with one LED or three series LEDs, the circuit still works for the most part, but with severely altered (lower) drive current.

I have read the datasheet (http://www.onsemi.com/pub_link/Collateral/NCP3066-D.PDF) but still have some questions for anyone who has worked with this sort of driver or has some sort of electronics/circuits background:

1) Why isn't my driver outputting constant current independent of the load?

2) How can I alter the current? Right now I'm only getting ~60mA with 2 blue LEDs in series, but halving the resistance of either 1.5ohm resistor does not seem to do anything to my current

output...despite what the datasheet and common sense says should happen



p.s. I'm doing all of this on my college breadboard, could the internal resistance/hole resistance of that be overriding the small resistors I'm using?

 

[RockStacker]

What are the voltage, current and resistance ratings on the individual LEDs you are using?

 

[nyquist]

What are the voltage, current and resistance ratings on the individual LEDs you are using?

Shouldn't matter in theory, but they are misc chips I just wanted to run with 100mA or so. Each one is around 3.0V @ 100mA, so around 30ohms per LED.



I'll post more info tonight, but I setup the driver in a different SMPS setup (now its in a boost setup) and seems to have more promising results.

 

[fastrd400]

[ame=http://www.youtube.com/watch?v=JCBoFEKxaSw]Constant Current Switching Regulator for LEDs with ON/OFF Function: NCP3066 - YouTube[/ame]

 

[nyquist]

Constant Current Switching Regulator for LEDs with ON/OFF Function: NCP3066 - YouTube

yeah I saw that, but that video is literally a person just reading the entire datasheet for the driver device



so I didn't make a schematic for my new setup, but Im basically using more circuit protection, more caps, and using an inductor between Ipk and common collector

seems to be working alright, but I don't have a zener diode with high enough zener voltage to test it with my 12v power supply, so I'm technically not satisfying the minimum supply voltage for this configuration at the moment but it is working as expected now

 

[nyquist]

So, here is my new "constant current" setup (basically a clone of DWZM's setup here):

I have tried the above using between 7 through 13 LEDs. What happens is this:

(V1 = voltage across 1 ohm resistor; Vled = voltage across LEDs)

7 LEDs: V1 = 106mV, Vled = 23V
8 LEDs: V1 = 85.6mV, Vled = 25.6V
9 LEDs: V1 = 73.4mV, Vled = 28V
10 LEDs: V1 = 66.5mV, Vled = 30.5V
11 LEDs: V1 = 60.5mV, Vled = 33V
12 LEDs: V1 = 55mV, Vled = 35.6V
13 LEDs: V1 = 51mV, Vled =37.9V

so as I increase the number of LEDs, the current goes down and accordingly the voltage drop per LED goes down. Am I clearly missing something?

why isn't my constant current driver supplying constant current? (I also setup two independent circuits the same way, both behave the same, so I am quite sure I didn't just set something up wrong)

 

[bjohanson1234]

I think you are missing something. Since you're adding LEDs, the voltage across all of them increases. V=IR. So as you add more resistance, the voltage needs to increase to keep the current the same. Try measuring the voltage across only one LED as you add more. It should stay preetty constant.

As for across the 1 ohm resistor, it is the one between Ipk and Vaccine right, that isn't in the same current path as the LEDs. So the voltage across that may very well change as the voltage across the. LEDs changes.

 

[nyquist]

Since you're adding LEDs, the voltage across all of them increases. V=IR. So as you add more resistance, the voltage needs to increase to keep the current the same. Try measuring the voltage across only one LED as you add more. It should stay preetty constant.

true, the problem is each time the voltage does not increase proportionally to the number of LEDs, with each new LED the voltage goes up ~2.5V, but the voltage per LED goes down and down; the voltage is "relatively" constant, i.e. it goes from ~3.2V for 7 LEDs, to 3.0V for 12 LEDs, but the problem is, diode have an exponential IV curve so that 0.2V is a substantial change in current

As for across the 1 ohm resistor, it is the one between Ipk and Vaccine right, that isn't in the same current path as the LEDs. So the voltage across that may very well change as the voltage across the. LEDs changes.

aside from current supplied from the energy storage elements, it should be the same current the LEDs see, the problem is with constant current, the voltage should stay constant (back to V=IR); I also checked the 0.5ohm resistor and it has changing voltage depending on the number of LEDs hooked up.





it is so puzzling because everything looks and seems right, its just not controlling the current properly...

 

[bjohanson1234]

Remember that each LED is different than the others. Even if they are from the same batch. They could have different properties. So unless you are actually measuring the current through the LEDs, you aren't going to know for sure.

As for any of the resistors in this schematic, they are all on different branches than the LEDs. Therefore, they may or may not have a constant current going through them.

Granted I have no knowledge of this particular device, just a general knowledge of electronics.

What I would do is replace one of the LEDs with a risitor then measure the voltage across that as you add LEDs. This should remain the same as you add more LEDs.

 

[nyquist]

Remember that each LED is different than the others. Even if they are from the same batch. They could have different properties. So unless you are actually measuring the current through the LEDs, you aren't going to know for sure.

yup, but if they are in series they will all have the same current; and I was measuring voltage across on a 'control' LED (same LED that never got switched out) which got lower and lower each time I added another LED.

As for any of the resistors in this schematic, they are all on different branches than the LEDs. Therefore, they may or may not have a constant current going through them.

the 0.5 ohm resistor is effectively in series with the LEDs, it is the only pathway to ground for the series; the current flowing through all resistors other than the 1 ohm and 0.5 ohm resistors is on the micro amp scale. I also double checked with another resistor in place of and LED and it looks like the LEDs are seeing the exact same current the 0.5ohm resistor is seeing (any fluctuation is beyond the precision of my multimeter).

What I would do is replace one of the LEDs with a risitor then measure the voltage across that as you add LEDs. This should remain the same as you add more LEDs.

it doesn't :(



I appreciate your help bjohanson1234, maybe I can probe everything in the circuit again and find something :)

 

[bjohanson1234]

Damn, sorry I couldn't help. Hopefully you get the thing resolved.