DIY LED driver for reef lighting

The CAT4101 is limited to 25v on the LED pin. That basically means you can use any input voltage up to 25v. You're using a 24v supply, so you're under the limit. What you are limited to with a 24v supply is essentially 23.5v of LEDs (leaving half a volt of drop for the chip itself). So, if you know the Vf of your LEDs, you can keep adding LEDs up to (but not over) 23.5v.

Of course this is all theoretical. LEDs tend to vary significantly in Vf so the best approach is to undershoot a bit then adjust the power supply to get the minimal drop.
 
What is the voltage rating of the power supply and have you measured it (what is it)? Was it still turned down when you tested 7? What is the power output of the power supply?

FYI you only need .5 volts over the forward voltage so you should be abloe to go as low as 22.9 (or was 23.9 a typo).

Appears to work. YOu could ave stressed it heavliy and it may not actually behave properly in all conditions or may quit all of a sudden. IMHO replace it with a new one.
 
What LEDs? And what is the voltage across the 7 LEDs? I am guessing that 22.9 is only enough voltage to drive them at 400 ma. So check the voltage across the the 7. I expect it will read 22.4. If so turn the supply up to 24 (ok the 23.9 you wanted sub consciously :)) and see what current you can get.
 
Anyone have any spare CAT4101 triple PCBs they would be willing to sell? Shoot me a pm if you have. If not I'll be placing an order in Feb. Thanks!
 
Howdy :)

Been reading through this thread.. very cool! :)

I'm planning on building some 3066 circuits, but have a few quick questions. While playing with the 3066 spreadsheet tool, it has IOUT, and labeled as "desired average led output current". Then there's also the Rsense values. I understand that changing the Rsense values, determines the current output.. but it seems to be independent from the IOUT.. whether I specify 350mA or 500mA for desired led output current, the resistor values don't change. Yet if I just put desired current as 350mA, but change the ESR value, I'm able to get outputs of 350mA, 500mA, 700mA, and 1000mA (with 0.167ohm resistor). That's with an input of 12v, and set for 36v output.

Am I configuring this right? Is the IOUT average the lowest it will do (350mA), with 1000mA being the max? I was planning on making the circuit so the current was adjustable.. maybe use an lm317 in "current" mode to regulate the output current from between 350mA and 1000mA using a pot. At the very least, I could use all 4 resistor values, and select using a 4-POS DIP switch.

Second question... a little confused still on how constant current driver actually works. I understand that the current adjusts itself according to voltage fluctuations, but I've also read that a driver can run a string of 12 led's.. or just 1 led.. from the same driver. So.. what exactly happens to all the extra voltage? A few volts (less than 5) I can see as fluctuation.. but what about a 20v surplus?
 
OK.. deciding on smaller strings (8 per) and reduced the max output to 32v from a 12v input. But I need mosfet and transistor wired in.. never used them before. And although the design tool.xls shows a block schematic, I need wires and pins :p. I emailed OnSemi and they sent me a pdf of the ncp3063.. lower voltage version of the ncp3066.

NCP3063BoostWithExternalSwitch.jpg


So, trying to modify DWZM's schematic, I'm not sure if I'm doing this right...

ncp3066driver-withextrnalswitch.jpg



Does it look correct? I'm doing my digikey order on Monday, but need to make sure I understand the external switch part. Everything else is easy enough.. just not sure about the mosfet and transistor.

Also, not sure if anybody's tried, or even thought of.. I was planning on having an "adjustable" constant current.. like the Meanwells. I originally thought of a trimpot in place of the RSense resistor, but less than 1ohm would be difficult, and a rheostat for that low would be expensive. But.. using resistors in parallel.. 3x 2ohm equals 0.66667ohm. 0.68ohm gives 350mA. So adding 2 2ohm 1/2w resistors in parallel with a 10ohm 1/2w 25 turn trimpot, "should" allow for a base of 350mA, or even a little lower. And at 25 turns, that's roughly 400mOhm per turn.. should be enough to fine tune.. hopefully. :D

Code: 
POT     10 ohm 1/2w 25 Turn    3299P-100LF-ND
The trimpot has a minimum of 2ohm.. at the most.


Any help with the external switch part would be greatly appreciated.


Cheers.............. :)
 
Hello all,

I wanted to check in with the resident experts here. I got my boards yesterday and finished soldering what I needed to. I hooked everything up and it seemed like I did the work correctly as I got everything to light up with the internal pot turned down.

I"m using a meanwell sp-100-24 with DWZM's driver and CREE XR-E's in a series of 6 for 4 strings. I tested a string with the multimeter in series and with the pot all the way down it reads .19 mA and turned all the way up it reasd .7 mA. I found it kind of odd that at max it was already at .7 mA.

Regarding the cat4101 boards I read that the chips should be okay to touch but not for an extended period of time. When doing so I can press my finger on the chip for 1-2 seconds before it gets too hot to touch.

Does anyone see a red flag that something is wrong with the pot turned all the way up and already reading .7mA and the temperature of the chip?

Any help would be greatly appreciated.

Thanks!
 
What is the range of the pot? What is the suppl voltage (24 volts)? What is the voltage of the string?

The loer current will be determined by the max of the pot. The low end possibly by the incoming voltage. I would add a resitor in sereis with the pot so you can not reach beow the minimum speced which is 550 IIRC.
 
I think it might be easiest if I post a pic of the spec sheet for the Meanwell SP 100-24:

SpecSheet.jpg


I'm not too sure what the voltage is in each of the 4 strings. How would I calculate that?

The pot is an internal one so I'm not sure how I would go about adding a resistor to that. Maybe I'm using the wrong terminology. I'm talking about the voltage adjustment screw on the power supply.

Hope that helps you help me? =]

Thanks Fishman!
 
Use a meter and read it. What is the sense resistor value? You set the maximum current of the CAT4101 with the sense resistor. It will not supply more current then the sense resistor allows. By turin the voltage of the power supply you limited the voltage that culd be supplied by the CAT and therefore the current. I am guessing your sense resistor was picked for 700 ma.
 
That makes more sense. The mouser part number for the resistor is 660-RK73H2BTTD7680F and has a resistance of 768 ohms.

If this part is limiting the to 7mA then its safe to assume that no matter how far up i adjust the internal pot it will always cap at 7mA correct?

This entire build has been a huge learning process for me.
 
I just typed a nice long replay and lost it.

768 ohms does limit the urrent to 700ma. Measure the string voltage and adjust the PS to .5 over for optimum efficency.
 
Fishman is helping me out with a current curiosity i have but i was wondering if anyone has had experience with this. Maybe DWZM can chime in.

The CAT4101 chip is rated up to a 1a current with a 549ohm resistor as stated in the CAT4101 data sheet. I'm currently running the 768ohm resistor in the original design but find that I have to turn my PS pot all the way up to reach the desired .7a current. This is resulting in the CAT4101 running fairly hot (1-2 second touch before burning my finger).

So the question is if i swap out the 768ohm resistor with the 549ohm one and adjust my PS pot to reach the desired .7a current, will that result in a cooler running CAT4101 chip? Also will that decrease the life of the chip or will it be fine?

Thanks in advance.
 
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