DIY LED driver for reef lighting

[RCBrooke]

I thought 2' up sounded like the perfect distance as well, with 40 deg optics.

I had thought to mount a aluminium box to the heat sink with the drivers inside, along the lines of soundwave's (around post 295) only two independent from each other (no L channel frame). As for the heat from the CAT4101's, I wonder if C-channel thermal epoxied across the chips then to the housing could be arranged so as to provide a heat sink for the CAT4101's.

I was also thinking of buying the tank mount stand for the Ecoxotic Panorama but read a review that that the upper arms come with the light fixture not the stand.

Well off to work!

 

[DustinB]

I haven't seen a need for a heatsink on the drivers at 700mA. I have mine in a small currentusa t5 fixture with my led heatsink. I have 2 fans blowing through the fixture and that is plenty to keep the drivers fairly cool.

 

[TheFishMan65]

RC L channel would help. I had some extra material from my build (check my album) and it did wonders. Remember anything you can do to keep the electronics cool will make them last longer.

 

[der_wille_zur_macht]

Switched to a double CAT4101 and added a LM317 & pot to hit that +0.5v sweet spot with mix LED's better (thinking of the soon to be out XM-L's with XP-E's). Mostly just practice - I used spice (p_spice?) about 20 years ago.

Thanks.

You're probably well aware of this but putting an LM317 in front of a CAT4101 is just transferring load from one linear to another - you're going to be taking an efficiency hit either way, the only advantage is dissipating heat from two packages instead of one. Also the LM317 has a pretty big minimum dropout specification, so in order to run a circuit with one of them inline you're going to need a big overall voltage drop anyways which means a pretty big efficiency hit (At 50c and 1A the minimum dropout is something like 2v). You're probably going to be near the thermal limit of the LM317 in order to meet it's minimum voltage drop at the current you'll be running your LEDs at.

If you want to be able to run a bunch of CAT4101's at different voltages off the same power supply and don't care about the efficiency hit of dissipating the extra power in a linear device, you might as well just experimentally determine an appropriate value of power resistor and put that inline with the circuit. That way you're able to get the exact voltage drop you need, instead of having to hit a minimum drop in the LM317. This approach is actually what's recommended in the CAT4101 datasheet for voltage drops greater than what the IC can handle.

Of course another option is to just get multiple DC power supplies and "dedicate" a supply to each Vf you need.

Regarding heatsinks and mounting the driver boards - I haven't had to put a heatsink on one yet, but most of mine are running at low dropouts and all of them are essentially in free air (the most "constrained" I've had one is in a 2' x 1' x 1' plywood box with a large vent in the top.) If I were to mount one in an enclosed space I would absolutely provide some air movement and/or heatsinks. The good news is you can determine this experimentally. Build your setup, and run it, and see if they get hot enough to trip the thermal protection feature. If they do, add more air movement and/or heatsinking. If they don't, you're good to go.

 

[TheFishMan65]

dwzm, Can you leave your hand on it? IIRC I could barely leave my finger on the CAT4101. I ran for a couple hours and never hit the thermal protection.

 

[der_wille_zur_macht]

I can touch all of mine and not get burned. Those that have higher drops are not comfortable to touch for more than a few seconds. I know this is highly subjective but I'm guessing this indicates it is well below the maximum junction temperature of 150c.

 

[TheFishMan65]

"not comfortable to touch for more than a few seconds" that is what I am remembering. I think when I tested one I did not have a problem. When I ran several with different Vf from the LEDs is when I noticed the heat. Since heat is the killer of electronics I would recommend some sort of cooling, but I also agree that we are probably below the 150C point.

 

[DustinB]

Have you guys adjusted your power supplies til you just get the 700mA or so(based on whatever your RSET resistor is)? When doing this I can hold my finger on the drivers indefinitely with the fans blowing through the fixture. It's pretty warm, but not to the point of having to remove my finger.

I'm running my LEDs all at 700mA with 6 per string on each driver FYI...

 

[TheFishMan65]

How many strings? What is the voltage of the power supply and the voltage of each string, please and thank you.

 

[der_wille_zur_macht]

Yeah, pretty much Dustin. It's kind of a six-of-one situation. You can EITHER adjust the Vin upwards until you see the target current on your LED string, OR adjust Vin upwards until you see the drop across the CAT4101 start to increase (it'll stick at .5v when you're below this point). These two events should happen at the same time, and at that very point, the chip will be regulating in a true constant current mode with the lowest possible drop. At this point the IC will be dissipating the smallest possible amount of power and will be as cool as possible given your drive current.

 

[kcress]

Barely touchable(metal) is about 108F, painful is about 110F, which is well below the typical 158F limit.

 

[artemm]

I am sorry, but i just can't read all 53 page. just cannot. i tried and i miserably failed
Maybe it was somewhere along these pages?

Why not do the following:
1) Power supply - buy a well build 100-150W notebook power supply or convert a more
powerful ATX power supply to be a standalone ps.
2) Make strings of leds. For example, for 12V and leds with 4V Vf the strings will have 3 leds.
3) For each stings put a MOSFET with ultra low resistance (there are plenty cheap ones on ebay)
4) Connects gates of all mosfets all together and route to a PWM output or your preferred MCU (powered by a separate voltage regulator)

Positives: no hassle with power supply, no led drivers in common sense, very cheap
schematic, high efficacy (no wasted current on heating extra components like Rs), no
Rs for strings because V fall 100% on the leds.

Negatives: any?

I did not do it for aquarium, but i did it for fun lights with this summer for my wedding It works
http://www.youtube.com/watch?v=xZQBRB3fuP0
Since i powered the whole thing with 3.3V li-ion battery i could put only one high power led there (in the video it is the white led) and many piranha leds (greens) - it works w/o any problems.

 

[der_wille_zur_macht]

Negatives: any?

I can think of one major and obvious negative. You're not actually regulating or limiting the current through the LED! You're driving it in a highly imprecise and unstable manner. When the MOSFET is on, the LED is exposed to the power supply with nothing to limit the current. Because of the unnatural V/I relationship of the LED, it basically looks like a dead short to the power supply, which is trying to regulate output voltage, NOT output current. Hence current can and will vary significantly, and if you try to provide enough current to light the LED near it's limits, you will get a runaway situation and something will break.

This might work if you're targeting a very low current and don't care about precision, intensity, or lifetime of the LED, but in a fishtank, I care about ALL of those things.

A better solution would be to put a current limiting resistor inline with the LEDs in addition to the MOSTFET. Then you'd have control over the current, but it would be static control - if things changed temperature, or your LEDs' Vf was different than you expected when you calculated the resistor's value, your current wouldn't be what you thought it was.

 

[der_wille_zur_macht]

Since i powered the whole thing with 3.3V li-ion battery

Keep in mind that a li-ion battery will respond very differently from a DC power supply. The internal resistance of the battery effectively acts like a current limiting resistor. Try to run the same LED on a 3.3v DC wall wart and you may blow the LED and/or pop the wall wart.

 

[RCBrooke]

I was thinking that the per board voltage adjust would be a good thing but the point about just transferring the load from one device to another makes sense. No point in extra components.

Another point you could clear up for me DWZM if you would, on your design the three caps (C4, C5, C6) are all tied to the 24v trace which then goes to each of the L1+, L2+, L3+ headers, are all three necessary or a product of replicating the schematic? (also the C1, C2, C3 on the 5v line). Are they there for ripple?

Thanks much - you guys rock.

 

[der_wille_zur_macht]

The CAT4101 datasheet specifies a capacitor across the LED string. Of course, the datasheet doesn't go into details about putting multiple such drivers on the same DC supply, much less on the same PCB, so I extrapolated a bit and put one such capacitor on each LED string. I'm sure they could be combined since they're essentially in parallel across 24v and GND but I don't know enough about the properties of capacitors to know exactly how to go about doing that (i.e. I don't know if it would be as straightforward as using a single 3uF cap instead of three 1uF caps).

 

[SpacedCowboy]

I am sorry, but i just can't read all 53 page. just cannot. i tried and i miserably failed
Maybe it was somewhere along these pages?

It may have been useful to read the first page, however... I don't see any current limiting in the circuit you describe, and that'll cause problems. The whole point of the driver chip is to control the current, otherwise we might as well link the LEDs directly to the PSU...

Simon

 

[der_wille_zur_macht]

I was thinking that the per board voltage adjust would be a good thing but the point about just transferring the load from one device to another makes sense. No point in extra components.

The most efficient way to do that would be to put a switching regulator on the board. But at that point, you might as well run the switcher in a constant current mode and do away with the CAT4101!

 

[SpacedCowboy]

The CAT4101 datasheet specifies a capacitor across the LED string. Of course, the datasheet doesn't go into details about putting multiple such drivers on the same DC supply, much less on the same PCB, so I extrapolated a bit and put one such capacitor on each LED string. I'm sure they could be combined since they're essentially in parallel across 24v and GND but I don't know enough about the properties of capacitors to know exactly how to go about doing that (i.e. I don't know if it would be as straightforward as using a single 3uF cap instead of three 1uF caps).

I'm reasonably confident that the 1uF cap is just there to help the PSU cope. I've not had any problems running without any (equivalent, because it's in my own circuit), when using a well-regulated PSU.

The caps on the input 5v line are needed "in noisy environments", and I would guess are just there for decoupling purposes. It's pretty standard practice to decouple Vin on digital electronics. I'd keep them because I'd reckon there's enough digital noise going on that there's no harm in having them.

Simon.

 

[RCBrooke]

SpacedCowboy, just for curiosity's sake, if the caps on the 24v line were to be changed, does it stay at 1uF or does its value change?

Thank you.