MC-E + single cell = what driver?

[AaronM]

I'd like to use an IMR18650 with a MC-E modded KL-4 head and would like to get even more output than the stock driver can provide.
Any suggestions? Perhaps the fatman?

 

[mudman cj]

I have been trying to contact Milkyspit to get an Acorn for the same application. It can deliver 1.5A wired 2s2p according to this post. At least I think that means emitter current and not battery current. This option would give you multiple output levels.

The Fatman won't be able to give as much output as the Acorn from a single Li-ion cell because of the input current limitations. You should wire the LED 2s2p to keep the input current down (input current should not exceed 2A, 2.2A max with heatsinking). (Edit: Actually, you could wire the LED in series and the circuit will still draw about the same battery current when set to max stable output level.) This will keep the LED die current below that which could be reached with direct drive because your power is limited to 3.6V*2A=8.2W. You also won't be able to make use of the external pot for a dimmable light using that small of a host. The Fatman would be a way to get brighter than the stock circuit, but it just won't be able to drive the MC-E to its limit using only one cell.

You could drive it off of two primary or Li-ion cells though and then wire the LED dies in series. Then you just set the output current to 700mA and you would expect an input current of about 1.5A from two Li-ion cells, so you would want to use IMR16340 for that. Current draw from primaries would be higher, about 1.8A.

 

[mudman cj]

OK, I think the Shark boost converter from the Shoppe is what you are looking for. It can power the MC-E wired all series with a trimpot-adjustable output up to 980mA with an input current of up to 4A. Running the LED at 4*.700*3.5=9.8W with 90% efficiency means a battery draw of (9.8/.9)/3.6=3.02 Amps. Keep in mind that these high current drivers require heat sinking - it says so under specifications.

 

[mudman cj]

I found another one. The MaxFlex driver could also work here, and it has some nice features like configurable UI, multiple output levels with adjustable current limit, and overheat protection with adjustable trip temp. It too requires heatsinking for this application.

Edit: There is a snag with the MaxFlex. Its diameter is about 0.020" larger than the stock KL4 driver board. I think it could be trimmed to fit...

2nd Edit: Oh, bigger problem. That board is supposedly 0.25" thick. And while there are no pictures of the other side of the board, I bet there is an inductor that accounts for that thickness. So, the thermal pad is on the wrong side of the board for it to mate with the stock KL4 heatsink (using a spacer of course). And just when I was thinking of trying it myself. :mecry: Then again, just maybe the tall components of that board on the side not shown are centralized enough to fit down into the plastic dome that holds the anode contact of the KL4 head assembly. :thinking:

3rd Edit: OK, I did find a picture of the other side of the MaxFlex board, and there is no inductor or other large component to contend with. Chips populate both sides of the board, so that must account for the thickness. Some of the components are pretty close to the edge, so some clearance needs to be machined out of the mating plastic part in the KL4 assembly.

 

[AaronM]

I was was looking at the fatman 'cause you can run the emitter 4S (all series) at 430mA on a 4V/1.5A input. As per this info:

http://www.taskled.com/techfatman.html

But that's preliminary testing, I wonder what the poduction unit can do? Could it take more battery current? I wonder if the minimum Vin of 2.7V wouldn't allow the driver to make full use of the capacity of the IMR cell or maybe it's just a good saftey feature for an unprotected battery?

The Acorn driver would be the obvious choice for me as well if I could just order it online and make it single output.

 

[mudman cj]

The final version of the fatman can go up to 2.2A max. So, the emitter current could get somewhere in the neighborhood of 510mA. To get this number, I assumed 90% efficiency, 3.6V battery voltage, and Vf of 3.5V. A safer region would be around 460mA (2A input current). With this, total power dissipated by the circuit is around 0.1*2*3.6=0.72 Watts (assumes 90% efficiency). This isn't a lot of power, but it is in the range where heat dissipation is a concern. Even with the fatman, it is advisable to provide a thermal path for the back of the PCB. This is the problem I keep coming back to. And it makes me wonder how the Acorn handles it - either Milkyspit provides a thermal path for the driver or it just doesn't dissipate that much heat because the max input current is 1.5A rather than the output.

I think the Acorn actually can be configured for single stage, but the problem, as you said, is availability.

And as for the 2.7V in requirement, you don't want to discharge Li-ion cells that low - including the IMR cells. You would get the full capacity out of them.

I think the fatman and the shark can do about the same thing here, but the issue with either of them is still heat sinking the board to keep it cool. If you can work that out then you are good to go.

 

[gswitter]

Since you're planning to use an 18650 (IMR or otherwise), why not just wire the MC-E 4p and skip the converter?

 

[mudman cj]

You make a good point. The drawback of course is that without regulation the output curve follows the battery's discharge curve of voltage with time, but it is much simpler than stuffing one of these drivers in there and figuring out how to sink it properly.

Direct drive will almost certainly require a resistor or resistors to prevent over driving the MC-E - they tend to have pretty low Vf. The resistor needs to be matched to the cell type because of their different loaded voltages and then only that cell type or one with a lower voltage under load can be used.

I really like the idea of being able to use any combination or type of cells in a Surefire lego head, so I would love to be able to stuff a maxflex in my KL4. I think the problem would be that components on the thermal pad side would interfere with metal in the KL4 head. I think you are right to focus on the Fatman among available drivers. It looks like its components might be far enough away from the edges to fit in the KL4, though I still have a lot of doubt about it fitting - perhaps it would with a few clearance cuts into the edges of the inner heatsink piece.

Then again, maybe I will install a D2Flex so I can get full power along with 800Hz PWM dimming...

 

[AaronM]

Well Mister Mudman sir, thanks the brainstorm and the heads up on heat-sink issues. I think I'll go grab a couple Fatmen:green:...the driver I mean!

P.S. Did you get the KL4 heatsink yet?

 

[mudman cj]

This brainstorming has been fruitful for me too. I has led me to a PWM direct drive circuit that a member custom makes. It will easily fit at 17mm diameter, and if I design the light to keep total LED current at or below 2.8A from an 18650, then I can interchange bodies to also run from 17670, 18500, or 16340 cells.

And I hope that heat sink gets here today...

 

[AaronM]

So it's buck circiut?
Can it handle more than one cell?

One of the benefits of the fatman to me is it'll run 1 to 3 cells if the MC-E is wired series...I think.

 

[mudman cj]

No, it doesn't boost or buck. It just passes the current along and turns it on or off at high frequency. It offers a medium and low using this PWM but that will just be some fixed percentage of whatever the total output is at any given time. That will depend on the cell type and battery voltage. This type of circuit would work for a single Li-ion cell wired to either a P7 or MC-E in parallel configuration possibly with a small resistor to limit current on a fresh 18650 especially (depends on particular Vf of LED).

For use with two cells or more (5.5-15V in), there is an option I found at KD that bucks to drive a parallel MC-E at 2.8A. I took a look at the size of this board, and I think it will fit in the KL4. Although it is small in diameter, it looks to be about the same size as the stock version in thickness. I really want to run single Li-ion cells mainly, so I am not interested in a buck circuit.

Another day without a heatsink...

 

[mudman cj]

I also just ran across this post saying that a single Li-ion cell can be used to drive an MC-E 2s2p with a Badboy driver. I don't think Wayne is talking about the BBNG, because that has a max Vout of 6V. The Badboy is limited to 1.5A input current though. So, you can drive at higher power levels using 2 IMR16340 cells in series for maximum voltage (but only if Vf is greater than or equal to 7.2V). For particularly low Vf, you would have to use 2 primary cells. Using two Li-ion cells, power would be 7.2*1.5=10.8W, which is more than needed. Two primary cells could deliver 6*1.5=9W. A single Li-ion cell is limited to 5.4W.