as current rises, forward voltage does too, so the circuit has to balance out somewhere right?
now whether the leds go poof before that happens…
I assume you meant 2 cells in series? 3 cells will kill them all unless you have a buck circuit in there somewhere.
I was thinking maybe it wasn’t a mistake, 4x XM-L2’s and an MT-G2 is gonna put a LOT of strain on a single string of batteries, without low modes I can see the voltage sag being high enough, even at start up, to keep the emitters safe.
Still tho it seems awful risky, that’s almost $40 worth of emitters your gambling with, to rich for my blood just to see it it’d work.
Edited my previous post. Not sure what I was thinking.
That’s what I was wondering about. I still haven’t got my teeth around what exactly is going on when you take a number of cells in series wired to parallel and then to series. So two cells would be doable with this setup?
What if I used this setup with 3*XML in series x2 and 1 X MTG. I guess most of the strain would then be on the MTG? I’m assuming it’s the DD on high that is the great decider of what’s going to blow.
I don’t really have a light for this XML/MTG mix. The idea has been in my mind for a while and this is the first time I’ve put it out there to find out if it’s possible. I don’t think it’s been done yet. Maybe it’s not possible, but it would make a really cool light.
I would highly recommend against putting a MT-G2 and a XM-L2 in a series string, going with 3 cells and strings of 3s XM-L’s would be fine, but you shouldn’t mix different voltage components in a series string even if the total voltage adds up correct.
I’d say your best bet is 2s, maybe 2 or 3p cells and 2s2p XM-L’s with another parallel string with a single MT-G2, that’s doable, mixing a MT and XP is really asking for it IMO.
I never planned to mix MTG with XML in the same string, I knew that would be bad. So basically, the number of XML in a series string has to equal the number of cells?
Does that apply to the MTG on a parallel string?
If I had three cells in series wth three strings, two of which are 3 x XML and one which is 1 x MTG, would the MTG be running at dangerous level, or would that setup be acceptable. (basically like the OP diagram, but with an extra XML added to the two strings)
That would fry the MT-G2 pronto; it would still be seeing the full voltage from three cells.
If your trying to use more than 2 cells your next option would be 4s cells. From three cells you can’t run a single MT cause its to high and you can’t run 2s MT’s cause it’s not enough.
With 4s cells (16.8v) you could run parallel strings of 4s XM’s and 2s MT’s. But really your best bet is the original diagram minus one cell.
Couple of things going on here:
Ouchyfoot - assuming that you can get the cells to keep sagging the more current you draw is bad. (Frankly this is one of the reasons I wish DD drivers didn’t exist, I think they serve to confuse people with limited knowledge of electronics.) Voltage sag beyond what HKJ shows in his discharge graphs is bad - hard on the cells. HKJ is already pushing many cells beyond reasonable limits (and he says so). What’s more relevant here is that there is a limit to battery sag. They will only sag so far and they will only sag so quickly. This is where you get poofing happening.
Now, I disagree with Cereal_killer about mixing Vf in series. That’s actually one of the major benefits of series setups - you don’t get current imbalances from mixing emitters. All emitters run in series will necessarily see the same current! If that current is too high then you still have problems, but it will be the same current. Hint: with a real driver that has any kind of regulation guessing about this is not required.
If you wire strings of two XM-L2's parallel to a MT-G2, according to my (undoubtedly not perfect) voltage measurements I did on one XM-L2 and one MT-G2 :
-at 3A through the MT-G2 the voltage is 6.4V, the XM-L2 string at 3.2V/led will receive 2A.
-at 3A through the XM-L2's the voltage over the string is 6.8V, the MT-G2 will receive 5A
So it is not really a nice balance, but the led-wiring should work if the driver can deliver the provided voltage and current.
In my OL-fromscratch build from last year I wired a XR-E, a MT-G2 and a XM-L in series (direct drive on a 9V block battery :Bigsmile: ) , again not a nicely balanced light output between the three, but it does work without a problem.
This is getting interesting, and I am understanding the pitfalls. My reasoning for starting this thread is because I’ve never heard or seen any discussions involving a multi emitter XML light that incorporates an MTG. I would have thought someone would have posted some kind of results by now, even if it were a failure.
Is there any kind of buck driver setup that is realistic.
At least when Scaru was around, he would have risked life and limb to just to see what would happen .
Like I said, with a regulated driver of any kind you just run them in series. Done deal - no problem. A buck driver is regulated, so you simply put the LEDs in series and go for it. Make sure you have a higher battery voltage than the combined forward voltage of all emitters at the desired drive current.
For example, a 3A buck driver running 2x XM-L2 and 1x MT-G2 in series would be driving 3.4 + 3.4 + 6.6 = 13.4v or so at that current (according to djozz’s crash tests). So feed that buck driver with 4-5 Li-Ion.
I still don’t really see why you’d want to do this. If I wanted to have a light with both XM-L2 and MT-G2 emitters I would certainly think I’d want to be able to control them separately.
it may work, you would have to test how current spreads between xmls and mtg. you wont know for sure until you try
I’m interested because it hasn’t been done before. Isn’t that enough.
Not for me. Lots of things haven’t been done before - that by itself does not give them value.
In parallel the voltage is the same across each of the chains. Current gets split up between chains. With the MT-G
having lower Vf than the 2 serial chains (assuming near typical Vf) it will get more than it’s share of current. You might look at putting a small resistor in series with the MT-G to get a little bit better balance between the chains.
Im fairly sure I saw a 5 emitter light with 1 MT-G2 in the middle and 4 XM-L2s around it here on BLF...
Thanks. I get that. Voltage stays the same and current is shared.
So current is determined by Vf. Lookup Vf in the appropriate chart to see what current you’ll be getting. Account for sag at that current with the batteries you are looking at. Lookup in the chart again. Go back and forth until things make sense. If what you want is just to light up those 5 emitters then it can be done with 2s1p batteries instead of 3s1p batteries. The draw of one MT-G2 seems to track the draw of 2s2p XM-L2 more closely than you might imagine. Each emitter is at a more or less appropriate-to-one-another current when you put a given voltage across the whole shebang.
Use these two posts:
https://budgetlightforum.com/t/-/19600#comment-417559
https://budgetlightforum.com/t/-/19546?page=1#comment-416063
and compare with the numbers from HKJ’s KK ICR 4200mAh review (these were the best results of the KK cells he tested):
http://lygte-info.dk/review/batteries2012/King%20Kong%20ICR26650%204200mAh%20(Gold)%20UK.html
There are a lot of variables here, but really it's just an exaggeration of the normal DD issues. I assume that this hypothetical situation involves normal cells, such as the KK's you used on the 9x Yupard. Let's scale up the normal single-emitter/single-cell DD problems to a situation with a battery which only handles around 10A but emitters that can draw a combined 30A+ before you account for voltage sag at the battery (real numbers, not a miscalculation)... Of course the voltage nose-dives and then you get to redo the calculation at some sort of unhappy medium. If you allow for an initial voltage sag of 3.6v per cell, you are still looking at a combined 16A draw at the battery. Still more than it can handle, but I feel like that's only slightly higher than where it would settle, at least for the first 10-30sec or so. Probably/maybe it would settle at around a 13A draw, like your Yupard. I wouldn't make a habit out of it. Note the tracking issues that start to show up later in HKJ's 10A test, this is due to the chemistry not supporting 10A very well. Compare that with what happens here and here in HKJ's tests of less impressive KK cells.
When you get a lot of voltage sag, that is the battery's way of saying "don't do that". When the battery gets hot, that's the battery's way of saying "no means no".
If you want regulation you could do a master/slave mod. Zener mod the master to control the current to the mtg and one XML and control 3 other XML’s with the slave. All running from 3s(x)p. Not sure how the Zener does with 12V but it is doable with a voltage regulator for the mcu instead.