I ordered a direct drive driver circuit board from FastTech.
I always thought direct drive was just an LED being driven directly from the battery. If that’s the case, what is the circuit board for? This board has Hi and Low, how can you get two modes from direct drive? I’m sure it just me that hasn’t really grasped the technical concept, but if I’m going to put it in a light, I guess I should make an attempt.
A simple tutorial on direct drive would be appreciated.
Hi,
I thought the same as what you said, so I’d be interested in responses also.
The other possibility is that FastTech’s description may be wrong, i.e., this may NOT be a “direct drive”. If so, you can go to their website and open a support ticket, and they will correct the description if it’s wrong, and very quickly in my limited experience.
Jim
I think a lot of confusion stems from casual use of the phrase. We casually say that putting a 14500 in a light designed for 1*NiMH operation "goes into direct drive" if the circuit is boost only. I think we commonly mean that current output to an emitter and therefore brightness _directly_ correlates with cell voltage in such cases--hence, why it's very enticing to simply type "direct drive".
When drivers do not regulate current when Vbattery is greater than LED Vf (or at all), the emitter is not on the receiving end of a Li-ion cell's full voltage. Rather, drivers that "go into direct drive" or simply "are direct drive" on 1*Li-ion cell limit current with SMD resistors. Modes come from PWM.
I would say "direct drive" means that on high mode the current is unregulated; as much current as the emitter wants is passed thru the driver. The only real reason for the driver board is to provide modes (i.e. a medium at 50% current level of what it pulls on high, and a low at 25% level).
Resistance in the current path (springs, emitter wires, switch, etc. . . ) will play a part in determining the current draw on high (and battery ability).
-Garry
I think it uses PWM to give the low mode.
will direct driving an xre-q5 from a 14500 cell (wired directly from battery to light, no driver board) burn it out?
Depends on the quality of the 14500. Better cell will hold higher voltage under load than a lower quality one. There is also less of a chance of frying an XR-E than, say, an XP-G because the former consumes more wattage for a given current (has a higher Vf). In other words, at 4V an XR-E may draw 3A, while an XP-G might instantaneously draw 20A at 4V before going up in smoke*. Dialing things back a bit, an XP-G may drop only 3.4V at 2.8A (the numbers are imaginary but should demonstrate what I mean).
*although sufficiently otherworldly heatsinking makes nearly anything possible 
The cheap drop-ins I have seen are true direct drive on high, meaning with a crappy old ~1200mAh laptop pull 18650 it draws whatever the cell can deliver, with mine that's about 1.8A. With better cells (but still laptop pulls, the best ones are unmarked but I guess to be 24-2600mAh), those in the same light pull 4.5A.
I have built some with no driver at all, just a contact board with a spring in the middle. An XM-L will survive that just fine, at least with the mediocre cells I use. The weaker the cell is, the more the load will cause the voltage to sag, which keeps the LED from popping. Something like a good 26650 would probably be an instant-death situation.
What would happen if I replaced the wires on the above driver, and ran an XP-G2 with a brand new 18650.
Depends on the cooling. I haven't played with any XPG2s direct drive with lithiums, but do have a few here, one with a 3.04A Nanjg 105c and another direct drive in a 3D Mag, but with just plain alkalines, which only pushes about 1850mA.
I would assume it would be fine. I have done the same with a T6 and I know you can do it for a short period of time on a Q5. Check out the thread on the Jacob A60’s those are a Q5 XRE and people have direct driven them. I guess it would all depend on heat sinking and resistance. With good heatsinking you won’t get that much thermal sag and with higher resistance you are less likely to pop an led. With heavy gauge low resistance wires and a fresh battery you might pop it though.
LOL, I have pushed +12 amps from two 18650s in series through five XR-Es wired in parallel and they survived just fine. Too much heat for the rest of the light, but the LEDs seemed happy. No weird tint changes, no dimming, nothing asploaded...
Okay. Not Direct drive…say…a 1.4A Nanjg driver with XP-g2, would there be any real benefit if I replaced the stock wires with 20AWG wires.
I would say no, not at that current level. What are the stock wires, 28AWG?
I dont know. The usual flimsy wires that come on most drivers.
(I think we were just spammed, I got rid of it.)
I would have to agree with comfychair. You may see a slight increase with 20AWG but not enough to warrant disassemble to replace them. If you where pushing 3 amps then yes, you would probably see noticeable gains.
I’m just new beginner at building and modding. I figure, if I’m building a light, why not upgrade whatever i can before I install it. I thought it might help in a small way.
If its not installed yet, go ahead you have nothing to loose and you might even gain a few lumens. I never use the supplied wires. I have a assortment of high strand count silicone wire I replace them with. I like the silicone wire because its really flexible and doesn’t melt when soldered to.
This is the wire size chart I usually use. Looking at the current I will be operating at for a particular size wire in the chassis column. I will usually go with the next common larger size. Looking at 28AWG it will carry 1.4a in short runs. I usually choose the next common size up, 24AWG good for 3.5a in short runs.