Dale,I think it's simpler to just measure parasitic resistance of each driver.And based on that number you can tell which driver will provide more current on high(if all other factors like battery,led Vf,springs wires are the same).
For DD drivers this is very easy:you need one ammeter and one relatively precise voltmeter,battery or power supply(better),led or resistor as load(not important,xm-l,xp-g...).
Connect driver,led and battery(or power supply) as usual(you don't need host),and ammeter in series for current measurement. At the same time measure voltage(mV range) across driver's "power" connections-GND and led cathode pad.
Parasitic resistance is then: R(par)=U/I
And that's it,repeat the same procedure for other driver(important thing is that it's best to use the same battery at same voltage and same load,because mosfet parasitic resistance Rds, which is dominant part of total parasitic resistance in DD drivers, depends on gate voltage).
You’re saying if I measure the off amperage at the negative end of the cell, and the Voltage between LED negative and ground, then divide which by which to get parasitic resistance?
Touching the LED negative to ground will normally give direct drive, right? So this is where I take a Voltage reading? While taking an amperage reading at the tail?
I’ll either pull the light engine and do this, or remove the bezel and use the host as a holder for the assembly of light engine/cell. I can use my older DMM with short 12ga copper leads to do the amperage reading, my clamp meter with probes hooked up gives very small Voltage readings.
Edit: Is the lower parasitic resistance driver the one that will make the highest power output?
First,as RMM said,you must measure current and voltage on highest(no PWM) mode.
Parasitic resistance for first driver is 6.22mOhm (miliOhm) which sounds about right,slightly higher than mosfet Rds;
but something is wrong for 2nd one, 21.34mOhm is too high for that mosfet? Value should be similar to first driver.
For fair comparison,it's important to measure parasitic resistance at same battery voltage,because mosfet's Rds usually starts to grow rapidly at voltages <4V(of course,that depends on mosfet)
My not so able bodied and not so willing partner in crime made it a big issue this morning. Geesh, just wanted a little help! lol
miliOhm huh, I was guessing on the Ohm connection studying Wikipedia on Ohm’s law. lol
I did both repeatedly, the Clamp meter is Auto sensing and gave me Voltage on the first one, then mV on the next and wouldn’t switch over. So I did them each repeatedly, arguing with the cranky wife, we both yelled at the kid, and ultimately I tested them again by myself holding the amperage leads on the cell with my tongue. Now it’s me that’s cranky! Vroooom! I started! Woohoo!
I’ll charge up the cell I used and try again. With peace and quiet the prevalent factor.
Nice job Dale! Sorry, very busy at work now - no time for BLF during working hours and long days. Was just about to try a Mattaus v1.0 driver in the X6 - not sure if it's worth it now, but using 20 AWG of same exact length each, it will still tell me how they do in the exact setup, same light, same LED, etc.
Not a thread - just finished some tests with the BLF17DD v1.0 using the de-domed XP-G2 S2 and got about 0.15A to 0.18A higher, depending on the cell (SAM 25R or LG HE2). Best #'s for each driver:
A17DD-S08: LG HE2 @4.23v: 3.78A @tail
BLF17DD: LG HE2 @4.22v: 3.96A @tail
Same host, same LED (didn't even remove the LED - it's screwed down), same 20 AWG wire lengths for LED wires, same 22 AWG wired driver spring setup (different spring but don't think that should matter), same batteries.
Conclusion:
Well, with just one sample of each, there can be variables I'm sure: quality of reflows, tight driver mount, battery charge differences, FET variations, etc.. Seems slight edge to the old BLF17DD which is kind of what I suspected, but since I only tried one A17DD-S08 up to this point, I can't say anything definite from this - too little data, not enough samples tested, don't 100% trust my methods, measurements, etc. It's not lab level for sure... I would trust djozz more than myself - absolutely .
Several other options are discussed in this thread. Did you look for those? Rufusbduck suggested one I really liked and I think there were a couple of others which sounded OK too.
Exactly. We would need very precise measurements and tightly controlled test conditions if we were actually out to prove that one is better than the other. But to prove that the info from the datasheet applies in the way we thought it would? No problem, you and Tom E have already done that, in spades!
All that said, anyone wants to play around with testing different FETs, here are 3 which all appear to have some superior characteristics:
PSMN0R9-30YLD - rufusbduck pointed this one out, it has very similar characteristics to PSMN3R0-30YLD but is better. Slightly lower Rds(on) and slightly lower Vgs. Also significantly higher “total power dissipation”. We expect the lower Vgs to ensure that the FET remains more “fully open” on single cell setups.
SiR800DP - comfychair pointed this one out to me in reference to this driver by PM weeks ago. I forgot about it until I went looking for FETs which I thought might work properly with the QX7136-based A17LDQX driver and it turned up again in that context. Much lower Rds(on) than our selections so far and even lower Vgs than RBD’s selection. Again, the low Vgs should help keep the FET fully turned on.
SiS414DN - Another one which turned up during my search for good FETs for the QX7136. Low maximum current handling, 20A “package limited”. I’m not really sure exactly what a package limit involves, since we know that PowerPAK® SO-8 can handle at least 50A (the SiR800DP is spec’ed for that) and the graph in the datasheet goes way beyond 20A. Even lower Rds(on) than SiR800DP, but only slightly. Similar Vgs characteristics to SiR800DP. Lower Qg than SiR800DP, but I doubt that that will help us any. Costs less than the other two FETs in this list.
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