xml t6 8 aa nimh nubers dont add up ????(pics added)
I built a t6 6 volt old school flashlight. Used at first 4aa nimh batteries with a 1 ohm 10w resistor . Vf was 3.1 at led. Pulled 1.25 amps . Switched to 8 aa's and getting 3.18 volts and 2.80 amps. Light is bright . With the online led resistor calculators the 1 ohm was what it said to get 3.5 fv 3.0 amps off of 5.6 volts (4aa) batteries. Why are my numbers so off from the calculator? I used 18 gauge auto wires, short lengths to keep resistance to a minimum. I just don't get it. Are my numbers good. Or can I change the resistor to get more out of it. Resistor get real hot even on a video card cooler. Led gets hot but not as bad as resister. Led is on a Northridge heatsink for now. Thanks in advance for your help.
Last edited by nicko42004200; 08-24-2012 at 07:38 PM.
Reason: pics added
4 AA NiMh in series should give 4.8v. Also, batteries have different voltages under load since they also have internal resistances. Even though the open circuit voltage might be 1.4v for the battery, the loaded voltage is probably different. Try loading the batteries and measuring their voltages under load.
The characteristics of the specific LED is also important. One could be 2.9v at 1A, another could be 3.3v at 1A. Also, you probably won't notice the brightness difference between 2.8A and 3 A.
It would mean that even assuming that cells sag to 1V under load you have 4.82V on resistor! At 2.8A it means 13.5W wasted on resistor. From that numbers it also looks that your resistor is closer to 2ohm than 1 (4.82/2.8). Something wrong here. They may be going down with voltage pretty far. What kind of cells are these?
Anyway, you should stick to 4AA NiMh, they will have enough voltage for 1 XM-L under any conditions (unless they are discharged completely and assuming they can deliver that kind of current like Eneloops can). More cells, more voltage has to be wasted on resistor. Than lower resistor value to get current you want.
Sometimes these online calculators are completely wrong for power LEDs.
With LED and resistor you have very simple circuit. Voltage across both of them is what you have from battery (4 of them in series in that case). As Gunner said voltage will be lower under load than open circuit. You have to measure it because it depends on load current and particular cells (internal resistance). Current flows from battery thru resistor and LED, it is the same for all components. Voltage from battery will be divided across resistor and LED. Your goal is to find voltage that goes to LED to get particular current and disipate rest on resistor.
Let assume that your 4AA have 4V under load (when loaded close to current you want, little tricky here since you probably measuring voltage sag at different current that you want). Say you want 3A on XM-L and you need 3.5V on LED to get that current (you need to find out how much voltage you need, it depends on particular LED, spec gives you a ballpark but each LED is little different). 4V input, 3.5V wanted on LED means that you need to lose 0.5V on resistor. At 3A it is 0.16ohm resistor.
Adding more cells may be necessary only if your NiMH cells have pretty low voltage when loaded with 3A. But then I would look for different cells because you loosing more and more power for resistance (resistor and internal resistance of cells).
If you had 3.1V and 1.25A on LED for 4AA then instead of adding cells change resistor to 0.5 ohm and see what happens (add second 1 ohm in parallel with first one).
Assuming you resistor is even 1 ohm, voltage from these 4AA was 4.35V (under load).
There are many variables here but changing resistor to 0.5 ohm would cause LED voltage go up (because of higher current) and battery voltage goes down (bigger loss on internal resistance again because of higher current). Say, Vf will be 3.2, battery 4.3V and LED current should be in 2.2A range.
Lowering resistor value further will increase current.
+1 on smaller value resistor, or you can use 3 cells with no resistors at all, 3nimh will drop voltage enough to be safe to use in direct drive, and current wont be over 3A, i have done that few times, the most current i got this way was 2,4a on fresh nimh cells, i used eneloops, energizers 2300mah, and some no name cells that died in a month and didn't put out more than 1,3-1,4a.
the only thing i would be carefull is using high current cells like titanium and elites, they can put out up to 10A.
The 8 cells are old, I mean old energized 2500. They still work though. The 4 others I used were new duraloop 2000. The resistor measured tip to tip 1.4 ohms. Leads tip to tip .4 ohms. Resistors one of the big 10w ceramic ones.I got from radio shack in a 2 pack. Should I put the 2 in series and use just the 4 duraloops? Would that give me about what I'm getting now with 8 cells? The lamp is amazing for throw and flood. It by far kills my 3 c8's with t6's ranging from 1.5-3.4 tailcap readings. It's not the prettiest though with my quick dremell square cut the get the heatsink through. I'm at work for another 10 hours today. What will a .125 ohm package read in no radioshack didn't have any that sais that. Would it be 1.25k ohm??? Also what watt resistor like a 1/2 with the 4 aa's .
Last edited by nicko42004200; 08-23-2012 at 09:06 AM.
Reason: auto correct
I would say for 4AA you definitely need less than 0.5ohm. Connecting resistor in series will increase resistance, you don’t want that. You can use 4 duraloops and connect two 1ohm resistors you have in parallel to get 0.5ohm. Most likely you will not get 3A though; resistance will be still too big.
You can get another pack of these 1 ohm resistors and connect third or all four in parallel. 3 in parallel will give you 0.33 ohm and 4 will have 0.25 ohm.
If 3-4 10W resistors are to big then you can try to find 1ohm 1W resistors at your local RadioShack. If you use 3 or 4 of them all connected in parallel 1W will be enough since they will spread power across all of them.
To parallel resistors do they need to be in a ladder like configuration or can I v them with 3-4 wire tip soldered to the wire? Like twisting 3 wires together and soldering them to 1 wire. Do u get where I'm going with that? Sorry for all the dumb questions and you guys are helping this newbee out tremendously . I love this new hoby of mine. My next project is a old Colman spot light that had a 12 volt battery in it. It will get a u2 and it got a massive metal reflector in it. It's got battery power thinking 2 18650s and a cig. Lighter plug for direct usage. This i think will be awsom. That is why I'm doing this cheap 6 volt first to get all the bugs out. And work on my technique. Cuz the Coleman will have 2 power options and 2 different power systems for the led. But a good thing about the massive reflector is it is used as a heatsink from Coleman and I will do the same eather for the resistors or drivers what ever I will end up using . I think it will be a sweet light when it's done. I got a few q5's would those be better for a spot light. I think the xml light output will be better when properly focused and have more flood to light up the surrounding area as a bonus. Thanks again guys you are the best here for advice and friendliness.
Well I added a third resistor off of 4 duraloops it's pulling 2.5ish amps and with 4 it Pulls 2.75ish. Took the fourth out made it get hot to quick. Between 1.5-2.2 I get good runtimes without it getting to hot to touch. Can go over 5 min. No problem. With my eyes can't tell real diff. From 1.75 to 2.75. Even after put loops with energizers 8 total with 2 resistors shot up to over 5 amps Vf was 3.38. I need a water cooler setup I think with the rite. Cooling I could go 6+ amps
base street iso 200
4 aa xml t6 at 2.5ish amps
backyard base about 40 yard to tree.
tree sorry for the blurr
and the piece
i no not the prityest inside or threw the lens. but hey it's my first ground up build and i'm happy with it . for now. laying on the table you cant tell the monster that lerks inside
Yea I was surprised too. I was just joking on the water cooling . 2-2.5 is best to keep the heat down. Only thing with the beam the spot is yellow if I put a c8 reflector over the led its white. Oh well I throws great. And floods awesomely.
Last edited by nicko42004200; 08-25-2012 at 11:35 AM.
Re: xml t6 8 aa nimh nubers dont add up ????(pics added)
i just was thinking i might add a 40 or 60mm fan on this to keep the heat down im thinking making a few slots in the front behind the lens cap and having the fan in the tail to pull hot air out and it will vaccum cool air in rite at the heatsink. should keep my run times alot better. no way to feal how hot it is without takeing the light apart and touch it because its plastic.
Re: xml t6 8 aa nimh nubers dont add up ????(pics added)
if you plan on doing water cooling, or active cooling, imo you should forget your plastic $5 host, and either build a light from ground up, or use a better host (something i've learned hard way, batteries don't like to be in hot enclosure, insulate them, or separate from hot parts).
it seems like you willing to put in great effort,would be a waste of it if all is in cheap body.