yet another 3D MAG DD question

[737mech]

I know this topic has been covered over and over but just want to make sure this set up is ok before I my new P7.

3d Mag
1 DSWOI P7
H22 heat sink
3 D "industrial" energizer alkaline batteries free from work! (may also use 3D Tenergy 10000 MaH Ni-Mh from time to time as well)
22 guage teflon wire

Is this a good setup? Anything I need to worry about? This is my first try at a mod so I am worried I am going to ruin my P7!

 

[dspencer]

That is the same light I just made. It was also my first attempt at modding a light and it came out OK.
My soldering of the led stuck out far enough that I had to cut too much off the reflector. I will think that part out better on the next light. Do the wire bending before soldering.

I tried the light on some Energizers before the Nimh EvergyON 9000 mah batteries. They are noticeably brighter on the rechargeables.

Good luck. Its fairly easy. I do now know how the switch works after taking it apart to solder the positive wire to it. Next time I'll take the advise in the pictoral threads and solder that wire without taking the switch apart.

David

 

[737mech]

yea, there is a lot of conflicting posts on here. Some say you will blow your led others say you will get 4 hours runtime while other say you can only use this bin or that bin ect ect...

 

[Justin Case]

Direct drive using alkalines isn't going to work very well. They just can't deliver the necessary ~3A of current for very long. See for example, this thread (look for the graph titled "D Battery Comparison at 3.0 Amp Rate").

The potential issue with using a DxxxI-bin P7 is that the forward voltage spec can range from 3.25V to 3.50V, while your NiMH cells could hold their voltage at 1.3V or more each for several minutes before sagging to closer to 1.25V steady state. that means a power source voltage of potentially at least 3.9V and perhaps about 3.75V steady state. If the NiMH cells are hot off the charger, they could deliver 4.2V initially.

The reason there are conflicting reports on or no is because it depends on the conditions of the DD setup. If you have a DxxxI-bin that happens to be at the low end of the Vf range, you are at greater risk of frying the LED when combined with some hot NiMH cells and low flashlight internal resistance (i.e., from the switch, wires, batteries, spring contacts, etc).

This is why some suggest using a J Vf bin P7, which is rated between 3.50V and 3.75V.

Are you able to measure Vf? Based on that, you could then install the appropriate drop resistor to protect the LED. If you can't measure Vf, then you could do a worst-case calculation.

Assume the lowest Vf of 3.25V for your I Vf bin P7. For your NiMH cells, assume that the voltage starts off at 4.2V nominal. Thus, you need a drop resistor value of R = V/I = (4.2-3.25)/2.8 = 0.34 ohms. The resistor needs to be able to dissipate at least P = V*I = 0.95V*2.8A = 2.66W.

 

[737mech]

Wow thanks for the great response. I do have a multimeter and can measure voltage at the tailcap but I cant measure the amperage because it only has a 2 amp max reading capacity.

Can I get a resistor like this at Radio Shack? 2.66 watt?

 

[Justin Case]

You want to measure the voltage across LED's + and - leads to get Vf, not at the tailcap, which would give you the resting voltage of your battery stack. Ideally, you would also have an accurate measurement of the current to the LED, but that requires inserting the DMM in series with the LED, so you'd have to partially disassemble your flashlight setup. Also, many DMMs, especially the inexpensive ones, have significant internal resistance which will affect your current draw reading (you will read a current draw lower than without the DMM in series with the LED). Thus, if you have two DMMs to make your Vf and LED current draw simultaneously, that might be the easiest route to take.

Most likely with alkalines driving your light, you will not have any problems with frying the P7. The alkalines just can't deliver the voltage and current to cook the LED. It's the NiMH (and Li-ions, if you use those) cells that have that potential. If your P7 is well heatsinked, however, it probably can withstand fairly significant overdriving beyond the rated 2.8A. Probably not wise to push too hard though. In theory, the P7 is like four P4s, which can take 1A. In reality, it's not quite that simple and you can't say that a P7 should be able to take 4A.

I don't know what Radio Shack has for resistors. The key is to get the right resistance value and then pick the wattage of at least 2.66W. If you get a 5W resistor, and the price is right and it fits the light, then that's fine.

 

[Justin Case]

A quick Google search found this ebay resistor:

"8 x Smart 0.33 ohm 5W 10% non-inductive power resistor
Item number 120396256578"

$4.99 for all 8.

 

[kz1000s1]

I've built 2 3D's with J vf bins tht were ok on alkalines running DD. Also a few running on an 18650 Li-ion at 4.2V hot off the charger with an I vf bin that are fine so far, one in constant use for 2 months. Next I have 5 more DSWOI's that will be DD builds, but I'm not expecting any problems.

 

[Justin Case]

When you say that the two 2D mods ran "ok" using alkalines, can you quantify that? What were the Vfs for the P7s? What was the current delivered to the LED for alkaline vs NiMH? Hot spot lux for alkaline vs NiMH? Hot spot lux vs time for alkaline vs NiMH?

You do realize that the I vs Vf curve is extremely sensitive to small differences in Vf? For example, going from Vf = 3.3V to Vf = 3.4V can change the input current to the LED from 2000ma to 2800ma. Such a P7 could hit 5A at about 3.6V, which would probably be unhealthy without excellent heatsinking. On the other hand, a high Vf P7 would see much lower currents.

The crux of the matter is that it is a Vf "lottery". A high side I Vf bin P7 at 3.50V could easily withstand the initial (and presumably brief) 4.2V input from an 18650, then run fine as the Li-ion sagged to its nominal 3.6V or 3.7V. A low side 3.25V P7 might have some issues depending on how well one heat sinks the emitter and the resistance in the rest of the flashlight.

See the discussion here for example. Note also that Vf can change with time and-or temperature. Thus, the Vf bin letter is a useful guide, but ideally you should actually measure the Vf of your specific P7.

 

[kz1000s1]

Ok as in they didn't fry or overheat running direct drive on alkalines and NiMHs. Output visually with fresh alkalines was around 3/4 the brightness (with fresh cells), of NiHM or my 18650 P7. Better than I thought it would be. I can't get amperage right now, my meter is acting up. They were J vf bin, but I don't know actual measured vf.

I know it's a lottery, but I do all I can to help the odds with the best thermal management possible. The 3D lights I build use the stock tailcap spring for a some resistance and more margin of error with high capacity NiHMs. The 18650 lights have ground wires on the spring.

 

[shdwkeeper]

So are you saying if I have a 3D MAG and I want to make it a DD, its better to use the D***I bin instead of the D***J bin if I want to put 3xNIMH D Cells 12000 or does it really matter if its I or J as long as its heatsinked properly.

Also in a 2D MAG can we same the same thing if we use 3x NIMH Cells?

Just curious

When you say that the two 2D mods ran "ok" using alkalines, can you quantify that? What were the Vfs for the P7s? What was the current delivered to the LED for alkaline vs NiMH? Hot spot lux for alkaline vs NiMH? Hot spot lux vs time for alkaline vs NiMH?

You do realize that the I vs Vf curve is extremely sensitive to small differences in Vf? For example, going from Vf = 3.3V to Vf = 3.4V can change the input current to the LED from 2000ma to 2800ma. Such a P7 could hit 5A at about 3.6V, which would probably be unhealthy without excellent heatsinking. On the other hand, a high Vf P7 would see much lower currents.

The crux of the matter is that it is a Vf "lottery". A high side I Vf bin P7 at 3.50V could easily withstand the initial (and presumably brief) 4.2V input from an 18650, then run fine as the Li-ion sagged to its nominal 3.6V or 3.7V. A low side 3.25V P7 might have some issues depending on how well one heat sinks the emitter and the resistance in the rest of the flashlight.

See the discussion here for example. Note also that Vf can change with time and-or temperature. Thus, the Vf bin letter is a useful guide, but ideally you should actually measure the Vf of your specific P7.

 

[shdwkeeper]

Not sure where my other post went. I'll try this again.

So are you saying that if you want to build a 3D MAG DD its better to use a D***I bin the a D***J bin? If you are going to put 3xNIMH D cells 12000 in there is that good? Or will both Bins perform pretty equally?

How about a 2D MAG with 3xNIMH C cells? Same question.