Reducing resistance at the tail spring on a Mag

[Battery Guy]

Greetings Everyone

I have read in a few threads that the resistance between the spring and the tail cap on a Maglite can be a problem with high wattage/high current hotwire mods. Does anyone have a summary on clever ways to solve this problem?

Thanks in advance.

Cheers,
Battery Guy

 

[AnAppleSnail]

Greetings Everyone

I have read in a few threads that the resistance between the spring and the tail cap on a Maglite can be a problem with high wattage/high current hotwire mods. Does anyone have a summary on clever ways to solve this problem?

Thanks in advance.

Cheers,
Battery Guy

Wrap thick wire, braided ribbon, or a strip of sheet metal from the top to the bottom of the spring. Of course, you've cleaned the contact points too, right?

 

[Battery Guy]

Wrap thick wire, braided ribbon, or a strip of sheet metal from the top to the bottom of the spring.

Does that mean that the spring itself is a source of significant resistance?

Of course, you've cleaned the contact points too, right?

Er, um....well of course I have Why don't you tell me exactly which contact points you are referring to so that I can confirm that I cleaned them all.

Thanks again for the advice.

Cheers,
Battery Guy

 

[AnAppleSnail]

Does that mean that the spring itself is a source of significant resistance?

Mostly this is to make a larger contact between the top of the spring and the bottom of the battery- all you've got now is most of a circle, not really even an area. Some people get clever and just solder a plate to the top (level) ring of the spring. I've seen braided copper ribbon stuff used to good effect.

Er, um....well of course I have Why don't you tell me exactly which contact points you are referring to so that I can confirm that I cleaned them all.

I hear good things about DeOxit gold - it makes sure you aren't trying to conduct through aluminum oxide or dirty metal. Top of the spring, bottom of the spring, and the bottom of the tube, to start with. Hopefully other people will chime in with their hotwire experiences?

 

[Battery Guy]

Thanks AnAppleSnail!

From what I have been reading, eliminating or minimizing sources of resistance is really critical in a successful high power hotwire mod.

Cheers,
Battery Guy

 

[Justin Case]

http://homepage.mac.com/rouses/circuits/PhotoAlbum373.html

It's not clear to me what the actual impact is. I've not measured the diam of the tail spring wire, but let's call it 0.1", which is about 10ga. 10ga wire resistance is 1 ohm per 1000 feet. If an unwound Mag tail spring is a foot in length, then the total resistance is about 1 mohm. At a 10A draw, the voltage drop is about 0.01V. Even if the spring wire is 2 feet long, you are looking at 0.02V drop.

If you insert a short piece (say 2") of 10 ga wire as shown in the link above, you are adding a parallel resistance of perhaps 1/6 mohm to 1 mohm, or about 0.14 mohm. So a 10A draw now results in a 1.4mV drop. Yes, you've reduced the drop by about 10X, but was the original 0.01V-0.02V drop significant in the first place?

Perhaps the contact resistance of the stock tail spring to Batt- is high and is significantly reduced by making a copper contact, as shown in the link above. I've made a few of these tail spring mods to add resistance to protect a WA1185 from instaflashing when driven by 3xIMR26500. A convenient plastic disk to use is the lens from a AA Mini Maglite. In your case, you don't want to add a voltage drop resistor.

Here is an example. You can see the black resistor, which is soldered to the bottom turn of the spring. A 22 ga white/black wire goes from the resistor to the copper contact glued to the center of the plastic disk.

 

[^^Nova^^]

It was my understanding that the high tensile spring wire has more resistance than "normal" wire. This goes for the spring underneath the bulb also (seen here).

Cheers,
Nova

 

[Justin Case]

Good observation, and something that I forgot about. Steel could have an electrical conductivity 10X less than that for copper. So move the decimal point over one place, and the voltage drop could be around 0.1V-0.2V. Now that looks potentially significant.

The bulb spring probably has high resistance. BigChelis has apparently had his bulb spring lose its spring temper when running an FM1909 bulb at around 5A-6A. I guess the resistance was high enough to cause the spring to heat up hot enough to go soft.

 

[RoBeacon]

Is the copper braid being used de-soldering braid?

 

[smokeychris]

Yes i believe it is!
:thumbsup:

 

[RoBeacon]

Sweeeet. Got plenty of that, good bye resistance!!!!

 

[mrartillery]

http://homepage.mac.com/rouses/circuits/PhotoAlbum373.html

It's not clear to me what the actual impact is. I've not measured the diam of the tail spring wire, but let's call it 0.1", which is about 10ga. 10ga wire resistance is 1 ohm per 1000 feet. If an unwound Mag tail spring is a foot in length, then the total resistance is about 1 mohm. At a 10A draw, the voltage drop is about 0.01V. Even if the spring wire is 2 feet long, you are looking at 0.02V drop.

If you insert a short piece (say 2") of 10 ga wire as shown in the link above, you are adding a parallel resistance of perhaps 1/6 mohm to 1 mohm, or about 0.14 mohm. So a 10A draw now results in a 1.4mV drop. Yes, you've reduced the drop by about 10X, but was the original 0.01V-0.02V drop significant in the first place?

Perhaps the contact resistance of the stock tail spring to Batt- is high and is significantly reduced by making a copper contact, as shown in the link above. I've made a few of these tail spring mods to add resistance to protect a WA1185 from instaflashing when driven by 3xIMR26500. A convenient plastic disk to use is the lens from a AA Mini Maglite. In your case, you don't want to add a voltage drop resistor.

Great link, and also a great explanation. A must for your Mag if you want to get the most out of it.

 

[PCC]

The middle line from this post is good information concerning resistance between the steel spring and aluminum.

 

[Justin Case]

It can be tricky to take the contact resistance for one setup and try to generalize it to a different setup. Contact resistance can depend on factors like surface roughness, material hardness, and contact pressure, all of which can be completely different from case to case.

I recently blew 3 1185 bulbs in a 3xIMR26500 Mag mod. If just the Mag tail spring contact resistance was the same as the SF Z41 at about 160mohm, I could have gotten about 0.6V drop just from the tail spring contact resistance alone. The fact that I instaflashed 3 1185s tells me that the total Mag system resistance, much less just the tail spring contact resistance, probably didn't add up to give 0.6V drop. Probably not even 0.3V drop.

Once I mod'ed the tail spring to insert 0.15 ohms resistance (using a 5% Vishay-Dale resistor), I stopped instaflashing the 1185s on freshly charged 3xIMR26500s.

 

[mrartillery]

Similiar story of mine, is that I currently use a 3854 H in a 2 C with 2xIMR26500's with never any instaflash. Others have stated that they have had problems with this bulb on 2 of these cells fresh, but I haven't done any resistance mods, I guess mine is just one of the Mags with a high enough resistance to keep from flashing it.