What Features Make A Tail Cap High Amperage Capable?

[GreyShark]

With the IMR's and high draw bulbs around now I've heard that not all tail caps can handle the high amp currents so I'd like to know what features are required to handle the amperage. It seems to me it would mostly be a resistance problem so being made of thick metal would seem to be enough. Anybody have an insight on this?

 

[baterija]

If you think of the tailcap as a bunch of resistor in series it might help. Resistance is related to the inverse of cross sectional area. The smaller the area the more the resistance will be in a given contact area, resistivity being the same. Small contact areas in a switch will contribute more to the resistance than the tailcap to body connection which has higher surface area. Those small areas may be less able to deal with the heat lost due to resistance in that area.

If I recall correctly this is in keeping with some of the IMR setups where clicky switches and their small contacts are less capable than twisties.

 

[Kestrel]

I imagine that a small electrical contact area, particularly if it is thin such as a component of a pcb or similar device, would also erode at a greater rate from the substantial spark created in a high voltage circuit upon connection. I gather that the necessarily small contact areas in a clicky switch are somewhat susceptible to this, for example.

 

[oregon]

Helpful for higher current are highly conductive contact materials and shorter paths to circuit completion. Both are seen here in this simple modification (braided copper cable shortening the gap from battery carrier to flashlight body otherwise bridged only by the metal spring) to a Maglite tailcap done for ROP (hotwire) purposes:

oregon

 

[EngrPaul]

The best spring is a copper alloy with gold plating. The copper bypass shown above is good as long as it stays in place.

Stainless steel springs are nice and strong mechanically, but poor electrically. Plus you can't solder to them, it takes a crimp connection or high force fit in the tube.

If a flashlight is regulated, go for the higher voltage source possible. More voltage means less current is needed to produce the same output.

 

[GreyShark]

Thanks a lot for the info. The bypass is a neat idea and I may just have to try it on my IMR setup.

 

[oregon]

The bypass material is inexpensive, highly conductive, flexible and easily obtained. Get two spools. I used a double bypass on my 1185 hotwire.

oregon

 

[peteybaby]

Isn't arcing a major problem too? I've never taken a tailcap apart to see how they're constructed, but if it's a mechanical switch, then switching a DC current is going to produce some arcing. When I worked in the electronics industry, some of the better switches, the ones with higher DC current ratings, had self-cleaning contacts, meaning that they would "wipe" against each other as the switch was closed. That helped to wipe away any soot from the arcing and ensure a better contact patch.

I just bought an MC-E light, and the max. current draw is supposed to be just under 3 A. At that kind of current, I expect to have problems with my tailcap switch some day, so I'm just hoping that day doesn't come until this light is obsolete.

Contact material makes a difference too. I don't know which materials are used, but I would guess that if the contact metal is too soft, it will pit more easily from the arcing, and eventually the contact surfaces would be rough and more susceptible to soot build-up, and eventually they won't make electrical contact at all.

Also, when the contact patch shrinks, all that current flows through a smaller tunnel, meaning increased resistance and heat build-up.

 

[GreyShark]

It is fairly easy to remove and replace the spring in a Surefire twisty tail cap. Is there any reason not to make a custom copper spring to replace what appears to be the stainless steel spring that comes stock? The stock spring appears to be about 16 gauge so I'd use 16 or 14 gauge copper wire to replace it. This wouldn't be as direct of a path as the bypass but it might be easier to keep in place.

 

[peteybaby]

I tried making a spring out of copper wire before. I found that the copper wasn't hard enough to act as a good spring. It would compress, but not spring back. I think you need some kind of copper alloy.

 

[GreyShark]

If it would work otherwise I'll give it a try. I inherited all kinds of weird stuff when I bought my house and among them was a lot of copper wire. I've already used some to make a spring. It's not quite the same thing as a steel spring but it does *sproing!*