Resistored split path question

I've got a plastic light formerly hosting a direct driven TY0K, which ran nicely at about 400 ma (li-on DD) and heat was manageable.

I swapped it out with a Cree XR-E (heard the Vf of these are around 3.4v at 500ma) and now it is driving at 800ma or so. Very nice and bright, but too hot for a plastic housing. Heatsoaks in minutes.

Everything is pretty much fixed in, so I can't install a converter now. The only thing I can think of is to add a resistor somewhere, and I plan to add one to the tailcap spring.

Original, current will flow through the spring along its coils. I know some people add solderwicker or wires from the top of the spring to the bottom to REDUCE resistance by providing more flow.

Now I want to do the reverse; can I add a resistor connecting from the top of the spring to the bottom? Will this add any significant resistance or will the current bypass the resistored path and still go on its merry way originally?

For e.g. __(10ohm)___
(-)-----|___________|----(+)

Will the above provide any resistance? Or will the current bypass the resistor and still continue to flow thru the open path?

I may be wrong, but I was always told when there are two paths to the same terminal, electricity will flow over the path of least resistance.

No, it will not add any resistance at all.

Is there a metal strap running down to the spring? Can it be cut and the resistor bridging the cut? Or, can you glue an insulating disc on the top of the spring, with a metal contact on top wired to the resistor back to the spring contact?

By the way, I think 10 ohms is too much, perhaps more like 2 ohms.

Crux

Thanks for the info. I might be able to do the resistored disc setup...

I haven't calculated the resistance (was an example) but thanks for the pointer! Basically I don't mind it running at around 400-450ma (batt draw), which should still be decent lumens based on the Cree specs. How many ohms would I need to get?

Easy (and cheap) resistor in the tailcap spring mod:
Get 2 brass/copper washers just big enough to sit on top of the tailcap spring without falling through. Buy/make a plastic washer the same size. Take your resistor and bend one of the leads at 90 degrees and solder the lead to one washer so the resistor is centered and perpendicular (imagine a thumbtack, the resistor is the pin and the washer is the head) Insulate the other resistor lead and bend it 180 degrees so that it runs along the resistor body and up through the hole in the washer. Make a washer sandwich with the plastic one in the middle and solder the lead coming through the hole to the top washer. Insulate the resistor and glue the washers so they don't shift. Install by putting the resistor down into the center of the spring with the washers sitting on top. Current flows from the battery to the top washer, down the resistor lead, through the resistor and out to the bottom washer and to the spring.

here's a good LED resistor calculator: http://led.linear1.org/1led.wiz

Nice idea mahoney! The resistor sandwich sounds like a good plan. Thanks for the tip!!

btw, as you can see I know little about electronics, I assume the resistor will really help to lower the current draw on the batt and subsequently reduce the current to LED (and heat).

Or will the current draw on the batt still be similar with a significant amount lost to heat at the resistance points?

Carpenter said:

I may be wrong, but I was always told when there are two paths to the same terminal, electricity will flow over the path of least resistance.

Click to expand...

It will travel all paths but most will flow along the "easier" path
Here is the equation and if you calculate it for a large resistor and the near zero resistance of the wire, you find the answer is very near the resistance of the wire.

This is why I'm not a electrician. I have a hard enough time doing V*A=W :thinking:

Mike Painter said:

It will travel all paths but most will flow along the "easier" path
Here is the equation and if you calculate it for a large resistor and the near zero resistance of the wire, you find the answer is very near the resistance of the wire.

Click to expand...