Spring bypass current

buck91 · 2019-11-20T01:19:51.000Z

Pretty straight forward, but at what current draw should we be looking at bypassing the springs in a light? Specifically, I have a Lumens Factory D36 Turbo Head spec’d with the XHP50.2 and compatible with 7.2-18v input which is rated for 1500 lumens. Currently running off 4xCR123 but I will also run 2x18650. Unfortunately my MM blew its fuse so I’m waiting for a replacement fuse to arrive tomorrow but at what battery current draw should I be looking at bypassing the springs?

moderator007 · 2019-11-20T02:46:33.000Z

Your driver is probably a buck driver with a set constant current of about 1.7 amps or so. You can look at djozz test with the XHP50.2 and see were 1500 lumens are at in current to the led assuming its 6v version.

Testing a Cree XHP50.2 J4 3A led Flashlight Modding and DIY Parts

[image] [image] A new generation of XHP50 from Cree, they called it XHP50.2 . The appearance of the led is in line with several other recently updated Cree leds, with rougher phosfor layer that is applied differently. Let's test it. I bought two XHP50.2 leds from Kaidomain, of which the J4 3A bin is tested here, so a neutral 5000K tint. The J4 output bin is the highest bin in the datasheet, both for XHP50 and XHP50.2, but at this moment Cutter sells XHP50.2 leds in K2 bin (as often hap…

Thats just a rough estimate but should be some what close to the led. With a buck driver the voltage is higher than the led vf. So if you have twice the voltage of the vf of the led the driver would only need to pull half the current from the cells to operate the led at twice the current drawn.
12v battery x 1 amp draw = 12 watts to a 6v led
6v battery x 2 amp draw = 12 watts to a 6v led
That means your probably only drawing around 1 amp with the cr123 and around 1.4 amps with 2 18650’s, just a guess.
Adding a bypass wire will probably do little to nothing for this light.
Most of the time a bypass wire is added when your using a fet driver where its direct drive and ever little bit of resistance determines the current as well as the battery used.
Anytime you have a resistance you have a voltage drop and looking back at djozz graph you can see you need a specific voltage to reach a certain current. If you loose to much voltage from resistance (voltage drop) in the circuit you dont have enough voltage to reach but a certain current.
I have used boost drivers that could draw upwards of 15 amps from a single cell and that is a good place to use a bypass wire also. Its having to boost the voltage and any place you can avoid to much voltage drop helps keep the voltage high for good output. It also can heat the spring up to a melting point if a good low resistance spring isn’t used.
I would guess to say anything running a single cell or multiple cells in parallel would benefit from a bypass wire around 5 amps or more. With a buck driver its not going to be needed unless its putting out some serious output.

buck91 · 2019-11-20T03:09:59.000Z

Thanks for the detailed explanation! I did see djozz’s graph after posting but its very interesting that at least one person isnt worried until a 5A draw. I have some lights which pull around 2-3A off the batts so I won’t really worry about it for now.

SammysHP · 2019-11-20T06:03:10.000Z

Side note: If you want to measure currents, use a clamp meter. Then it won’t blow your fuses and doesn’t add additional resistance.

Lexel · 2019-11-20T06:42:43.000Z

current measurement with DMM will always show way too low currents
the higher the more it affects the reading
100mA is fine,
1A will be slightly affected depending on meter internal resistance and leads quality
anything above 2A is too low with any budget meter/leads

just keep in mind a meter+leads has more resistance than most good (copper alloy) front and tail springs together