Spring bypass question

I’m just wondering why small brass pills aren’t soldered directly to the driver vs a spring (get ’em by the dozens at FT and the like). Have a big-ass bypassed spring in the tail, but the solid flat brass pill soldered to the driver should be seriously low-resistance.

1000% agree with you.

(Sheesh I haven’t used FT in years. Forgot all about them)

But there may be more (with me there usually is)…

Depending on the material used to make the spring, it may be difficult or impossible to make good bypass-wire solder joints. Assuming that the spring is solder-able I would prefer something like a #14 brass washer (or something like that) soldered to the spring (using a washer thin enough to allow the continued use of protect batteries, if necessary) much more contact area, which can’t be a bad thing.

Cheers

It’s not efficiency we are concerned about here, it’s output. If you look at djozz thread on springs you will see that a normal steel spring will have a voltage drop of near 300 millivolts. Add another 100 mV for driver and wire resistance if everything is done well. Taking into account the battery sag you now have a maximum of 3.7 volts available to the emitter.

An XPL-Hi at 3.7 volts will take roughly 4.5 amps producing about 1500 led lumens. Done properly, the same led given 4.0 volts will take about 6.5 amps giving 1800 plus led lumens.

For me candela is more important than lumens as I build primarily throwers and I can tell you from personal experience that the difference is substantial. For leds with a higher forward voltage like XP-G2 I like to use the 45% IACS springs because in my applications I can accept a roughly 11 mV drop.

By all means do the tests on various springs yourself and please post the results. We are always happy to have more resources.

Mountain Electronics sells brass buttons specifically for this purpose although I find it difficult to solder both the button and a wire bypass on the tail spring. If done properly a dome of solder can be applied to the top of the spring and sanded flat giving good contact.

The stock springs are solder-able as they need to be soldered to the switch board.

I respectfully disagree.

When efficiency is increased all kinds of good things can happen. Goodness = efficiency.

You’re assuming that supply voltage and emitter output are directly proportional. The electronics in between the battery and emitter negate the possibility of the proportional relationship you’re assuming. Of greater consideration should be emitter efficiency (lumens/watt) and/or efficiency of voltage/current regulators and/or efficiency of the processor itself along with the quality of PWM and PWM pulse integration and/or the on resistance of the low-side mosfet switch, along with transfer of heat from the emitter to the outside world.

Cheers

Ok, correct me if I’m wrong but the battery is connected directly to the emitter on the positive side (at least in the lights I build) and the electronics follow after on the negative return from the emitter. Or am I missing something here.

Edit: for clarity

As I mentioned earlier we are not concerned with efficiency when we do a bypass, just raw output.

V+ goes to voltage/current regulators . From there, regulated Vcc (usually 3.3 volts) goes to the processor. Buck regulation is easy. The processor looks at Vcc through a simple voltage divider. Next, the processor delivers PWM based on battery voltage and user input. That is the best case scenario, based on buck regulation for the electronics and an emitter that can be directly driven by the battery. All of that goes out the window when the emitter requires a supply voltage higher or lower than what the battery provides.

Next comes thermal sag. This is a very big deal as flashlights become brighter and smaller.

The point of diminished returns can happen when emitters are over driven. It can become impossible to remove heat fast enough, and the emitter dims as a result. I’ve proven this by driving a loose XHP70 on my bench. Driving the XHP70 at 12.2 volts resulted in more heat than I could remove using a fan-cooled heatsink. This is why turbo is usually limited to just a few minutes, but it doesn’t end there. Thermal sag must always be factored in.

Getting this stuff right requires a thoughtful, sensible design, especially now when flashlights are sold based on output lumens. When all of this is considered, soldering a bypass wire across the spring becomes a non-issue. The watt or two that’s lost across the spring is not a big deal when overall input wattage is considered.

Battery voltage stays flat up until the point when voltage drops very quickly. Bypassing the spring may result in an additional handful of seconds of useful light. IMO not enough justification for me to trash my springs.

So there you have it. Now harry steps off soapbox and slithers away. Cheers.

For a FET/direct drive situation bypassing the springs can result in significant performance increase, which is what Swib and others are referring to when they talk about performance increase. The spring resistance is a significant portion of the total circuit resistance. A good 18650 cell has 15 to 30 mOhms, FET has ~5 mOhms. So when you take out the 70+ mOhms of the springs it makes a big difference. I don’t think contact resistance is usually more than a few mOhms.

Edit. To summarize, bypassing the spring can result in more current to the emitter, which usually means more output.

Contact resistance is usually in the 1-4mOhm mark depending on materials used.

In that case, gold is usually 1mOhm due to its high conducivity and softness, nickel is usually 2MmOhm, and steel can be even worse at up to 3,3-3,9mOhm, or even higher in some cases.

Or even worse. A copper alloy spring which is oxidized can be worse than that with no coating

This is why I chose a nickel plating: most cost effective while still having low contact resistance.

In most cases, like the BeCu spring I made, I chose mechanical properties over pure conductivity. I could have went with BeCu C17500 instead of BeCu C17530.

I could have gotten an even lower voltage drop at 0.045V at 6A had I changed the material, but at that point, it would be way worse mechanically, and a 0,007V difference is quite small.

That’s the whole point of why I made this specific spring: to get a spring good enough for 95% of cases where you don’t need more than 13-14A of current, and easy to bypass copper alloy spring.

Using higher conductivity parts is also crucial in FETs, but also regulated linear drivers, but most importantly, boost drivers.

TLDR: A nice phosphor bronze spring, or even BeCu spring if you want even higher power, is usually great in most cases.

If you want uttemost performance, while sacrificing mechanical properties, go with a spring bypass!

Take a look here and you will see that in fact V+ does go directly to the led+ in the builds I do.

We all understand the issues with heat and this has been addressed by one of our own members (Toykeeper) who has done a phenomenal job programming driver software to include thermal management.

“In the builds you do”. That’s what you said. I thought you were referring to products designed by engineers. The products that we pay good money for, I was not referring to designs created by electronics enthusiasts. How, exactly, would you drive a 6 volt XHP70 from a single cell? How would you make that regulator small enough to live inside a flashlight tube? I commend you for getting your design to work. Did you us a a spring bypass wire?

You know a lot more about this stuff than I do, stuff for me to learn from, so thank you kindly for the information

Question- my guess is that a small LDO regulator is used only to provide the 3.3V or 5V the electronics require. I can’t imagine a linear regulator anywhere near the emitter.

Cheers

To begin with, I have never considered building a single cell XHP70. I build lights primarily for the marine industry. They pay good money for these lights because they give them what they need, namely a pocket-able light that will show them unlit channel markers, shoal markers and shorelines from 700 meters or greater. Generally the lights are on turbo mode just long enough to illuminate what they are looking for. As long as the battery lasts for a half hour on turbo they can navigate home safely.

Please don’t commend me on getting “my design” to work, it is the hard work of others on this forum that have given me the opportunity to utilize their designs to create amazing lights for my industry.

I do use a bypass on the driver spring to connect to led+ but only use tail bypass on high current lights.

Depends on the driver, cell and LED setup in your light.

Some emitters can be damaged or destroyed by too much current. For lights with FET drivers and those emitters you may wish to avoid spring bypass. Doing so means there’s more resistance and less current to the LEDs.

This is typically only a problem for certain emitters known for not being able to take high current when used in single-cell lights with FET drivers.

A properly coiled wire bypass never breaks.

I don’t understand why use a spring at any end…I have several springless lights from DQG and Peak and they do well while offering lower resistance, smaller size and good compatibility with batteries of different length. Springy lights that have fixed body length and for that reason sometimes designers choose to save several mm by not supporting protected cells. Springless requires no compromise, the light is always as large as it needs to be.

Your batteries aren’t going to be happy if you attach a springless light to your shotgun. :smiling_imp:

But I agree for a pocket EDC I really like how DQG did it with the 18650 Tiny III and IV. Springless battery compartment that auto-adjusts to fit any length 18650. Simple and elegant solution to the problem. I wish other manufacturers did something similar.

best is to use some high flex silicone wire, I use some with very thin insulation and about 250 strands per mm²
the finer strands the less likely is that it breaks
the best version to get mechanical stress relieved is to have a driver or contact board with an hole so the wire can move without bending

usually brisge with 0.35mm² or 0.5mm² depending on spring size

having the wire inside the spring reduces short issues a lot

To the OP, “efficiency” isn’t a single dimension, and those dimensions are interdependent with other design parameters and constraints. Moreover parameters and constraints are not continuously variable, they change in steps.

To lightbringer’s question, one reason to use a spring instead of a button at the driver-end of a light is that it means the weight of the battery is fully suspended in both directions.

If a jolt (like a recoil) makes it move backwards, relative to the rest of the light, a driver-side button will loose contact, whereas a spring will likely keep contact. A stronger tail-spring can mitigate this, but then you’ve got a chance of more wear on the battery.

The copper will work-harden at the interface with the solder blob. At some point, a strand will break, then another, then another. Coiling isn’t a panacea. While it reduces strain in t I’m not suhe body of the wire, that isn’t necessarily the case at the ends, particularly if the wire emerging from the solder joints aren’t oriented to minimize movement.