Pixel Blades, Wire Gauge, and You!

[jbkuma]

Here is a very convenient tool to figure out if your wiring is appropriately sized in your high draw pixel blade saber. I've book marked a typical application.
https://www.calculator.net/voltage-d...s=10&x=95&y=19

Using Watt's law you can take these numbers and sort out the initial* power dissipation by your wiring.
For example, using 6" of 28awg wire, at 3.7v, with a 12 amp load we would see a drop of about .78V which is a 21.05% drop. Our guideline is that we should not exceed 3% and we should really aim for less than that.

P=IV, P = 7 x .78, P = 5.46 Watts

We can calculate the initial* resistance imparted by the wiring using Ohm's Law.
V=IR, 0.78v = 7a x R, R=0.111ohm

As you will see below, this will put your wire somewhere between "melting" and "catching fire." The wire is insulated so the heat doesn't dissipate, and as the temperature increases the resistance of the wire will go up. This creates a feedback effect wherein as the wire gets hotter the resistance goes up and it gets even hotter until an equilibrium is reached with the heat it's able to dissipate, or the jacket of your wire melts and/or catches a flame. This is why I noted these as the "initial*" values.


Strydur performed experiments to demonstrate this effect:

I did a quick test using a power supply and a load generator. Keep in mind this is all tested with the wire we sell and could be different for the same thickness wire in a different brand. You would also get different results with longer or shorter wire, etc..

3" long 24 gauge
10A - Ok
15A - Hot
20A - Melt Casing

3" long 26 gauge
10A - Ok
15A - Melt casing
20A - Catch Fire

3" long 28 gauge
10A - Melt casing
15A - Catch Fire

source: http://forums.thecustomsabershop.com...l=1#post276869


In Tim's example of 3" 28AWG at 10A & 15A the power dissipation was: 10A x .32Vd = 3.2 Watts; 15A x .49Vd = 7.35watts

Keep in mind that if this was packed inside a saber, rather than a bench experiment, the effects would be worse. In any case a melting jacket will lead to a great sadness, and possible injury. Longer wires will also impart an increased resistance.

There will be complicating factors like the contact resistance of your connector, the actual charge level of your battery, and the color of your blade, but with these examples and the mathematics I hope you have been convinced to appropriately size your hilt wiring for your pixel blade.

[jbkuma]

As far as using multiple conductors to compensate for the drop:

The calculator does include a calculation for drop over parallel conductors. In general my advice is to design your wiring to the saber's capabilities rather than your expected configuration.

In the example I linked, it shows a fairly typical use scenario of 7A and about 5" of wire. With 22AWG wire you would have a voltage drop of about .13V or 3.5%. According to the guidelines for this calculator that is actually already a bit out of spec. For 2x 28AWG it would be about .26V per wire. I used multiple 28 AWG wires in the past, but I have long since replaced the wiring in all of my sabers with 22.

The situation is actually a lot more complicated than the simply saying "it dissipates about 2.2 watts." While this is true, the dissipation is over the length of the wire. There are applications where this resistance is actually counted upon, such as some string lights which use a long conductor on one side that loops back to drop the voltage into the range of the LEDs and balance the paths. In other situations the resistance of the leads is problematic not because of heat, but because the drop degrades a relatively low power data signal. This is why we use switches, routers, repeaters, etc. for our data communication lines. (you may already realize this, but it's worth mentioning for those who don't)

In this case we are using short wires though, and the overall dissipation can be treated as effectively as we would a resistor. Thinner wires will also have less mass and surface area for that heat. Multiple wires will have more surface area to dissipate heat. We will see losses in other parts of the circuit as well. Tinned vs bare copper (you do NOT want to use aluminum wire!), type of solder, quality of joints, jacket material, etc will also factor in, some things more than others. These calculated values are mostly useful describe the situation and use as a guideline.

I would say, based on experience and experimentation, anything more than a calculated 1 Watt will not be a good idea. The guideline of a calculated 3% drop stated on the calculator page is probably a good mark to follow.

[Tom Tilmon]

I follow JBKuma's advice.

[Kooky Mawnster]

In my entry to the most recent contest I used 20 ga for the mains to and from the neopixel strips and accent neopixels, and 26 ga wire everywhere else (speaker wire, momentary switch, data line). The strips are Adafruit minis so if I recall the theoretical draw for each pixel is 35mA full white, x120 x2 = 8.4 A for my setup. The data lines come to a common point at the PCB but each strip has its own +/- wires to the PCB.

It's tricky using 20 ga, especially connecting the PCB connector to the strips - 1" length 20 ga wire between PCB and strip becomes very stiff when soldered and the version 1 PCBs were not robust enough (pads from 2 PCBs were torn off by sparring 8 year-olds).

[Kooky Mawnster]

At what voltage was the test below done? A single 18650 is about 4.2 V fully charged.

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Quote Originally Posted by Strydur
I did a quick test using a power supply and a load generator. Keep in mind this is all tested with the wire we sell and could be different for the same thickness wire in a different brand. You would also get different results with longer or shorter wire, etc..

3" long 24 gauge
10A - Ok
15A - Hot
20A - Melt Casing

3" long 26 gauge
10A - Ok
15A - Melt casing
20A - Catch Fire

3" long 28 gauge
10A - Melt casing
15A - Catch Fire

[jbkuma]

At 3.7v the pixels should still be pulling around 47ma each at full white. 35ma might be with the brightness capped in firmware (FX-SaberOS details to cap at 230/255) or could be the the result of losses and battery sag.

You can read a full characterization report here: https://github.com/Protonerd/DIYino/...on_report1.pdf

You can design to a lower standard, but as long as you saber has the ability to go into those higher regimes there is potential for disaster.