Pixel Blades, Wire Gauge, and You!

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.
Voltage Drop Calculator

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 of The Custom Saber Shop 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: TCSS Forum

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. Longer wires will also impart an increased resistance. In any case a melting jacket will lead to a great sadness, and possible injury.

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.

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.

Using Multiple Wires
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.

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