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Author Topic: Pixel Blades, Wire Gauge, and You!  (Read 288 times)

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Offline jbkuma

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Pixel Blades, Wire Gauge, and You!
« on: January 18, 2019, 07:41:23 AM »
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:
Quote from: 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
source: Wire Gauge for Neo Pixel


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.
« Last Edit: January 18, 2019, 07:54:55 AM by jbkuma »

Offline JakeSoft

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Re: Pixel Blades, Wire Gauge, and You!
« Reply #1 on: January 18, 2019, 08:07:44 AM »
Thanks for posting this.

I assume that in a pinch, two smaller gauge wires could be twisted together at the end points thereby doubling the effective width of the wire. Assuming the two wires were of similar length, the load would be split evenly between them. So, for example if you were to twist together two 26 gauge wires you should have no problems unless your blade is insanely long and you are running high-drain mixed colors all the time. Does that sound about right?

Offline jbkuma

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Re: Pixel Blades, Wire Gauge, and You!
« Reply #2 on: January 18, 2019, 08:49:15 AM »
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.

Offline Kolgrima

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Re: Pixel Blades, Wire Gauge, and You!
« Reply #3 on: January 18, 2019, 10:46:59 PM »
Good post, and good tool. as a Pixel newbie, this helps me confirm the math I've done my self and I'm glad I went with 22AWG copper, fire Lithium-Ion Batteries don't mix well, and I'd rather craft lightsabers than pipe bombs and you know... keep my hands.

Offline JakeSoft

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Re: Pixel Blades, Wire Gauge, and You!
« Reply #4 on: January 19, 2019, 09:19:00 AM »
Now that I have read this, I am questioning if the wire on the TCSS 10A 18650 battires is adequate up to the PCB'S rated cut-off current.

https://www.thecustomsabershop.com/Panasonic-Li-Ion-18650-37V-10A-3200mAh-PCB-Protected-Rechargeable-Battery-P1282.aspx

All the current for the entire saber must pass through those wires, so the amp draw from sound board itself is also included in that. I have one of these and it certainly doesn't look like 22 AWG wire. I hope it's copper.

Offline jbkuma

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Re: Pixel Blades, Wire Gauge, and You!
« Reply #5 on: January 19, 2019, 12:21:50 PM »
In reality the connector isn't rated for 10A either. I think these batteries and connectors have proven themselves out, but that shouldn't be the end of the design considerations.

A lot of sabers end up losing power through various incremental losses and end up dropping the actual delivered power significantly. This is probably why we don't see as many meltdowns as you might expect. Between the solder joints, MOSFETs, connectors, sag, and wiring losses the blade might end up seeing up to 1V less than expected and the current draw from the LEDs drops considerably.

Instead of relying on poorly chosen components, we should be designing for better performance: use as few connectors as possible, wiring gauge as short and heavy as is reasonable, and a soldered in battery.

I've been debating going to 20 or even 18 on the power side, and I think I will do that.  The calculator was also useful to confirm that a few accents are fine on thin gauge wires (I was only really concerned with performance there).