It's come up on one of the Facebook groups I follow that JST connectors, which battery manufacturers commonly install on high current batteries, are not rated to carry the current which the batteries are capable of supplying. This raised the related issue of the current carrying capacity of the wires themselves.
I wanted to post a response which is more suited to a forum post than a FB post. So here I am. First, an introduction: I am completely new to the saber building scene. I'm a mechanical engineer with a background in robotics, and I'm currently a contractor at NASA Langley Research Center. Over the years I've heavily dabbled in the world of electric powered RC aircraft, and I've been a mentor on a high school robotics team for going on ten years. Connector and wiring selection are important in all these fields. I'm approaching this issue using the theoretical knowledge of an engineer, mixed with a healthy dose of real world application in similar, but not identical, applications.
Let's start the conversation with the observation that overcurrent failures of connectors and wiring are exceedingly rare. It takes extreme current draw for a sustained period of time to trigger the relevant failure modes. That's because those failure modes are heat and temperature related. And any mechanical engineer can tell you that heat transfer is a black art which only Sith can fully master. Essentially, the issue is that electrical circuitry is constructed of copper and solder, and non-conductors like plastic and phenolic circuitboard materials surround the metallic conductors and guide the electricity on its intended journey. As long as the electricity stays where it's intended to go, there is harmony in the universe. But, in metallic conductors there is resistance. When flowing through resistance, current leads to heat; heat leads to melting; and melting leads to electricity deviating from its harmonious path. When this happens, the magic smoke is released, and without the magic smoke, sabers die.
So, in summary, we need to keep things cool so the insulation doesn't melt and let conductors short together or against the metallic case. It's interesting to note that if the circuit were suspended in air, such that the wires wouldn't touch, the insulation could completely melt away and the circuit could continue to function up until the conductors themselves melted. As the temperature increased, the solder would melt first, as it has a lower melting temperature than copper. The failure modes we need to prevent, in order of likelihood are: melting insulation (leading to shorts), and melting solder joints.
When proper soldering technique is used, it is exceedingly rare for solder joints to fail from melting. The plastic in wire insulation and connector housings go first. That said, poorly soldered joints fail all the time, usually from mechanical strain popping the joints loose. If that happens while there is power applied to the circuit, very bad things happen. To prevent failures like this:
1) Practice your soldering technique to the joints are made properly.
2) If possible, strain relieve your wiring so when the wires are flexed during installation, they flex in the middle, not at the solder connections. This is actually a huge issue with spacecraft systems, where extreme vibration during launch and re-entry can vibrate unsupported cable bundles and cause them to work harden and fail. Robots, aircraft, and automobiles have similar issues, though the vibration environments are less extreme. Unsecured wires are bad!
Now, let's talk about melting plastic. When you see the current carrying rating of a connector or a wire, this is the failure mode which is prevented by that rating. Those ratings are based on a whole host of assumptions, any of which can be different in the specific application. Understanding how the actual application is different from those assumptions allows you to make good design choices to either push past the rating, or use an overrated component.
Let's talk about the relationship between electrical current, heat and temperature. Remembering that our failure mode is melting plastic, we need to prevent the temperature of the electrical conductors from reaching the melting temperature of the plastic. Think of the wire as an isolated chunk of material. If heat is added to that chunk faster than it can be removed, the temperature rises. The temperature is a measure of the level of heat energy in that chunk of material at any particular time. If you remove heat faster than it comes in, the temperature will drop. The primary input of heat into a chunk of wire, connector, pogo pin, or switch contact, is the electrical current flowing through it. All conductors (except superconductors) have electrical resistance, which is simply a way to calculate how much of the electrical energy is converted to heat when it flows through a chunk of conductor. When you're working with electricity, it's easy to remember that high resistance means loss of electrical energy available to power your device, while forgetting that the lost electrical energy isn't actually lost. It is being converted to heat, which will raise the temperature of the high resistance chunk of conductor unless it is removed somehow.
Most component current ratings are essentially saying: if you don't flow more than this much current through the device, and the device is sitting in open air, you can do this as long as you want without the temperature of the component rising to the point where it will fail. Check out this chart for rating current carrying capacity of wire.
Current Carrying Capacity of Copper Conductors | Multi/Cable Corporation [nofollow]Notice that each wire size has multiple current ratings depending on the melting point of the insulation plastic, and the ratings are based on "Single Conductor in Free Air 30°C Ambient Temp." This chart is telling you about the relationship between the electrical energy being converted to heat, which raises the temperature of the wire over time, and the rate at which heat is removed from the wire, which lowers the temperature of the wire over time. The chart give you a rule of thumb for the amount of current above which you may experience problems, under specific operating conditions. Ratings for connectors and switches work the same way.
So how does that circle back to saber building? One obvious takeaway is that the enclosed interior of a saber is not open air. Open air allows heat to be carried away by "natural convection". That means that in open air, warm air rises, carrying away heat and being replaced with cool air. In an enclosed saber hilt, the air can't freely circulate, so heat transfer via natural convection is less. On the other hand, there is other solid stuff which may be in contact with the wires, which can remove heat through conduction. The ultimate goal is to get the heat out of the metallic conductors, though the saber body, and out into the air. If you can do this faster than the heat coming from the electrical current, we are good. If there is poor heat transfer, the temperature will rise, and things will start melting.
Remember how I said that heat transfer is a dark art?
So, when building sabers, you need to recognize that the operating conditions of the components used are probably worse, heat transfer wise, than the conditions assumed by the ratings. That said, there is no substitute for experience. If people are been successfully using a particular component with no problems, that's a sign that reality is different than theory. As a newby to the scene, that's why it's important to learn from those who have gone before you. And, if you really want to take a conservative approach, just oversize some things if you have room. That's way better than learning that something is underrated when it smokes, taking an expensive soundboard with it.
Topic: Connector and wiring current ratings (Read 1077 times)