So TCSS and the like sell ‘high amp kill switches’ and charging plugs, but it’s not clear to me what the current rating needs to be? Do these interrupts need to handle the full current drain from the board plus leds, plus whatever else, or do they just need to handle the current into the board, and if so what is that?
This is a long and sordid tale, with no satisfactory answer, so sit back and get comfortable.
In theory, the answer is that you shouldn’t use a switch or charge plug rated for 1A to conduct 3A. Current doesn’t care about voltage, 1A @ 5V will heat up your component just as much as 1A @ 100V.
If you’re installing a switch that cuts the battery power to all components, including the blade, this means that your switch will experience up to 10A, so that’s what your switch should be rated for.
If you’re installing a switch that only cuts the + side to the board, then 1A is sufficient.
In practice, it’s much more complicated. Most switches we use are rated for 100 volts or more, and they also have a rating for how many operations they are good for. The tricky part is that those operations are assumed to be at full power. When you operate a switch at full power, some part of the switch surface always connects “first”, and that part will experience the full voltage and current until the rest of the switch surface connects. If the current and/or voltage is high, that small part may sustain heating, oxidation, pitting and scorching from excessive current. Eventually, the damage will increase the resistance of the switch to the point where it cannot operate within the specified parameters anymore.
BUT
That’s not how saber switches are used. In a saber, we don’t use 100+ volts, which reduces damage to the switch considerably. Also, the sabers don’t tend to draw full power at the moment we turn them on. That only happens when we ignite the saber. Because of this, the contact surface of our switches remain undamaged for much longer, and this should lend us some headroom beyond what the switch is actually rated for. How much headroom you ask? No idea…
Ideally, switch manufacturers would rate their switches separately for low-voltage use, but I’ve never seen that in a switch data sheet, which means that all we can do is guess and/or measure. Ultimately it comes down to resistance. Most switches have a resistance somewhere between 0.01 and 0.1 ohm. The lower the resistance, the less heat is generated in the switch when we pull current through it. Also, high resistance would mean that we would loose power to the switch, which isn’t ideal.
Finally, I want to point out that a single FET can be used to bypass the current limits of a switch. You can buy tiny FETs that can handle many many amps, and by hooking up the control pin to the switch, and have the other two legs of the switch hooked up to + and - on the battery, you get a switch that can control very high loads. I use this in my graflex helper pcb as the tiny switch I use is only rated for 250mA. The FET on the other hand is rated for 4A.
tl;dr; get the high-amp switches unless you know what you’re doing.
I smell a phd thesis in here somewhere…. So, for typical ‘high current switches’, what’s a normal current rating? Is 10A sufficient? I ask because I haven’t seen it specified and I want to see what options mouser etc have for different form factors.
I believe the “high current switches” that are sold for sabers specifically are in fact NOT rated for 10A. However, they do seem to work just fine. Same is true for the JST connectors we sometimes use.