eli5. How do table saws with an auto stop tell the difference between wood and a finger?
In: 6214
Capacitance.
But before we discuss that, let’s touch on conductivity.
If those safety stops worked on conductivity, then wearing rubber soles boots, which are a better insulator than wood, would disable this safety.
Rubber soles boots don’t disable this safety, so that’s not the correct answer.
Capacitance is the correct answer. Let’s get into this.
A capacitor holds an electric charge (like a battery), but losses that charge quickly (unlike a battery).
Most things act like a capacitor, holding a bit of charge. This means if you touch most things to a battery, there will be a tiny, but measurable, current flow out of the battery.
dry wood is a crappy capacitor. It holds very little charge.
The human body, or a hot dog, or a bucket of salt water, all of these are also crappy capacitors, but they are orders of magnitude better than dry wood.
So if you electrically isolate the blade, and tricke a bit of voltage into the blade, and measure how much current is flowing, most of the time it’ll be almost none.
But if you get a little jump in current, that means something with better capacitance than dry wood has touched the blade, like maybe a finger, or a hot dog. Trigger the brake.
This is irrespective of how well insulated your boots are, since it’s measuring capacitance, not conductivity.
It’s also why wet wood can sometimes trigger that safety. Wet wood has a higher capacitance than dry wood.
If for some reason you want to test this, hold a hot dog to the blade, not your finger… 😉
Latest Answers