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Bad Idea of the Week: Big Inductors
Oct. 28th, 2017 07:58 pmThis is a blog about a terrible, horrible, no-good idea that I recently saw on Youtube, and why it's so unsafe when you might think otherwise.
The tl;dr is this: LOW VOLTAGES ARE NOT SAFE IF AN INDUCTOR (a coil of wire) and HIGH CURRENT IS INVOLVED.
Here's the video. It looks cool (though you'll probably cringe at the lack of gloves), but it's one of the most dangerous things I've ever seen online, and I know way too many people are going to try and replicate it. https://www.youtube.com/watch?v=6aNm3qWkW8A
See, I know a thing or two about inductors and low voltages. I'm an electrical engineer, and I happen to have taken quite a few classes on these sorts of things. The thing they teach you when it comes to inductors is that /they don't like the current through them to suddenly stop/. Much like capacitors, they store energy, but in the magnetic field of a coil rather than between two plates.
If we have a setup like the one in the video above, with an output of 2V at around ~900A, you could safely touch those contacts, assuming nothing else was bridging them. I definitely wouldn't, in case anything goes wrong (insulation issue, etc), but you probably could. Here's why: Human body resistance is usually in excess of 10kOhm hand to hand, meaning the most current you're likely to see pass through you is less than 200 microamps. It'll take about 70mA, or 350 times that amount, to stop your heart. (Note that this also assumes your hands are dry, not covered in lotion, etc... Bare hands only.)
Now, 200 microamps is pretty low, so the magnetic field in the secondary (the big thick wire) of the transformer will be pretty small. There won't be a lot of energy stored there that can discharge into something. 900 Amps? Oh boy are you in for some fun.
Here's some quick math I put together based on what I saw in the video. I'm assuming a 1 meter length of wire, which I read off the insulation is 2/0 AWG. There are two complete turns when the current is flowing.
Inductance of that secondary loop is calculated as follows:
L = u0 * N^2 * A / l
L = 4* π * 10-7 * 2 ^ 2 * 0.785398163 / 1
L = 0.00003947841
L = 34.478 microhenries
Okay, let's calculate the energy stored in the coil in joules in the case of 200 uA, and 900 A.
E = 1/2 * L * I^2
200 uA: E = 0.5 * 0.00003947841 * 0.0002 ^ 2 = 7.895682e-13 Joules
900 A: E = 0.5 * 0.00003947841 * 900 ^ 2 = 15.98875605 Joules
That is a WHOPPING FOURTEEN ORDERS OF MAGNITUDE higher, and represents a massive amount of energy available to be discharged into anything that gets in the way!
Now, the voltage spike created by tiny connection and disconnection events at the contacts, which you see as sparks is due to EMF (electro-motive force). That's calculated by:
emf = L * di/dt, where di/dt is how fast the current is changing over time. Let's try cutting our current from 900A to the 200 uA that can flow through the human body normally in 1 microsecond.
emf = 0.00003947841 * (900 - 0.0002) / 0.0000001 = 355305 VOLTS
Holy crap, you say! That's a lot of volts, you say! Yeah, I say, it is! It's a lot of lot of volts! It's the reason you're seeing spikes in the first place, as that voltage is ionizing the AIR so current can flow, and it's WAY more than enough to burn a path through the dead skin cells on your hands (the primary reason humans have such high resistance) and create a path straight through your watery insides! What's that you say? We have built in electrical conduits called nerves? Oh! That's not going to end well, is it?
No, no it's not! And it didn't for me back in highschool when I almost fucking /killed/ myself. I was messing around with a 12V motorcycle battery and a 2A automotive solenoid when I accidentally triggered a mechanism that caused repeated current cycling. I can't even fucking tell you how much that hurt, and I'm pretty sure I *still* have permanent electrical scars in my back.
In summary: NO. ALL OF THE NO. NO NO NO NO.
Equation sources: http://www.electronics-tutorials.ws/inductor/inductance.html
