The Greatest Thing Since (Toasted) Sliced Bread

That’s right. [Colin Furze] just made a household appliance obsolete. Who needs a toaster when you can cut your bread… and toast it at the same time!

Leave it to [Furze] to make something out of the Hitchhiker’s Guide to the Galaxy a reality. Submitted as an idea by one of his subscribers to his new series called Furze’s Invention Show, he took it upon himself to make the long revered lightsaber bread knife. We were waiting for this day.

Unfortunately, it’s not exactly a light saber. In fact, its more of a light-saw-ber, which, pronounced with the right accent could be easily mistaken for the real deal. Using a re-wrapped microwave transformer — much like home-made spot welder rigs — [Furze] is pumping a ton of amps at low voltage through a hacksaw blade, making it red hot and ready to toast bread.

The build is pretty simple, but as always, very entertaining to see how he’s made it.

What will he think of next…

[via Gizmodo]

55 thoughts on “The Greatest Thing Since (Toasted) Sliced Bread

  1. I’m sure someone will pipe up soon about how dangerous this is. But… cool! Every time he touched the thing or even put his hands near it I was panicking inside.

    1. I really don’t get why timid people post here. Or uninformed people.

      The voltage across the blade is between 2 and 3 volts (MOT primaries are wired at around 1 volt per turn) and it’s galvanically isolated from the mains.

      There’s no chance of electric shock from the blade.

      Most MOT capacitors have a built-in resistor. It takes a few seconds to discharge, but it takes you much, *much* longer to take the oven apart to get at the capacitor.

      (Some don’t, so discharging is prudent. Still… is this really scary? Capacitors are in a lot of things, and we’re supposed to be hackers. Total energy (1/2*C*V^2) is on the order of a joule – not much of a spark there.)

      Is it because the blade is hot? Is this somehow more dangerous than an electric carving knife? Or a soldering gun?

      People use circular saws and even sometimes (*gasp*) chainsaws! Is a hot knife that much more dangerous?

      I really don’t understand the atmosphere of fear shown by some posters. Do these people ever do anything of note?

      Stay home, you’ll be safe there. Be careful with your shaver, and use it responsibly.

      1. It’s just a numbers thing. Safety concious post and others less concerned complain but as time goes on there are more of the safety concious and less of the complainers for obvious reasons. When you die you loose your vote in the safety argument.

      2. I hear discovery channel featured an event where a guy died by microwave capacitor discharge
        You don’t actually need to burn to death you know, a nice good jolt can stop your heart and then what? Better write a script to automatically post daily how it’s not dangerous and run it beforehand.

        As for ‘it automatically discharges in no time’, you better not bet your life because you won’t be around to say ‘oops I was wrong that time’..

        1. I heard that t.v. is fake, and I think it was real life that told me that.

          The key is voltage, and microwave’s operate at about 2100v, which is more than enough to kill you under the right conditions. The typical human body has a resistance of 100k ohms, and it takes 75 mA across the heart to stop it. At 2100v, and dry skin you’ll get hit with close to 21mA, which means you’ll definitely feel it, and probably hurt, but it wouldn’t kill you. However, if you were wet, or sweaty, your resistance drops to 1k ohms, which is 2.1 A, which not only would kill you, but probably severely burn you, and cause lots of other problems. People forget that people are hit with lightning and still survive. Electricity really only kills, when it stops your heart, or cause an organ to die, both of which are pretty unique scenarios.

          For people reading this, don’t fuck with poly-phase or three-phase electricity, like 240v, and 480v, its a whole other ball game, and both can kill you pretty easily.

      3. Just sharing my visceral reaction. I suppose I should have included in my comments that I did have a feeling that due to the low voltage you probably wouldn’t by at a great risk for electrical shock, but I wasn’t sure. And regarding the microwave cap, I suppose I stand corrected there as well–I just always understood that discharging a large capacitor the wrong way is a good way to get killed. But perhaps I’m confusing the size/type of caps in TVs with those microwave ovens.

        So in spite of your condescending tone, I appreciate the education. Thanks.

      4. I think the issue here is it’s 2 – 3 volts and a ton of amps running through a *knife*. While your skin needs a much higher voltage to pass the current to your heart, your blood is literally filled with electrolytes. Once you get past the skin, 2 – 3 volts is easily enough to stop your heart at the amperage in a MOT. If you cut yourself with an electric knife or a regular knife, that sucks. This one can kill you with a nick.

        1. No. Electricity travels between the negative and positive terminals. The terminals are each side of the blade. The human body as at least 100k ohms resistance. The worst case scenario is if you are wet say from sweat or water, your skin becomes 1k ohms, yet even at that resistance, only 2 milliamps would be traveling through you, in other words, you probably wouldn’t even notice it. The heart requires 75 milliamps from traveling through it, to stop, meaning not even the thing directly attached to both sides of your heart, would be enough to stop it.

          1. The knife blade is a conductor. It’s iron. The question is whether – once you break the skin with the blade – your electrolyte-laden blood is a better conductor than the iron in the blade. There’s a hell of a lot more than 75 mAmps running through that blade. 75 mAmps would not make it glow red-hot. A MOT can melt small pieces of steel at 3 volts. Many amps. Touching the blade would – I believe – not be dangerous, apart from the second or third degree burn you’d get. I’m talking about getting cut by it, because it’s sharp, which is the whole point of the project. I’m not saying it’s not a neat or creative application of a MOT.

      1. No, pay attention, he rewound the transformer, the blade is probably about 3V AC or so, there is no risk of electrocution unless you somehow get in contact with the primary side.

        1. You mean like the sequence where he burnt all the skin off his hand,arm ? nice Youtube that one. (one of the jet engines or pulse jet IIRC) and the gas pipe came off and leaked…. Exactly as how you might imagine it would in your worst safety nightmare. Go Colin :)

    1. I’m guessing a couple of dozen blades might draw a little too much current even perhap too much for a poly phase supply.
      Perhaps temperature control might be in order.
      Thinking of cauterising, maybe an ultra sonic version might make for an interesting alternative.

      1. It might be possible to put two blades in series and still have enough heat to get the job done. Parallel may still work though. Either way it’s ALMOST a direct short even with a single blade. Just limited by a bit of resistance from the steel blade, a tiny bit of resistance from the wire, and the core saturating.

      2. For this application you need to look at the power.

        MOT’s are rated to the capacity of the oven, which can go as high as 1500 watts. Anything above this and you risk popping a circuit breaker and overheating your home wiring.

        That’s the maximum that the transformer *can* supply, but the amount that it *does* supply is determined by the voltage and the resistance. The hacker gets to choose the voltage by using more or fewer windings: MOT primaries are wound with about 1 volt per turn, so 2 turns gives you about 2 volts.

        This is low enough and safe enough that you can just put a DVM on the outputs and measure voltage or current. (And yes, I’ve done that. Within the past month, even.)

        The amount of power you draw, I^2 * R, or V*I, has to be less than the MOT power rating.

        That said, the blades of the saw have a specific resistance and R factor (resistance to heat energy being transferred to the bread) which is easy to determine using a DVM.

        If you use two blades in parallel you’ll get half the resistance and twice the current, but 4x the power per the formula above, so you’ll want to reduce the voltage by using fewer turns.

        Blades in series is twice the resistance and half the current, but 1/4 the power so you’ll want to increase the number of turns.

        It’s easy to measure the power drawn by one blade, then you can see how many blades will fit into 1500 watts.

        I’m guessing that the blade is equivalent to a 16″ space heater core (which gets red hot), and there are typically two of these in a heater, so it seems likely that a 2-blade system is feasible.

        1. unfortunately MOTs don’t have an inductive shunt like Neon transformers, so the amount of current in a direct short is not current limited. I’m waiting for someone to rewind Neon secondaries and set the maximum short circuit current that way. Bit harder though…

          1. The power is limited by the resistance of the load (the blade in the project). At the small voltages he’s using, the tiny resistance in wires (even thick ones) limits the power to a safe level.

            Or more directly, at 2 volts a short circuit should push 750 amps, but it never does because the wire resistance limits the current.

            To get the kind of shorted failure you’re thinking of, you need more voltage to overpower the resistance. Ten turns won’t do it, even at that current (150 amps) the resistance still limits the power to a safe level.

            When making a spot welder you have to sort through these issues. It’s not enough to have a unit that can supply 750 amps, you need enough voltage behind it to supply enough power to the welding tips.

    1. And it leaves the inside of the bread soft: you can see in the video that it’s not really toasted, just burnt on the surface. I don’t think anyone would actually want to use this, though.

  2. This knife can be dangerous, but is nothing in comparison with the gas heated shoes, the Jet kart, the Jet bike, the weaponized tuk tuk and many other sick projects this guy done so far.

  3. Wow, so many people getting nervous about a couple of volts of electricity. And here I thought we were on Hack-A-Day, where people make and modify things all the time. Maybe that’s why logic voltage is trending lower: too many people scared of 5 volts.

    Colin Furze does dangerous things so you don’t have to. Stop whining about it already.

    1. A million internet points for this.

      TBH apart from his on-camera excitability and the odd accident, everything Mr Furze has ever done has been relatively well-executed and reasonably safe, probably helped by him being a qualified engineer (gas & plumbing if memory serves, in the UK gas engineers MUST be certified) and at least understanding the risks even if he then chooses to ignore them.

      He may appear mad but the man’s no fool. I’d trust him & his creations over a hell of a lot of stuff that’s been featured on HaD over the years.

  4. You know one bad thing about this, is if you use heavy metals at high temperatures, they might leave deposits of heavy metals like nickel, or something from the stainless steel, or even just the steel coming off in little bits. I wouldn’t recommend anyone actually using this as a regular tool eat. Just use it as a really cool novelty tool. Neat idea though, I wonder if you could use some type of ceramic knife or something, with iron leads inside to heat up the knife, that’d be awesome.

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