Power Resistance Isn’t Futile

As [Electronoobs] points out, everything has resistance. So, how hard can it be to make a high-power resistor? In the video below, he examines a commercial power resistor and how to make your own using nichrome wire.

Sure, in theory, you can use a long piece of wire, but normally, you want to minimize the amount of space occupied. This leads to winding the wire around some substrate. If you just wind the wire, though, you get an inductor. This can cause nasty voltage spikes when there is a change in current through the resistor. You can get “noninductive” wire wound resistors that use either two opposing windings or alternate the turn direction on each turn. This causes the magnetic fields to tend to cancel out, reducing the overall inductance.

Nichrome wire has more resistance per millimeter and can dissipate more power. Modern digital meters can measure the resistance of a wire if you account for the test leads. To make a substrate, [Electronoobs] got creative since he anticipated generating a lot of heat. The final product even uses water cooling.

Why do you want a big resistor? Maybe you need a dummy load, or you want to drain some batteries. If you want to recycle nichrome wire, it is much more common than you might expect.

16 thoughts on “Power Resistance Isn’t Futile

  1. They’re surprisingly complicated devices when you aim for the higher power levels and making them non inductive is kinda tricky if all you have is wire to work with.

    There are wirewounds that are specced up to approx 30MHz which really surprised me.

    1. Bifilar coil and Ayrton–Perry winding https://en.wikipedia.org/wiki/Bifilar_coil are the classic ways to make non-inductive windings. The video shows what I think would be a very simple Bifilar coil design that just doubles back onto itself. A-P would theoretically perform better (lower inductance) but it’s harder to DIY. So I think 30 MHz is really excellent result.

      I would imagine keeping the resistance high per meter would lead to a shorter winding, and fewer turns, on top of using some type of non-inductive winding strategy which already has only a tiny bit of inductance in “the real world”.

  2. The company I work with has a datacenter, where the the generators need to be tested. There’s something impressive about standing in the path of the exhaust of a 2 megawatt power resistor/space heater.

  3. As surprising as it might sound, toilet tank lids make for excellent HV insulators. With proper machining they are often used for making high voltage, high power resistors.

  4. somewhere around here there is a resistor bank from an old EDM machine made of several hard carbon tubes about 1 inch OD x 6 inches long. They snap into holders like a cartridge fuse. I want to say they were labeled 13 ohms each IIRC. Just thought they were cool….

  5. @02:02 The ceramic tube is a quite good thermal conductor. For those big power resistors, the ceramic soak the heat out of the (low mass) wire to improve the power rating of the resistor.

    @03:40. Yes, bifilair wound resistors do exist, (See Wikipedia link from Jon Mayo above) but this is not it. In this example the two wires are just connected in parallel. Both ends of the resistance element are connected to each other (and to the terminal)

    @15:00. If you want to wind bifilair, then first double up the wire before you start winding. You can also use some comb to keep the wires at equal distance.

    Also, in the first two minutes he started with an explanation for the cost of a commercially made resistor, then cut in to the advertisement, and did not come back to the price of those resistors.

    Adding those things up, this video just has too many errors, and I did not manage to see it all. (I skipped most after the first 3 minutes).

    Some other notes:
    For test setups, dunking the Ni-crhome wire in a bucket of water greatly improves the power limit of a resistor. MikesElectricStuff has made a video about this.

    For measurement of low value resistors, a current source is handy. You can make a 1A current source (or sink) with an LM317 and a 1.25 Ohm resistor. (or 100mA with 12.5 Ohm resistor). This lets you easily measure low value resistors. Downside is that it’s a bit finicky to calibrate the thing. (you have to combine multiple resistors).

  6. Just zig-zag some kanthal across the front of a box fan. Make a wood frame and suspend the wire with nails so you don’t melt the plastic. Works better than whatever this guy is doing, and now you don’t have to waste 20 minutes watching the video.

  7. Just curious, would it not be easier to DIY a load resistor by using a carbon rod? many many years ago I learned to use a carbon arc torch, this is where I discovered these wonderful things. I have done simple experiments/demonstrations with them, so I was thinking they might be easier to use and less worry about creating magnetic fields. Also they are cheap on amazon

    1. It depends on your application. It’s easier to dissipate the heat from a thin wire than from a thick carbon rod, and the carbon (graphite) has a negative temperature coefficient of resistance, so it tends to “run away” when it gets too hot. You need to limit the current if you intend to dump lots of power into it.

  8. “The final product even uses water cooling.”

    Eh, yeah, but he also used a layer of vinyl electrical tape between the copper pipe and nichrome wire. It’s going to be a gooey mess by the time that water cooling is necessary, and a whole lot of sparky fun once the wire touches the pipe due to the tape shrinking and melting. It’s a poor substrate, bottom line. There’s a damn good reason commercial power resistors use ceramics.

    I suppose using nichrome wire is fine if you just need “a load” without much precision in the resistance value, AND you have some method (better than that shown in the video) of mounting it. But it’s probably a better use of time and money to buy something like an electric stove heating element, or buy the correct value of ceramic power resistor for the application, from any reputable online vendor. Unless you already have a lot of nichrome wire on-hand, maybe.

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