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.

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Handy Tool Drains 18650 Cells So You Don’t Have To

Draining a battery is easy. Just put a load across the terminals, maybe an incandescent bulb or a beefy power resistor, and wait. What’s quite a bit trickier is doing so safely. Put too large a load on, or leave it connected for longer than necessary, and you can end up doing damage to the cell. Not convinced he’d always remember to pull the battery out of his jury-rigged discharger at the opportune moment, [Jasper Sikken] decided to come up with a simple tool that could automatically handle the process with the cold and calculating precision of silicon.

V4 used the protection module from a pouch battery.

At a glance we can see the major components you’d expect in a discharger: a fairly simple PCB, four ceramic power resistors, a holder for a single 18650 cell, and a rocker switch to connect it all together. But wait, what’s that a TP4056 charging module doing in there?

While its presence technically makes this device a battery charger, [Jasper] is actually using it for the onboard protection IC. With the charging module between the cell and the power resistors, it will cut the connection when the voltage drops to 2.4 V. Oh yeah, and it can charge the battery back up if you connect up a USB cable.

[Jasper] says his little tool works great, with the resistor array putting just enough load on the battery to pull it down quickly without getting so hot that they’re dangerous to have exposed. He estimates the BOM for this gadget runs around $2 USD, and is considering offering it as a kit on Tindie in the near future.

If you’re looking for something a bit more advanced, [Jasper] built a programmable load a few years back that can discharge batteries and test power supplies all while logging the data to your computer for later analysis.

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Building A Heat Sealer For Anti-static Bags

[Raphaël Assénat] needed anti-static bags for some boards he is selling. He had a lot of leftovers on hand (presumably from the components he ordered to assemble these boards) and wanted to reuse them. Instead of buying a heat sealer he built his own to cut them down to size.

His build starts with a transformer to drop mains voltage down to 9 Volts. From there, you can see the two power resistors used in series to limit the current. Without these, the wire would get way too hot. Just in front of those resistors is a momentary push switch which cuts the power by default. Here we can see that [Raphaël] is using a wood block to press the bag against the wire as it heats up.

The wire itself is a piece of straightened tension spring. Apparently this spring material is a poor conductor, which is why it gets hot enough to melt the plastic bag when you run current through it.