There are times when you might want an odd-value resistor. Rather than run out to the store to buy a 3,140 Ω resistor, you can get there with a good ohmmeter and a willingness to solder things in series and parallel. But when you want a precise resistor value, and you want many of them, Frankensteining many resistors together over and over is a poor solution.

Something like an 8-bit R-2R resistor-ladder DAC, for instance, requires seventeen resistors of two values in better than 0.4% precision. That’s just not something I have on hand, and the series/parallel approach will get tiresome fast.

Ages ago, I had read about trimming resistors by hand, but had assumed that it was the domain of the madman. On the other hand, this is Hackaday; I had some time and a file. Could I trim and match resistors to within half a percent? Read on to find out.

Metal-Film, Through-Hole Resistors

Your run-of-the-mill through-hole resistor is a metal-film resistor, made by depositing a thin layer of metal onto a non-conductive ceramic cylinder. The metal film is cut into a helix, and the length, width, and thickness of the resulting metal coil determine the resistance. Since the deposited metal is so thin, between 50 nm and 250 nm, you might think that trimming this down by hand is going to be a bit finicky.

Jumping straight to the punchline, when I was trying to change the resistance by small amounts, maybe less than 5% or so, it was trivially easy to land spot on the exact desired value. I had bags of 1 kΩ and 2 kΩ 1% resistors, and I figured I would make a whole bunch of mistakes while learning.

The reality is that I went over the target once out of seventeen attempts, and that only by one ohm. The rest of the resistors are trimmed as well as I can measure — down to the single ohm. (My meter and probes have a 0.3 Ω offset, but there’s nothing I can do about that.) I pitched the “bad” one, made one more, and had a perfect set in short order.

Here’s the whole procedure. I put the resistor into some insulated clamps, and clipped my ohmmeter to either end. I used a small round file, and just went at it. The first few strokes get you through the relatively thick coating, but once you see metal, or notice a blip on the ohmmeter, a very light touch with the file is the rule. Maybe blow some of the metal dust off between strokes when you’re getting close, but I didn’t notice that it made much difference. Seven or eight light strokes with the tiny little file brought the resistors to a ten-point landing.

Indeed, because it’s easy to go too far at first, I found that ideal candidate resistors to file were the 1,990 Ω ones. Many of my 1 kΩ resistors came in at 999 Ω, which makes it hard to get through the casing without overshooting the mark. I probably could have just left them. The good news is that most 1% resistors will be off by more than a few ohms in either direction, otherwise they’d be sold as 0.1% resistors. And of course, you need to pick source resistors with a lower resistance than the target — you’re not adding metal with the file.

So you only need to have one value of resistor in your kit, right? Absolutely not. Creating a 1.2 kΩ resistor from a 1 kΩ original is asking for trouble. I got it to work a few times, again down to the single ohm, by restarting the filing process in a different place rather than simply going deeper in one hole, but I don’t recommend it, and I can’t think of when you’d need to. Just add a 200 Ω resistor in series and trim that. Remember that you’re thinning down a metal spiral that’s only 100 nm thick to begin with. Easy does it.

Surface-Mount?

Filing down through-hole resistors to exact values was so much easier than I had anticipated that I decided to take on something harder. I tacked a 1206 2.1 kΩ resistor onto some stripboard. Wouldn’t you know it, it read out exactly 2,100 Ω, so 2,105 Ω became the target. That didn’t go well at all; I ended up with a 2,722 Ω resistor faster than I had expected.

The second 1206 started out at 2,103 Ω, and I just went at it without a goal in mind. By going very carefully, I got it’s resistance down to 2,009 Ω before it jumped to 2,600 Ω and beyond. Lowering the resistance doesn’t make sense at all. Maybe I was dragging some solder into the gap and effectively thickening the metal layer? I went looking for information, but didn’t get any further into the construction than Vishay’s datasheet: “metal glaze on high-quality ceramic” which doesn’t enlighten much.

After two more attempts, I couldn’t get the SMT resistors in trim at all; the layer of deposited metal is just too thin. And anyway, I’m not sure how useful it would be — the thought of soldering and de-soldering seventeen of these isn’t very appealing.

Conclusion

Trimming through-hole resistors is awesome. I made a complete set of matched better-than-0.05% (!) resistors for an 8-bit DAC in half an hour with nothing more than a file and an ohmmeter. And on my first try. You could easily make a 10-bit DAC this way. The result was an order of magnitude better than I had hoped, and it wasn’t hard at all. Amazing. And nothing says cool like a hand-made, artisanal DAC. (For odd values of cool.)

My attempt at trimming surface-mount resistors, on the other hand, was a complete failure. Anyone out there care to guess why? Is it just the tweakiness of trimming a super-thin film? Anyone with a precise laser cutter want to have a go and write us about it?