When Only A TO92 Will Do

As through-hole components are supplanted by their surface-mount equivalents, we’re beginning to see the departure of once-common component form factors. Many such as the metal can transistors became rare years ago, while others still hang on albeit in fewer and fewer places. One of these is the once-ubiquitous TO92 moulded plastic transistor, which we don’t see very much of at all in 2022. [Sam Ettinger] is a fan of the D-shaped plastic blobs, and has gone as far as to recreate them for a new generation to enjoy.

Though a TO92 was a relatively miniature package in its day, it’s still large enough to easily fit a SOT23 or similar SMD packaged device on a small PCB. So the tiny board with just enough space for the part and the three wires was fabricated, ready for encapsulating. Epoxy moulding a TO92 gave very poor results, so instead an SLA print of a T092 shell was made. It fits neatly over the PCB, producing a perfect TO92 package. We’re sure a translucent pink package would have raised a few eyebrows back in the 1960s though.

There will come a time when restorers of old electronics will use and refine this technique to replace dead components. We’ve seen the technique before, after all.

17 thoughts on “When Only A TO92 Will Do

  1. Most TO-92 transistors are side collector, but if you’re willing to go with center collector I don’t see the need for the PC board since it should be possible to fixture the leads and solder a SOT-23 or SC-70 directly to the leads. And the leads don’t even have to be flat … they were only flat because the leadframes were stamped out of flat stock. Wires would work fine since nobody is going to see them anyway unless they look really hard.

      1. But then, as a replacement part, if I search TO92 on eBay UK, I get 26,000+ matches, typically £2 for 10. So they are certainly not rare, and if you want the original “look”, surely original part is a better option, and if you don’t worry about the look and/or just want a quick fix, you would do as David says. And as Jan points out below, if you have to sacrifice a genuine TO92 to get the legs, then the whole thing makes even less sense: either you already have some TO92, or you’re going to have to buy some anyway. So this seems like a solution looking for a problem.

        1. So, there’s a guy posting an in-depth manual how he encapulated his own electronics/chip in selfmade TO-92 cans. For free, on the internet.
          And people like you and David just tear it up and make it look like a stupid idea. Why? Really I don’t get it. Some people (like me) have hands full of old TO-92s. If I cut off the legs of an old can to solder it to my new creation, WHY THE HECK is that “a solution looking for a problem”?

          Here’s an extract from his .io page:

          “My original goal was to make my own TO-92 sound ICs. The quick summary is that there are some super-simple music chips with only 3 pins: power, ground, and audio output. I’m really happy with the songs I’ve managed to play off an ATtiny10, which critically has just 1 kB of flash and 32 bytes of RAM. There’s so much I want to talk about with the ATtiny10 music chips, however, that I had to spin it off into another project writeup.”

          Still stupid?!

          1. Not tearing him down, just saying that if you need some TO92, given they are not rare, this solution seems a bit over the top. Besides you still need some donor TO92 + resin printer + ability to make custom tiny PCBS, so it’s not really a solution for the masses. But if it works for him, then great.
            If you don’t want to make/solder tiny PCBs, you could simply solder wires, and shuffle them in position. And if you don’t have a resin printer, you could still encapsulate in a blob of resin and form it with a dremel + some sanding, and maybe a coat of black paint. No capital required (except a tube of epoxy glue and a can of paint), so much more accessible to the masses.

          2. > “My original goal was to make my own TO-92 sound ICs. (…)”

            Oh, come on, that should be at least mentioned in the article. A project to replace TO-92 transistor with SOT-23 transistor is just “meh”, as others already suggested. A project to create a home-made, programmable, Turing-complete replacement for UM66 (look it up, kids!) is beyond awesome :-D

          3. WTF? Where did I tear anything up? I merely pointed out that you could accomplish the same thing without a PC board that the author admitted was a bit problematic, and that flat wires weren’t necessary. You can even do side collector with wires if you are willing to make a slight bend at the tip of the collector lead. I’ll post a diagram if you can’t figure out how. But since you brought it up, I will now point out that it does seem silly to me (as it does to others here) to sacrifice a TO-92 to make a lesser one.

        2. “if you have to sacrifice a genuine TO92 to get the legs, then the whole thing makes even less sense”

          Why not use the legs of the transistor being replaced?

        3. J3xx JFET, cough cough…
          Not obtainable anymore, but a SMD version exists. In some RF circuits, you can find them and if you need low-noise replacement, you just have to use the same exact part.

  2. What a neat little project, creating something (new) that looks like something else (old)
    So I’ve read the project page, interesting stuff… then I stumbled upon Step-3… THE HORROR!!!
    Innocent TO-92 devices have their legs amputated to serve as parts for the new breed of look-a-like silicon!
    I need some time to process this…

    1. That leg thing was completely unnecessary, some (early) TO-92 had round wires. But at least the author used cheap chinese transistors – the stuff where you get random transistors with random markings.

  3. An interesting use case could be making dual transistors with slightly bigger cases capable of containing two smd transistors, then select them in pairs for identical characteristics, and fit them in there along with with enough non electrically conductive thermal paste, so that they’re also thermally linked together.

    As for the legs, anyone using power tht resistors has plenty of them around to recycle:^)

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