Topic themes

Although in general I strive to cover a diversity of topics here in the blog, regular readers may have noticed that some amount of chronological theme-grouping still goes on. A few years back, for example, I wrote a fair bit about building PCs, both conceptually and in un-teardown (i.e., hands-on assembly) fashion. After that, there was a cluster of posts having to do with various still and video photography topics. And last year (extending into early this year) I talked a lot about lithium-based batteries, both in an absolute sense and relative to sealed lead-acid forebears, as well as the equipment containing them (and recharging them, i.e., solar cells).

Well, fair warning: this post is the kickoff of another common-topic cluster, having to do with audio. This isn’t a subject I’ve ignored to this point, mind you; consider just in recent times, for example, my posts on ambient noise suppression, interconnect schemes, lossy compression algorithms and listening gear (portable, too), microphones (plus on-PCB ones, tearing them down, and boosting their outputs) and exotic headphones, among others. But even more recently, I’ve obtained some “Chi-Fi” (i.e., built and often also directly sold by China-based suppliers) audio equipment—class D amplifiers and the like—along with audio gear from a US-based company that also does Stateside assembly, yet still effectively competes in the US and broader worldwide markets.

What is a wall wart?

More on all of that in posts to come through the remainder of this year, likely also extending into the next. For now: what does all of this have to do with a wall wart? And what is a wall wart, for those of you not already familiar with the term? Here’s Wikipedia’s take on the topic:

An AC adapter or AC/DC adapter (also called a wall charger, power adapter, power brick, or wall wart) is a type of external power supply, often enclosed in a case similar to an AC plug. AC adapters deliver electric power to devices that lack internal components to draw voltage and power from mains power themselves. The internal circuitry of an external power supply is often very similar to the design that would be used for a built-in or internal supply.

Today’s victim arrived via a Micca PB42X powered speaker set, purchased from an eBay seller:

The story behind the teardown

They’d previously belonged to her son, who according to her never used them (more on that later), so she was offloading them to make some money. Problem was, although she’d sent me photos beforehand of the right speaker (fed by an RCA input connector set and containing the class D amplifier circuitry for both speakers; a conventional strand of speaker wire connects its output to its left-speaker sibling’s input) powered up, complete with a glowing red back panel LED, no AC adapter was accompanying it when it arrived at my front door.

After I messaged her, she sent me the “wall wart” you’ll see today, which not only was best-case underpowered compared to what it should have been—12V@500mA versus 18V@2A—but didn’t even work, outputting less than 200mV, sometimes measuring positive and other times negative voltage (in retrospect, I wish I would have also checked for any AC output voltage evidence before dissecting it):

She eventually agreed to provide a partial refund to cover my replacement-PSU cost, leaving me with a “dead” wall wart suitable only for the landfill. Although…I realized right before tossing it that I’d never actually taken one apart before. And this’d also give me a chance to test out the hypothesis of a hilariously narrated (watch it and listen for yourself) video I’d previously come across, proposing a method for getting inside equipment with ultrasonic-welded enclosures:

Best video ever, right? The topic was of great interest, as I often came across such-sealed gear and my historical techniques for getting inside (a hacksaw, for example) also threatened to inadvertently mangle whatever was inside.

The teardown

I didn’t have the suggested wallpaper knife in my possession; instead, I got a paint scraper with a sharp edge and hammer-compatible other end:

And in the following overview shots, with the wall wart as-usual accompanied by a 0.75″ (19.1 mm) diameter U.S. penny for size comparison purposes, you’ll notice (among other things) the ultrasonic welded joint around the circumference, to which I applied my pounding attention:

Complete with a closeup of the (in)famous Prop. 65 sticker…

How’d it work out? Well…I got inside, as you’ll see, but the break along the joint wasn’t exactly clean. I won’t be putting this wall wart back together again, not that I’d want to try in this case:

Maybe next time I’ll use a more lightweight hammer, and/or wield it with a lighter touch

Anyhoo, with the damage done, the front portion of the enclosure lifts off straightaway:

Two things baffle me about the interior of the front case piece:

• What’s the point of the two glue dabs, which aren’t location-relevant to anything inside?
• And what if any functional use does that extra diagonal plastic piece serve?

That all said, this is what we’re most interested in, right?

The insides similarly lifted right out of the remaining piece(s) of the enclosure:

If you hadn’t already noticed, the heftier front of the case had survived its encounter with the paint scraper and sledge intact. The smaller back portion…not so much:

Here’s an overview of the now-exposed back of the wall wart’s guts. The transformer, which I’m sure you already noticed before, dominates the landscape:

Now continuing (and finishing) the rotation in 90° increments:

Let’s take a closer look at that PCB hanging off the bottom:

I am, as reader feedback regularly reminds me, not an analog or power electronics expert by any means, but what I believe we’re looking at here is visual evidence of a very rudimentary form of AC-to-DC conversion, the four-diode bridge rectifier:

A diode bridge is a bridge rectifier circuit of four diodes that is used in the process of converting alternating current (AC) from the input terminals to direct current (DC, i.e. fixed polarity) on the output terminals. Its function is to convert the negative voltage portions of the AC waveform to positive voltage, after which a low-pass filter can be used to smooth the result into DC.

 When used in its most common application, for conversion of an alternating-current (AC) input into a direct-current (DC) output, it is known as a bridge rectifier. A bridge rectifier provides full-wave rectification from a two-wire AC input, resulting in lower cost and weight as compared to a rectifier with a three-wire input from a transformer with a center-tapped secondary winding.

The low-pass filter mentioned in the definition is, of course, the capacitor on the PCB. And re the diodes, the manufacturer (presumably in aspiring to squeeze as much profit as possible out of the design) didn’t even bother going the (presumably more costly) integration route:

Prior to the availability of integrated circuits, a bridge rectifier was constructed from separate diodes. Since about 1950, a single four-terminal component containing the four diodes connected in a bridge configuration has been available and is now available with various voltage and current ratings.

Ironically, in looking back at Wikipedia’s “wall wart” page post-teardown, shortly before I began writing, I happened to notice this exact same approach showcased in one of the photos there:

A disassembled AC adapter showing a simple, unregulated linear DC supply circuit: a transformer, four diodes in a bridge rectifier, and a single electrolytic capacitor to smooth the waveform.

And it’s also documented in an interesting Reddit thread I found, which starts out this way:

Do inexpensive 12v wall warts usually use a transformer to step mains to about 12vac then bridge rectify and regulate to 12vdc?

Or

Do they use some minimal 1:1 transformer for isolation, rectify to dc then use a buck converter to drop to 12v?

Or some other standard clever design?

Look again at the PCB, though, specifically at the markings, and you might notice something curious. Let me move a couple of diodes out of the way to emphasize what I’m talking about:

Capacitor C5, the big one for output filtering, is obviously present. But why are there also markings for capacitors C1-C4 alongside the diodes…and why are they missing? The clue, I’ll suggest, appears in the last bit of Wikipedia’s diode bridge introductory section:

Diodes are also used in bridge topologies along with capacitors as voltage multipliers.

Once again to save cost, I think the manufacturer of this wall wart developed a PCB that could do double-duty. Populated solely with diodes, it (requoting Reddit) “uses a transformer to step mains to about 12vac then bridge rectify and regulate to 12vdc.” And for other wall wart product proliferations with other output DC voltages, you’ll find a mix of both diodes and capacitors soldered onto that same PCB.

Again, as I said before, I’m not an analog or power electronics expert by any means. So, at this point I’ll turn the microphone over to you for your thoughts in the comments. Am I at least in the ballpark with my theory (can you tell that MLB spring training just began as I’m writing this)? Or have I struck out swinging? And what else about this design did you find interesting?

—Brian Dipert is the Editor-in-Chief of the Edge AI and Vision Alliance, and a Senior Analyst at BDTI and Editor-in-Chief of InsideDSP, the company’s online newsletter.

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