Inside a clever power design

April 24, 2017

bdipert-April 24, 2017

I receive a blizzard of promotions from Newegg each week, most of whose temptations I successfully (albeit often reluctantly) resist, but I found irresistible one that recently showed up in my RSS reader. The EDIMAX EW-7438RPn, a 2.4 GHz-only "N300" Wi-Fi device that can operate as a wireless network extender, access point, or Ethernet bridge, was on sale for $14.99 with free shipping (versus its normal $21.99 price tag at Newegg, and a $34.99 MSRP). On top of that, a $5 manufacturer rebate (which I promptly received ... thank you very much, EDIMAX!) further dropped the price to $9.99. If you guessed I had a teardown in mind, you'd be right. I clicked on "buy," it promptly arrived, and I immediately set out to disassemble it.

First things first: some shots of the external packaging. The "hole" in one side of the box was caused by my removal of the UPC barcode; rebate requirements, don'cha know.

Notice that the same product is sold in multiple geographies. Perhaps obviously, this diversity has both operating voltage/current/frequency and power plug implications; hold that thought, and we'll revisit it shortly.

Inside the box you'll find documentation both on disc and in paper form:

The back side of the business card-sized piece of literature in the bottom right corner offers QR codes for Android and iOS app direct downloads:

Inside the box, of course, you'll also find the EW-7438RPn:

EDIMAX refers to it as a "mini" device, and they're not kidding:

Here are more views of the bottom, top (note in both cases what I presume to be ventilation holes), backside, and right and left sides; the latter revealing the 10/100 Mbps Ethernet port used in access point and bridge modes:

Here's a closeup of the stickers previously seen in the earlier backside and underside shots:

Prying open the case was pretty easy. I used a modest-size conventional screwdriver, beginning from the underside ventilation area:

I couldn't wriggle the top edge free, until I realized that there was a Philips screw underneath the sticker:

After removing it, the front portion of the plastic enclosure came right off. Note the "plunger" for the WPS button (which does double-duty for factory reset tasks, when held down longer), and the "lenses" for the five LEDs:

And here's the front side of the PCB, with which the button and lenses mate:

The WPS/reset switch is at the bottom, with the LEDs along the top half of the right edge. And dominating the landscape are two Realtek ICs, the RTL8196 network processor and, above it, the RTL8192E WLAN controller. Above the RTL8192E is a PCB-embedded copper "squiggle" that at first you might conclude to be the Wi-Fi antenna. Remember, though, that this is a "N300" wireless network device, meaning that two antenna are necessary for normal operation. My guess is that this particular trace instead supports the self-broadcasting "EdimaxEXT.Setup" SSID used for initial device configuration purposes (although why EDIMAX couldn't just use the "normal" antenna instead is beyond me ... readers, your thoughts?).

The main two antenna are (barely, at least from this particular angle) visible in the overview photo, too; to the right of the LEDs, as well as along the left edge of the PCB. Here are other views of them, from better vantage points:



Now let's take a look at the other side of the PCB. After removing another Philips screw (at the bottom in the overview shot), you first need to wriggle the PCB free from a connector holding it in place:

What's under the piece of plastic? Again, hold that thought. For now, let's peruse that PCB, newly freed from its captivity.

As usual with low-priced (translation: low-profit margin, therefore low cost, therefore highly integrated) consumer electronics devices, there's not much here aside from scattered solder points and passives. Below the label, and to the right of the wired Ethernet port, you'll find a diminutive Macronix 25L1606E 16 Mbit serial interface flash memory.

And under the label, with "FUN-JIN PC161S 1445" markings, is a chip that I can't find information about anywhere, even on the manufacturer's website. My guess is that it's a LAN transformer, although I've never seen one in this svelte a package before. Any ideas?

Finally, let's revisit that mysterious piece of obscuring plastic in the bottom half of the chassis. Remove it:

And underneath you'll find the AC/DC power conversion subsystem:

But of course! Remember how I earlier noted that the same product was sold in multiple geographies and countries? Separating out the power and "other" portions of the design, plug-connected together, enables a common foundation PCB to be used everywhere, simply by swapping in specific transformers-and-such as appropriate. Clever, EDIMAX!

This article originally appeared on Embedded's sister site, EDN: "Teardown: Wi-Fi device offers clever power design."

 

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