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Wall Warts – the good, the bad, and... the ugly

AC adapter, wall transformer, power pack – they go by many names, but the most popular seems to be "wall wart." Although they are indeed an "unsightly protuberance," they look pretty good to me most of the time…

When I was a high school student building my first ham radio gear, a low voltage DC power supply meant buying a transformer, aluminum box, power cord, terminal strips, hardware, feet, grommets, diodes, and capacitors. Then you drilled, assembled, soldered, and hoped it didn’t start a fire or shock anyone. Buying the whole thing ready to go for a few bucks suddenly doesn’t seem so bad, even if they do tend to clog up a power outlet strip – or look like a wart on your wall...

Most wall warts feed AC power from the wall outlet into a "step-down" transformer, which reduces the voltage to the desired level in its output or "secondary" winding. A few wall warts are much lighter than normal for their size and power rating, and they typically have fancier "switch mode" power supplies like the ones in your PC, TV, or laptop computer. Right now we’ll focus on the far more common warts with a traditional step-down transformer…

There are two major categories: those with AC output, and those with DC output. The warts with AC output don’t have a whole lot more than the transformer inside the plastic "brick." The DC variety has diodes – typically four – to "rectify" the AC to DC, and maybe a capacitor on the output to filter the voltage peaks left over from the AC.

One thing they almost never have is voltage regulation, and that’s very important to understand, since it can create questions and concerns… like: How come I’m reading 20+ volts from my 15 volt wart? Will it damage my xxxx? Is it defective? Actually, it’s perfectly normal. Let’s see why…

Any time we have current flowing through a normal conductor there is a voltage drop. More current, more voltage drop – as stated by Mr. Ohm’s famous law. And the wire used for the windings of your wall wart's transformer is no exception. I have a shiny new "AC adapter" rated at 15 volts DC at 500 mA right here on the bench. What happens when I plug it in to the 120 volt outlet and measure the output? About 20.3 volts, that’s what. So... what’s going on? Did they miss the specified voltage by over 30%? Nope…

This is the "open circuit" voltage – there is no "load" on the wart, so there is no current flowing through the resistance in the transformer’s secondary winding. Thus, no voltage drop.

What happens if we connect a "load" to the wart’s output which requires the rated 500 mA? Now there is a substantial current flowing, and a corresponding voltage drop in the secondary winding. How much drop? Enough to get the voltage down around the wart’s rating – 15 volts in this case. There will be some variation depending on the exact voltage available at your wall outlet, and other factors.

What if the load only "draws" about 100 mA of current? There will be some voltage drop in the transformer, but not as much as when it was supplying 500 mA. So the output voltage will be somewhere between the "open circuit" voltage and the "full load" voltage. The wart I’m using for this example delivers over 18 volts at 100 mA, with 120 volts AC at the wall outlet. What if the wall outlet voltage was lower… like when we have the vacuum cleaner on? Since the wart’s output voltage is not regulated, it will also be lower. Less voltage in, less voltage out.

So, when we select a wall wart for a particular application, several things need to be considered…

Will it be able to maintain the required voltage when delivering the maximum current demanded by the application – even if the wall outlet voltage is a bit low?
Does it have a little "breathing room" – rather than running at its maximum output specification? Since wall warts tend to be kind of chea… er, inexpensive, running them at their maximum rating might not be the best plan.
Is it UL listed, and does the appearance give the impression of quality rather than shoddiness? I’ve seen some pretty crude looking electronic components, and it doesn’t give me that warm and fuzzy feeling of solid reliability. Heck, it’s possible that they may spend a little to make it look nice on the outside and put junk on the inside, but the opposite seems far less likely. A Class 2 rating, which relates to shock hazard, isn’t a bad thing either.

So, let’s say you wanted a wart that would output 100 mA at a minimum of 16 volts DC for a PIC programmer. I wouldn’t select a wart rated at 16 volts DC and 100 mA for two reasons: One, it would be running at its maximum rating and might not be too happy about it. Two, there isn’t much margin for lower than normal wall output voltage (or if the wart doesn’t quite meet specifications).

How about the wart we’ve been using as an example? It can deliver over 18 volts at 100 mA. Is that too much? What if the wall voltage was below normal – perhaps a minor brownout. If the voltage output from this wart is 10% below normal, it still exceeds the required 16 volts. Not too bad! And, at the 100 mA needed by many PIC programmers, for example, we’re only demanding 20% of the wart’s maximum rated current of 500mA. So it’s not working very hard, and should therefore lead a long and happy life.

Next time we’ll explore a related topic – selecting and using those wonderful 3-pin voltage regulator ICs to convert unregulated wall wart output to the voltage our project requires…

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