Wall wart + battery = small UPS: Good idea
or not?
Bill Schweber - March 07, 2016
We’re all familiar with those self-contained AC/DC converters – often called “wall warts” – which
power many small devices, often running 24/7/365 at load levels ranging from quiescent to nearly
full rating. These devices, which typically have DC outputs anywhere between 5 and 30V and about
1 to 2A capacity, come with a bewildering variety of DC output plugs, and have a reputation for low
or indeterminate quality along with questionable long-term reliability. Still, they provide a
convenient solution to providing low voltage/power DC to a device while getting the AC side (and the
regulatory/safety issues that AC brings) out of the box, so to speak.
Fortunately, in many cases, the load that these AC/DC converters support is non-critical and loss of
power is a nuisance more than a crisis. Still, there may be times when the load is either somewhat
critical, or it would be nice to have at least a few minutes to finish a task or implement a preferred
shutdown sequence when primary AC power or the AC/DC converter itself fails. Think about all
those lab experiments you have heard about, where critical data was lost and long-term experiments
were ruined due to a local power failure; this might be a transitional solution, or at least a partial
one.
That’s why I thought “what a good idea – haven’t seen that before” when I saw a small project in the
January 2016 issue of QST, the official publication of the Amateur Radio Relay League. The article
“Done In One: Battery Backup For Your Wall Wart” shows how to add a modest circuit and small
battery to the output of a wall wart, thus making it into a small uninterruptible power system (UPS).
[Tap into power techniques in Power Management in Embedded Systems at ESC Boston]
UPSs are not new, of course, and you can buy one with capacity of 350VA/200W and run time of a
few minutes beginning at around $50 from vendors such as APC. However, I have never seen a
commercially available implementation that was just a small-scale wall wart/UPS combination with
low-voltage DC (not AC) output and low-end capacity.
The QST circuit (Figure 1) is fairly straightforward and can be built on a prototyping breadboard;
it’s all non-critical components and layout. The BOM includes some Schottky diodes, regular diodes,
resistors, capacitors, a 741 op amp (will that ancient op amp ever become obsolete?), and a Pchannel MOSFET, plus a small gel-cell battery.
Figure 1 This fairly straightforward, non-critical circuit turns a basic AC/DC converter into a miniUPS, and can also serve as a teaching tool for aspiring EEs. (Source: QST, January 2016, used with
permission).
The author, Paul Danzer, calls it “a circuit you can build in one night” and it not only serves a useful
technical purpose, but can also be used as introductory projects for beginners — and what better to
start with than something related to basic power? It’s a nice, tangible starter project that can offer a
sense of accomplishment after the student builds it: he or she can unplug the wall wart and see that
the load device continues to operate. It also encourages a try at some calculations to estimate run
time versus battery size, which is a function of maximum power consumed, load profile, duty cycle,
and more.
I did some basic online research and could not find an AC/DC UPS; I did see plenty of AC/AC units
with low-end ratings under a few hundred watts, of not-well-specified runtime, and for less than
$100. Still, it seems to me that a small-scale battery backup that connects in series between the
AC/DC wall wart and the load, perhaps with a selection of output voltages and connectors, might be
a viable product for specialized applications where even just 20 to 30 minutes of run time would
make a big difference. Don't we all have a drawer of these units sitting around waiting to find a new
role (Figure 2)?
Figure 2 Could one of these many wall warts from my diverse collection (and I have more) be repurposed as a small AC/DC UPS by the addition of a simple external circuit and rechargeable
battery?
Or maybe not: perhaps the sheer economics of dealing with the many DC voltage and plug
combinations makes the idea commercially impractical, and it is better to go with a small AC-line
UPS? Also, there's the uncertainty of the regulatory classification of the resultant AC/DC supply plus
battery combination. That would affect which of the new and fairly stringent regulations the unit
must meet, with tough limits on quiescent dissipation as well as overall efficiency mandates. It could
be a bureaucratic black hole. Perhaps using a USB AC/DC unit with a small accessory of an
independent battery and charger adjunct solves both problems.
What do you think of the idea of a “wall wart plus battery” for a small-scale UPS? Are there places
you could use one? Or are the limitations just not worth it?
Also see:
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When reliability hangs by a “wall wart” thread
Battery & connector diversity: Sensible design, or casual engineering?
Energy Availability vs. Power Needs