li">Working with Wall Warts

John H. Bryant, with Bill Bowers

Edited version, January 2005

Ive often been frustrated
by my lack of understanding of the simplest electronic device that any
of us possess: the ubiquitous wall wart plug-in power supply. 
I must confess that when I have co-authored technical articles in the past,
the emphasis is very much on the co.  In such projects, I generally
perform as scribe, editor, graphic artist and all-around cheerleader,
while relying on my partner to supply the essential technical expertise
and applied creativity. Thus, when faced with a continuing need for
small external power supplies of specific voltage or RF cleanliness,
I have been at the mercy of the rather insanely high prices charged
for such things at Radio Shack or other electronic parts outlets; all
the while, I was accumulating a whole drawer full of spare wall warts,
orphaned from various long-forgotten consumer and hobby devices.

In case you
arent familiar with the slang term wall wart, that refers to the
seemingly solid block brick that plugs into the AC mains electrical
wall outlet and provides DC power to various consumer devices from cell
phones through medium-sized radios to many computer peripherals. Typically,
a good consumer-grade wall wart contains a transformer to reduce normal
mains voltage to the range needed by the device to be powered. Once
the mains AC power is lowered in voltage, it is run through a solid-state
bridge (four diodes) or full-wave (two diodes) rectifier to be converted
to DC. This is usually followed by a single filter capacitor. The power
is then piped out through twin wires of appropriate size to carry the
current at the designed power level. It is my understanding that some
very inexpensive wall warts do not contain even rudimentary filter capacitors.
Avoid these, if possible.

Being rather desperate
for some small power supplies about a year ago, I finally worked up
the courage to admit my ignorance about such things to long-time friend
Bill Bowers. I asked him to develop some notes for me, so that I could
create both adjustable and fixed voltage DC power supplies from my drawer
of wall warts. This was about like asking a NASCAR driver to give you
tips on parallel parking, but Bill kindly complied. In the last few
months, Ive loaned copies of these notes to several hobbyist friends,
some of whom are technically astute and others, dummies like me. Both
sets of folks have found Bills notes very useful, so I am encouraged
to share them more widely. What follows are Bills notes, data and circuit
diagrams (redrawn by me) along with my running commentary.

Cautions

At its simplest, there are three possible outcomes
to a project like this and two of them are BAD. When energized, there
is some 120 volt AC (or more) electricity wandering around in what you
are working on Be careful, or you might fry yourself or at least knock
your heart out of rhythm. I hate it when that happens.  BE CAREFUL!

A second bad outcome
is the possibility of frying the device that you are hoping to power
with your newly modified wall wart. DC current has a positive and a
negative leg and it is very easy to get them reversed (reversed polarity.)
Often, this will literally fry the device that you hope to power. 
I take a very direct approach to checking for proper polarity at the
end of one of these wall wart projects: I keep an unmodified wall wart
around and I check its polarity first, using a small cheap analog volt
meter. I pay close attention to which meter probe, red or black, that
I use on the inner and outer parts of the DC connector and I note which
direction the meter pointer swings.  I then repeat the same test
on my newly modified wall wart. That approach may seem both simple-minded
and paranoid, but Ive made too many sad mistakes to do otherwise. BE
CAREFUL!

The third possible outcome
of one of these simple projects is VERY GOOD: you create a voltage-regulated,
RF-clean, fixed or adjustable voltage DC power supply for about $5.00
worth of parts and less than an hour of easy work. BE JOYOUS!

Discussion

Most wall warts are manufactured to power a single
small solid-state electronic device and are usually designed to supply
DC power. This article focuses entirely on wall warts that supply DC.
Unless the wall wart happens to be one of those few that also provides
voltage regulation, the DC voltage measured at the DC tip of the device
can be between 15 and 20 VDC, when there is no load on the wall wart. As load is applied
to the circuit, the voltage drops proportionally. Thus, if a wall wart
is rated at 12 VDC and 500 ma., the voltage will be significantly above
12 VDC when the supply is powering a device that only uses 200 or 300
milliamps of current. Similarly, the voltage will drop well below the
specified 12 volts if more than 500 milliamps of current is needed by the device
to be powered.

Cheap wall warts have
a second weakness of concern to us: although they usually have a single
filter capacitor inside the case, they are still somewhat dirty devices
from an RF point of view and can produce all sorts of buzzing and other
artifacts at the frequencies on which we normally DX. Most often, this
is caused by ripple in the DC current created by the rectifier Ripple
is a less than smooth, steady value for the voltage and or current produced
when plotted over time. It may be thought of and measured as vestigial
AC current downstream from the rectifier and is often a cause of serious
noise in RF-related circuits. Unless you are interested in DXing your
new power supply and QSLing yourself, adding filter capacitors to your
Wall Wart project is very worthwhile.

So, since we usually
want to power an auxiliary device at a steady specified voltage and
since we usually need a very clean power supply, what I wanted Bills
help on was in creating a filtered, regulated power DC supply. We accomplished
this by adding a module between the wall wart and its DC output plug
that contained a fixed or adjustable voltage regulator<span class="Alaviitteen-0020viite--Char"><sup>1
and a network of filter capacitors.

Voltage Regulators

Bill recommended
using two families of small integrated circuit voltage regulators for
our more common applications: the 78xx family of fixed voltage regulators
and the LM3xx family of adjustable regulators. Both of these families
are popular in commercial and hobby applications and are manufactured
in great numbers; hence, they are all quite commonly available and very low cost. 
A grid of reference information for each family which follows:

Fixed Regulators   

Type

I<sub>max

V<sub>out

Package

All Electronics

     Mouser        

 

amps

Volts

 

P/N               
cost

P/N                          
cost

7805T

1.0A

+5*

  TO-220

7805T        @
$0.50 

511-L7805CV      @ $0.40

7808T

1.0A

+8*

  TO-220

7808T        @
$0.50

511-L7808CV      @ $0.40

7809T

1.0A

+9*

  TO-220

   Not Available

511-L7809CV      @ $0.40

7812T

1.0A

+12*

  TO-220

7812T        @
$0.50

511-L7812CV      @ $0.40

7815T

1.0A

+15*

  TO-220

7815T        @
$0.50  

511-L7815CV      @ $0.40