The BasicDCC Project




Power Adapters




I found that the bins at the back of the thrift shops
are gold mines for suitable power adapters.





Power adapters can be divided into two major categories
Conventional and Switching


Conventional

All conventional adapters are of a basic design that has been
a standard building block circuit since the early Lionel and
American Flyer days. This is the circuit that for years, was
used on virtually every model railroad layout.

It is comprised of an AC step-down transformer, rectifier diodes
and a power filter capacitor, all encased in a somewhat bullet
proof housing. Some have a 4 diode, full wave configuration and
some are half wave (2 diodes). AC to DC rectification occurs on
the secondary (low voltage) side of the transformer.

The voltage produced at the output is unregulated and will be at
a somewhat higher voltage than rated, when no load is applied.
I found that most 12v adapters put out about 16 volts unloaded.
The voltage dropped closer to the rated voltage when the load
was increased to about 2/3 of the rating.

Although very few have conventional fuses, all that I ran into
had some sort of thermal fuse that triggers on the heat of the
transformer windings. Some were one-shot devices that opened
permanantly, rendering the adapter useless. Most were self
resetting. These fuses are meant for safety only and should never
be relied on for over current protection.

I found that very few adapters of this type had any sort of short
circuit protection and if they did, it was pretty old technology
that wasn't very good. Most adapters of this type provide raw
current with no short circuit protection. Care must be taken to
prevent the current draw from exceeding the adapter rating. The
first thing to go is usually a diode. This type of adapter is
more forgiving to over peak current draw and back EMF than a
switching adapter and almost always will work well with motors.

Compared to a switching adapter of an equivalent rating, a
conventional adapter will be much heavier due to the weight
of the step-down transformer. Most adapters of this type will
usually be rated under 2 amps. A 3-AMP conventional adapter
would weigh in at about 3 pounds.


































Switching

Conventional adapters, while simple and reliable, are a somewhat
inefficient way to do the power conversion necessary to produce
lower voltage DC at high currents.

Relatively recent technological advances have resulted in an influx
of high current, switching adapters that produce 3-5 AMPS with very
good regulation. Most have some sort of built in short circuit and
over current protection. It is now the preferred type being supplied
with 95% of the consumer goods sold today.

If you were to crack one open, you'll likely find that everything
(including the step down transformer) is on a single circuit board.
They work by doing a direct rectification of the incoming AC line
voltage, creating a high voltage DC (appx 180v). The DC is then
ran through a high frequency chopper circuit that converts it back
to AC at a much higher frequency than the standard 60HZ that is
used by conventional adapters. The high voltage, high frequency
AC produced by the chopper circuit is normally in the KHZ range
and at a few hundred volts. Many switching supplies make use
of voltage multiplication to supply an even higher voltage to the
chopper, to further increase efficiency. Through a combination of
carefully chosen parts and a well laid out design, a very stable,
well regulated output is obtained that is virtually independent
of the load.

All of this reduces the size and weight of the power adapter.
Adapters rated at 4 amps can weigh in at less than a pound.

Most switching supplies have built in short circuit and over-current
protection. Some protection circuits won't work well with the type of
load created by electric motors. For example, if the design of the
power adapter incorporates a protection circuit that responds too
quickly, it may trip on a quick short created by a damaged or dirty
commutator in the motor. For the most part, I found that the issues
were minimal and limited to a just a few of the many adapters that I
tested. Most were capable of supplying continuous current at their
marked current rating without any issues.

I found that lap top computer adapters will generally have very
good protection circuitry that works very well and won't trip on
momentary shorts or back EMF from the motors. Generally, the
newer models seemed to perform the best but I did find a few that
were built with older technology that were virtually bullet proof.



























I picked up a few power adapters that were originally supplied
with laptop computers that fit the bill nicely. I picked them
up at the local thrift stores for a few bucks each. Asking
around the local computer stores found similar units in the
$15-30 range (one guy dared to ask $50).





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