Note:
this project is a work in progress, but should be finished in a couple
of weeks.
A linear power supply is an ideal
first hands on project. We take one each of a transformer, rectifier,
capacitor, resistor, zener diode and a transistor and link them
together to make a useful
circuit. A few more components and we have a precision-regulated power
supply. A power supply is also a great first project
because we need it to supply power for our other projects. In addition,
most electronic devices need power supplies. Whether it's a television,
a radio, a DVD player or a computer, most devices run on fairly low
voltage direct current (exceptions are light bulbs, motors, toaster,
etc.). The power coming from the power grid is fairly high voltage AC.
That is great for delivering power over long distance but not what we
need to power electronic equipment. A power supply (or DC power supply
to be a little more exact) converts the high voltage AC from the power
receptacle to the low voltage DC needed by our equipment.
The stages of a typical linear DC power supply are:
1. Step-down transformer
In a linear power
supply we need to limit the voltage across the main regulating element
(usually a transistor). This reduces the power consumed and thus the
heat produced by that element. The step-down transformer serves this
purpose.
2. Rectifier
We need to convert the AC from the
power grid to DC for the power supply. This is done with one, two or
four rectifier diodes
3. Filter
The rectifier provides DC, but in the
form of pulses. This is OK for motors or light bulbs, but not
for most circuits. A large capacitor or bank of capacitors is usually
sufficient although inductors may be used too.
4. Regulator
Some DC power
supplies have nothing but a transformer,
rectifier and filter. A regulator will provide a precise output voltage
if such is required.
Our Power Supply Design
We will start by building an
unregulated power supply. This will be only a transformer, rectifier
and filter. After demonstrating the limitations of such a power supply
we will design and build a series of regulators to improve the
performance. At each stage we will demonstrate the advantages and
disadvantages of each type of regulator. Then we will design and build
a better regulator. Our final design will be a dual precision
regulated, current controlled 12 volt linear DC power supply. If you
choose to build the complete project you will have a great addition to
your workbench.
In the end
you can customize the power supply to meet your needs. For example, you
can use increase the output to 24 volts by using a 24 volt transformer
instead of the specified 12 volt transformer. However, this will
approximately double
the cost of the transformer. You can choose to make a single power
supply instead of a dual power supply. This will cut your cost nearly
in half. The design can be easily modified, at additional cost to
provide much higher current if you desire.
Here is the basic schematic for the design of the power supply
(everything will be duplicated for the dual power supply version):
The basic power supply design.
Component values will be calculated later.
The only component in this design we haven't discussed already is the
operational amplifier (op-amp [the triangle symbol near the middle]).
We
won't need the op-amp until we add the precision regulator. We will
have a brief discussion on op-amps when we come to that stage (we will
have a complete discussion on op-amps later in the course). This design
has a fixed current limit. For our final design we will add a variable
current limit. Don't worry about how the circuit works right now. We
will discuss that as we design and build the circuit.
The Enclosure
You need to decide ahead of time if
you will be building the full dual power supply or just a single power
supply. Everything will be duplicated in the dual power supply, so you
will need to buy an enclosure large enough to house everything. The
biggest parts will be the transformers and the heat sinks for the pass
transistors. We need to choose an enclosure that will fit these parts
along with the circuit board. You may need to choose an enclosure after
you have built the power supply just to make sure everything fits. Here
is the enclosure I will be using:
The enclosure I chose. Search
Amazon or eBay for "electronic project enclosures" to find this or
another enclosure.
