A linear power supply
typically needs a step-down trans former to convert the high voltage AC
from the power receptacle to a reasonable lower voltage.
A small step-down transformer
typical of ones used for power supplies.
WARNING:
The primary winding of the transformer has dangerous
voltages
applied to it. Use caution when working with live circuits.
Do not touch the connections
directly; have your hands on insulating probes.
Do not allow the probes to to
touch live connections and other parts of the circuit or metal chassis,
etc., at the same time.
Watch your probes carefully. Only touch the part of the
circuit you intend to touch.
Make sure the ground connection of an oscilloscope probe
is not connected to anything but the circuit ground.
Be sure your test equipment is
set to the proper signal type, range and connectors.
When testing voltage make sure you haven't left your
probes attached to the current input of your meter, etc.
Make sure your meter is set for volts and not amps, etc.
We are going to design this power
supply to deliver 4 amps at 12 volts, so the circuit starts with a 12
volt 4 amp step-down transformer. Here are some
examples you may want to buy:
These transformers are rated at 117
volts on the primary and 12 volts, 4 amps on the secondary. This
primary voltage is appropriate for North America and other parts of the
world using the 110 to 120 volt standard. If you are elsewhere you will
need to buy a transformer with the appropriate primary voltage. For
example, in western Europe an equivalent transformer will be rated at
230 volts on the primary and 12 volts, 4 amps on the secondary. The
Jameco P-8658 that I will be using is rated for up to 230 volts on the
primary. this will increase the secondary voltage to 24 volts, but
reduce the secondary current rating to 2 amps. You should be able to
use this transformer to build a 24 volt, 2 amp power supply in the UK.
You will find some transformers rated in watts or VA (voltamps) instead
of giving a current rating on the secondary. To find the current rating
of these transformers, divide the watt or VA rating by the secondary
voltage. For example, if the transformer has a secondary voltage of 12
volts and is rated at 50 VA, the secondary current is about 4.2 amps.
Many transformers have multiple primary and secondary windings. These
can be wired in series or parallel to get different primary and
secondary voltages. See Multiple Winding Transformers
in Transformers in AC Circuits
This project will be a dual power supply where nearly everything is
duplicated. I will be using two separate transformers so that the final
circuit will fit in the enclosure I chose. However, you can use a
single transformer with multiple primary and secondary windings. Such
transformers should have diagrams attached to show how to wire them for
different primary and secondary voltages. If you decide to build a
single power supply instead of a dual supply you will need only a
single transformer. You may still find that a certain multiple winding
transformer can be wired to suit your needs.
Here is the transformer as it appears on a schematic diagram.
The transformer as it appears on
a schematic diagram.
In the accompanying video we examine the primary and secondary voltages
with a digital multimeter and an oscilloscope.
Building a Linear Power Supply - Part 1 - The Transformer
