Download 01 Rectifiers

Rectifiers
Most electronic equipment makes use of a
steady DC Voltage to control all of its electronic
components.
The problem is that the power that is delivered
to our houses or places of business comes in the
form of AC Voltage from the power company.
So we need to be able to convert Full wave AC
power to Straight line DC power.
To do this we use a device called a Rectifier.
Basically a rectifier can be as simple as 1 diode.
As you know a diode will only pass current in
one direction and that is when the Cathode is
biased in a more negative condition compared to
its Anode.
So if a diode is connected to one side of a AC
power supply, the other end is connected to one
leg of a resistor (load) with the other leg of the
resistor connected to the other side of the power
supply the voltage would only be present over
the resistor when the diode is forward biased. If
you were to measure the voltage over the
resistor with an oscilloscope the wave would
look like the diagram below.
The above wave represents the output of what is
called a half-wave rectifier.
Notice that the signal is only positive and the
negative part of the wave is not being used at
all. Therefore the output is now called “halfwave varying DC”
Now this would be great if we wanted our
equipment to be turned on and off on the cycle
of the AC power.
As you can guess we need to have a device that
will give us constant power (DC).
This is accomplished by using 4 diodes
connected in a certain way to make use of both
the negative and positive parts of the AC wave.
This device is called a Bridge Rectifier and is
configured as in the picture below
It is very important to make sure the diodes are
connected in the right configuration for a Bridge
Rectifier to work properly.
A Bridge Rectifier provides a what is called a
“Full-wave varying DC” output and would look
like the wave below.
Now before we go any further what is important
to remember is that diodes use up .7 Volts each
out of the supplied voltage when they are
forward biased.
Therefore as voltage has to pass through 2
diodes to complete the circuit on each half cycle
the Bridge Rectifier will use up 1.4 Volts of the
power provided.
Bridge Rectifiers are rated in the following way
Bridge rectifiers are rated by the maximum
current they can pass
Also they are rated by the maximum reverse
voltage they can withstand (this must be at least
three times the supply RMS voltage so the
rectifier can withstand the peak voltages).
Now that we have a pulsating full-wave signal
we now need to flatten or smooth out the signal
(straighten to represent a DC signal)
To do this we use a capacitor across the load.