Download Series Voltage Regulators

Devices and Applications
Ctec 201.
Series Voltage Regulators
Supplement
Prepared by Mike Crompton. (Rev. 7 July 2003)
Series Voltage Regulators
Although the Zener diode makes a reasonable voltage regulator, there are still minor
variations in O/P voltage as the I/P voltage or load resistance changes, particularly when
the load current is high. This has led to the development of transistor controlled
regulators that have excellent regulating capabilities. One of these is the ‘series regulator’
shown below.
The concept of the series regulator is based on the simple
voltage divider law that states that the voltage drop across any
two series resistors is proportional to the Ohms ratio of the
resistors. If one resistor is 10 times bigger than another it will
have 10 times the voltage drop across it. This concept does not
change if one of the “resistors” is a Zener diode or a Transistor.
Both these devices can be viewed as variable resistors, and
when their resistance changes, the voltage drop across them
changes and the voltage drop across the second resistor also
changes. The circuit on the right is much simplified, but will
illustrate this voltage divider action that will maintain the O/P
voltage at a constant level (5V) even though the I/P voltage or
the O/P load resistance (RL) may vary. (RL is shown as a
variable resistor to indicate it’s ability to change).
The circuit can be divided into two separate smaller circuits,
each being a two component series voltage divider. One circuit
would be RS and the Zener diode fed by VIN, and the second
being the Transistor and RL again with VIN as the supply. Both
circuits are shown at right in their simplified form.
First look at the RS/Zener circuit. The Zener diode, shown as a
variable resistor, will simply change it’s resistance due to Zener
action every time VIN changes. This change in resistance will
maintain 5.7V across the Zener diode and this voltage will be
fed to the Transistor maintaining it’s base at 5.7V.
Looking at the Transistor/RL circuit, it can be seen that if RL changes it’s resistance, the
transistor, by changing it’s conduction rate (transistor action) will alter it’s resistance and
maintain the voltage across RL at a consistent voltage. In this case that would be 5V.
This compensates for changes in VIN or in RL as did the previously studied Zener diode
regulation circuits, but with much better regulation over a wider range of changes. Of
course there are limitations to the magnitude of change that the circuit can tolerate. For
example VIN must always exceed the Zener diode voltage or the Zener will never conduct
and no regulation will take place. The load resistance (RL) cannot drop below a certain
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value. If it does, the transistor’s resistance will drop to such a low value that the resulting
current flow will exceed it’s maximum and it will burn out.
In actual practice the series regulator has many more components than our simple circuit.
(The full circuit diagram is shown below courtesy of Motorola). Many of the extra
components deal with error detection and protecting the device against misconnection or
exceeding the regulating parameters. These regulators are usually very efficient, the 78xx
series for example is completely self contained, has only three pins, can handle as much
as 1 Amp of current and the O/P will not vary from the designated O/P voltage (5V in the
case of the 7805) by more than a few micro volts. To withstand the heat generated by 1
Amp of current requires that the device has a heat-sink that mounts to a large pad on the
printed circuit board, or to a metal chassis. Care must be taken to ensure that mounting
the heat sink to a chassis does not create a short circuit. In some devices the heat sink is
internally connected to an active part of the circuit, in the case shown below the heat sink
is actually connected to the ground.
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