by Gerard Fonte
In the first part,
we saw that there
are three main
components to an
analog power supply:
examined the first
two components and
in this article we
will examine the
We will concentrate
on the basic t
that are cheap and
easy to obtain.
However, it should be remembered
that many of the basic design
aspects of three-terminal regulators apply to any analog regulator
design. In this article, a zero to 30
VDC, zero to one amp, current limited
power supply will be developed. The
basic cost for parts is under $25.
Figure 1 shows a conceptual
design of a three-terminal regulator.
(We will limit our discussion to positive
regulators because the negative regulators are virtually the same, other than
polarity.) It is important to appreciate
how this works in order to properly
understand and apply analog regulator
concepts. Fundamentally, the regulator
is a negative-feedback servo system.
The resistor and zener diode provide
a stable voltage reference to the
non-inverting input of an op-amp. The
inverting input of the op-amp measures
the output. If the output is higher than
the reference, the op-amp slews
towards a lower voltage.
Conversely, if the output is lower
than the reference, the inversion of the
op-amp’s input drives the base of the
transistor harder to allow a higher
voltage to pass. This design concept
is pretty straightforward (although
implementing it can be fairly complex).
There are a number of points
about this basic design that need
discussion. The first is that the pass
transistor is acting as a variable resistor.
It reduces the input voltage by limiting
the current. As you can imagine, a
fixed resistor in this situation must be
able to dissipate plenty of heat, so it’s
the same for the transistor. We will
examine this in more detail later.
The second point is that there is a
voltage drop across the pass transistor.
All bipolar semiconductor
junctions display this
effect. This means that
the input voltage must
FIGURE 1. The basic
approach to the three-terminal regulator is a
simple negative feedback
op-amp circuit. Although
in practice, it’s a bit more
complicated than this.