Practical_Electronics-May_2019

The minimum input voltage for the 7805

regulator is specifi ed as 7.5V (see Table 3.1

on page 45 of EPE, February 2019). Thus,

for a maximum load of 1A, we can now

estimate the minimum required value of

reservoir capacitance from:

In practice a 4,700μF component would

be suitable with a working voltage of at

least 16V.

from a full-wave bridge rectified
12V RMS AC supply and requiring a
maximum current of 1A. The 12V RMS
supply derived from a mains transformer
secondary will have a peak value of
approximately 17V (12V × 1.4). Let’s
assume that we would not be prepared to
tolerate a voltage fall of more than 1V on
the DC supply rail when the maximum
load current is being supplied. We should
thus plan for a worst-case situation in
which Vpk = 17V and Vtr = 16V (in other
words, we are designing for a maximum
ripple voltage of 1V peak).
With a 50Hz supply the value of tdmax
would be 10ms and the maximum load
current will be around 1A. Hence:

As a general rule, reservoir capacitors
should have a maximum working voltage
rating of at least 50% more than the
expected DC voltage and with a ripple
current that is at least 50% more than
the maximum DC load current. Thus, a
component rated at 30V or 35V with a
ripple current rating of 1.5A, or more,
would be suitable.
In this simple example we considered
the bridge rectifi er to be perfect and didn’t
account for any voltage drop that might
be attributable to the rectifi er diodes
when in the conducting state. In some
cases (and for low-voltage supplies in
particular) it would be wise to consider
the additional voltage drop that is
developed across each semiconductor
junction when it is conducting. We
will illustrate this with a slightly more
complex example.
Let’s assume that we are using a linear
three-terminal 5V regulator based on
a 7805 and a mains transformer with
an 8V secondary and a bridge rectifi er
arrangement (as before). The peak
voltage appearing across the reservoir
capacitor will be (8 × 1.41) = 11.3V less
the forward voltage drops associated
with two of the four rectifi er diodes
(only two of the four diodes will be
conducting at any time). Thus, the peak
voltage appearing across the reservoir
capacitor will not be 11.3V but nearer
to (11.3 – (2 × 0.7)) = 9.9V.

Electrol ytic capacitors__

×

−

1 10 10^3

F 10mF or 10, 000μF

17 16

C

××−

==

−

−

×× × −

−

π

π

⎛⎞

⎜⎟

⎝⎠

π ××× ×−

1 1

×

×

× × ×

×⎛⎞

⎜⎟⎝⎠

× ×⎛⎞

⎜⎟⎝⎠

× ×⎛⎞

⎜⎟

⎝⎠

×

−

××−

−

1 10 10^33

F 4.17 10 F = 4,170μF

9.9 7.5

C

××− −

==×

−

π

π

⎛⎞

⎜⎟

⎝⎠

π ××× ×−

1 1

×

×

× × ×

×⎛⎞

⎜⎟⎝⎠

× ×⎛⎞

⎜⎟

⎝⎠

× ×⎛⎞

⎜⎟

⎝⎠

As you’ve seen, the vast majority of

capacitors used in power supplies

are electrolytic types that permit

relatively large values of capacitance in

a small volume. In the manufacture of

electrolytic capacitors, an oxide fi lm (the

dielectric) is grown onto a metal anode by

electrochemical means. The oxide fi lm is

very thin and offers a dielectric constant

of up to about 25. In normal operation,

the dielectric must be polarised by the

application of a DC voltage.

Electrolytic capacitors need to be

treated with care; they are not as widely

applicable as other types of capacitor for

a number of reasons, including the need

for a polarising voltage, wide tolerance,

poor temperature characteristics and

relatively large values of leakage current,

signifi cant effective series resistance

(ESR) and effective series inductance

(ESL). Electrolytic capacitors are also

signifi cantly less reliable than other

types and are prone to failure at high

temperatures (see later). This serves to

emphasise the need to choose the correct

type of capacitor for power supply

applications and particularly where

reliability is paramount. The properties

of various common types of electrolytic

capacitor are listed in Table 6.1.

Electrolytic capacitor characteristics

It’s worth spending a little more time

explaining why large electrolytic

capacitors need to be treated carefully. We

will start by explaining some important

characteristics that have a major effect

on the way capacitors operate.

Dielectric

A capacitor is an energy-storing device

made of two parallel conductive plates

separated by an insulating (dielectric)

material. When a voltage is applied

across the plates, the electric fi eld in the

dielectric displaces electric charges, and

thus it stores energy. It is assumed that

there are no free charges in the dielectric

(at least in the ideal case), and that while

they are displaced, they are not free to

move (as they can in a conductor).

Value and tolerance

The capacitance of an electrolytic

capacitor is usually stated in μF

(microfarads) and is determined by the

physical properties of the capacitor

(effective plate area and separation) as

well as the dielectric material. Tolerance

is the maximum allowable deviation

from the nominal capacitance value at

a specifi ed temperature. Typical values

of tolerance for electrolytic capacitors

can range from –20% to +50% and so

you can certainly expect some deviation

from any marked value of capacitance.

Maximum working voltage

The maximum working voltage for

an electrolytic capacitor is defi ned as

the maximum DC voltage that can be

applied to the capacitor continuously

at the rated temperature. Note that AC

and DC ratings are not the same and

the sum of the DC and any peak AC

voltages simultaneously applied to the

capacitor must never exceed the rated

DC voltage.

Equivalent series resistance

ESR is the internal resistance of the

capacitor expressed as a single resistance

value, modeled as connected in series

with the capacitor (which is then viewed

as a ‘perfect’ component (see later)). It

is important to note that ESR is made

up of the sum of the resistance loss in

the dielectric (important in electrolytic

components) and the resistance of the

conducting path between the capacitor

plates and its external connections. ESR

varies slightly with frequency, usually

falling to a minimum value in the range

associated with most switched-mode

Type of

capacitor

Typical range of

values

Typical working

voltage

Temperature

range

Typical ripple

current

Typical leakage

current

Typical

ESR

Life expectancy

(hours)

Miniature radial

lead electrolytic

1μF to 2,200μF 6.3V to 25V –55°C to +105°C 0.4A to 9.7A 0.01CV or 4μA^15 Ω to

350 Ω

2,000 at 105°C

Miniature SMD

electrolytic 1μF to 1,000μF 6.3V to 50V –45°C to +85°C 0.01A to 0.33A 0.01CV or 3μA

0.4Ω to

166 Ω 2,000 at 85°C

PCB-mounting

electrolytic

220μF to 22,000μF 16V to 400V –40°C to +85°C 0.7A to 4.1A 0.02CV 0.03Ω

to 4Ω

2,000 at 85°C

SMD low-ESR

electrolytic 100μF to 1,000μF 6.3V to 50V –55°C to +105°C 0.3A to 0.45A 0.01CV or 3μA

0.17Ω

to 0.4Ω 2,000 at 85°C

Low-ESR radial

lead electrolytic

2.2μF to 1,000μF 16V to 100V –55°C to +105°C 0.03A to 1.5A 0.01CV or 2μA 40mΩ

to 2.8Ω

5,000 at 85°C

High-grade

electrolytic 1,000μF to 47,000μF 25V to 385V –40°C to +85°C 5A to 42A 0.006CV or 4μA

5mΩ to

70mΩ 4,000 at 85°C

Table 6.1 Comparison of electrolytic capacitor types