FROM THE BENCH
E-mail: fromthebench@gcmagazine.co.uk
Alan Bennett sings the praises of modern switch-mode power regulators made in power-regulator design
line-scan-synchronous ones, with each set
in recent years, in perfect tune
THE HE
ROES, V
IC TIMS
AND VIL
LAINS
designer having his own bright ideas and
an oscilloscope connected – with care –
with the need for high efficiency in these
concepts and each service engineer tearing
at the primary winding of the chopper
days of hideously expensive energy. It wasn’t
his hair out as he tried to do his job.
transformer: it will show needle pulses or a
always so...
Ancient stuff
Today’s switching regulators are brilliant in concept and design. Their efficiency
slowly-repetitive ‘bump-bump’ effect. In the secondary circuit of the PSU,
generally ranges between 80% and 90%,
check by inspection and with an ESR meter
The further back in time you go, the less
sometimes more. Very simply they work by
for faulty electrolytic capacitors; with an
efficient and hotter-running was home
converting incoming AC mains energy to
ohmmeter for short-circuit diodes; and (if
entertainment equipment. I can clearly
DC where necessary, then applying this via
necessary with a magnifying glass!) for
remember the days of wire-wound mains
a very fast-acting repetitive semiconductor
dry joints. On occasion, a faulty diode or
dropper resistors, and of diverse designs of
switch to an energy reservoir having
electrolytic capacitor will not show up on
power supplies which operated at power-
inductive (magnetic) and capacitive
test instruments; here substitution by a new
line frequency, 50 or 100Hz. There were
(electrical) storage elements. The chopper
one is the only certain way to prove its guilt.
thyristor regulators, which took huge, sharp
transformer provides electrical isolation
Apart from the ferrite-cored transformer,
and short bites out of the mains supply, in
between the mains and the load, while the
many PSU parts are generic and widely
some types only on one half-cycle. There
energy fed out is proportional to the duty-
available from component suppliers and
was an ingenious design by Sony in which
cycle (ratio of on- to off-time) of the switch
wholesalers, while complete boards and
two large capacitors were charged in series
in the primary circuit. This is governed by
panels can be had from Grandata and
on one mains half-cycle and discharged
a feedback loop, monitoring the secondary-
into the load in parallel on the next. All very
circuit voltage or current. This PSU building
Shopjimmy. A brilliant producer of PSU repair kits is Phoenix, who research failure
well until something went wrong, which in
block forms part of all mains-powered home
modes in a wide range of TVs etc, and
those days it often did! Some of the early
entertainment kit, and provides light-as-
make up complete sets of components
types of switching regulator were fiendishly
feathers gadget-chargers etc.
which are in my experience almost
complicated.
Then and now
Servicing
certain to do the job reliably. Typically consisting of one or more fuses, a handful
Current and recent designs of chopper PSU
of semiconductors and some passive R
Very few TV sets had mains transformers
are reliable and reasonably predictable.
and C components, these inexpensive kits
because of their cost, weight and stray
Complete failure may be due to an open
include all the little villains, victims and
magnetic fields. In their place were a
circuit fuse, dry soldered joints, diode or
heroes of breakdown. They are available
huge variety of switch-mode designs,
capacitor failure, with the latter the most
on the web and from, for instance, Charles
some based on ICs, some using discrete
common. In the primary circuit, fuse failure
Hyde at www.chsinteractive.co.uk I can
transistors, often in large numbers. They
may arise from a short-circuit chopper
really recommend these kits which are
could be a nightmare to work on: you might
transistor or IC, or just fuse ageing/
now available for a very wide range of
replace a whole bunch of blown-up bits,
metal fatigue, while the small electrolytic
TVs etc. Charles Hyde also stocks a range
only to have them instantly destroyed again
capacitors here can cause the failure
of external PSU modules, used with some
at switch on, with a brilliant blue flash
of the chopper switch to operate. More
small-screen LCD TVs, laptop PCs etc.
from the mains fuse. There was a certain
common are problems on the secondary
Japanese 18-inch TV which had a frequent
side, often betrayed by a ‘pumping’ action
Caution
habit of destroying two expensive SG-type
of the circuit, the usual reaction to some
Always replace PSU components with
power FETs; unless you got the diagnosis
kind of overload condition, which may be
exact known equivalents to avoid fire and
and repair exactly right the replacements
outside the power unit altogether, i.e. in
shock hazards. Particularly in diodes and
would rupture as soon as power was
the circuits which form the electrical load.
fuses not all characteristics of would-be
reapplied with a raucous pzzzz from the
This pumping/sampling action may not
replacements are obvious from (sometimes
fuse as it blew apart. There were difficult-
be obvious, but is easily discernible with
superficial) published specifications.
AUGUST 2011
to-service Wessel designs, and other
GET CONNECTED
W
hat brilliant progress we have
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