depends greatly on the design
by ALAN WINSTANLEY of any printed circuit board or
interwiring layout. There are
This month, our in-house “Surgeon” examines some various ways in which ripple can
causes of power supply ripple and checks out the use be introduced or affected,
of buffer circuits. including:
1. Insufficient power supply
RIPPLE it. This can be caused by a smoothing
number of factors. The ripple
OF APPLAUSE content (in millivolts) is one 2. Poor regulation of a
Here's a query related to specification to look for in d.c. transformer
power supply ripple, which power supplies - the lower, the 3. Poor choice of transformer
highlights some issues better. secondary voltage
concerning the layout of power However it's uncommon to 4. Layout and wiring
supply circuits. Mr. S. Fox of see specifications for ripple considerations
Epping writes: quoted in regulator data
5. IC bypassing.
I have been interested in sheets, because ripple can be
making a variable voltage introduced by a combination In the case of (1), too small
supply for some time now, and I of factors, which are beyond a smoothing capacitor could
have been using a circuit with the control of the regulator provide inadequate smoothing
an L200 regulator IC. However, manufacturer. It's over to my and introduce ripple. Provided
for one reason or another, I get creaking bookshelves - SGS- that the regulator does not
30mV of ripple measured on my Thomson produce the L200 “bottom out” this is not a
oscilloscope, which is some- and luckily I have a copy of problem, because the regulator
thing I'm trying to sort out. In all the “SMART Power” Appli- will output a d.c. straight-line
the circuits I've seen, whatever cation Manual, which includes voltage.
method they use to obtain a d.c. the makers' design notes on It is possible though for a
voltage from the mains, there is this device. regulator to come out of regu-
no mention of ripple. The L200 is a positive lation if there is insufficient input
The other point I'd mention, variable voltage regulator voltage (to overcome the
while writing, is that these days which includes a current regulator dropout voltage), so
it seems one must have some limiter. It provides up to 2A at any ripple present on the input
sort of engineering facility since 2·85V to 36V. The output will also appear on the output.
most items are contained in voltage is fixed with two You should allow sufficient
metal containers. This is rather external resistors and the “headroom” on the input side to
beyond me so I construct most current with one low-value overcome the drop-out voltage
things in wood. I was doing this resistor. Hence, the regulator of the device under worst case
over 60 years ago when our is housed in a 5-lead conditions, to ensure that the
pocket money was in pence Pentawatt package. The device continues to regulate
(pre-decimal old money!) rather operation of the L200 was properly.
than in Pounds as would seem described in a special feature In (2), a transformer
today. article by Andy Flind in the regulation of up to 33 percent is
July 1998 issue. Also, see not uncommon on the smallest
Your query on “ripple” first.
Andy's Float Charger design in sizes, so when the load current
Ripple is the unwanted a.c.
the August 98 issue for a increases, the transformer's
variation superimposed on top
typical application circuit. peak output voltage will fall.
of a smooth d.c. voltage. On an
oscilloscope, a d.c. voltage will Usually the transformer
display as a constant straight manufacturer ensures the
Matters of Performance secondary voltage is as
line over time, but a close look
will often reveal alternating Like many regulators, the specified when running at full
peaks and troughs mixed in with performance of the L200 load. This means a higher off-
Copyright © 1998 Wimborne Publishing Ltd and EPE Online, November 1998 - www.epemag.com - 61
Maxfield & Montrose Interactive Inc
load voltage is present across a given frequency and a spec- BUFFERS FOR
the transformer secondary to ified value of bypass capacitor
compensate. on the reference bypass BEGINNERS
If the regulation is poor, if terminal. My thanks to
you're not careful with your In the case of the common Sivasubramanian
choice of transformer voltage LM317, ripple rejection is about Pathmanathan in Japan who
(3) then again there is scope for 65dB without any bypass asked for the low down on
the regulator to “bottom out” - capacitor, and improves (85dB) “buffers”:
come out of regulation - and when a 10µF bypass capacitor “What is a buffer?
ripple will appear on the output. is fitted between the “adj.” pin When are they used? Could I
Layout considerations (4) and 0V. A 10µF tantalum is have a brief explanation of the
are in the constructor's hands, usually the optimal choice for function and use of buffers?''
and this is where you can most the bypass capacitor. (By E-mail.)
influence the ripple content of
the voltage. If a circuit is being Tool Case
constructed on a PCB then at A buffer is an amplifier cir-
higher outputs, the layout of the Turning to your point about cuit which ideally has an infinite
copper track becomes quite instrument cases, it's true that input impedance, and also a low
critical if ripple and noise are to the cost of plastic molded boxes output impedance. They are
be minimized. can be daunting, and also that a used in between stages of a
bench and a modest number of circuit, usually acting as a
The L200 is a multi-pin
hand tools are needed to per- current amplifier to enable a
package with separate pins for
form some of the mechanical comparatively heavy load to be
the reference and current
work. (I illustrated the entire driven by the preceding stage.
limiting feedback. The manu-
prototyping process in my series Buffers include ordinary
facturer's data recommends that
“Build Your Own Projects” in the bipolar or MOS transistor
the track connecting pin 3
November 96 to March 97 switches that you might couple
(ground) and any resistor on pin
issues of EPE.) to a logic gate, in order to drive
4 (voltage reference) must be
kept as short as possible, and However, it can cost a an external load such as a
should not be crossed by the manufacturer several tens of relay, solenoid, loudspeaker,
load current. thousands of pounds in tooling indicator and so on. You could
costs to design and mold a class a MOSFET power
Also, there must not be any
complex plastic box, even transistor, hooked to the output
“impulsive” currents between
though the intrinsic value of the pin of a logic gate or a 555
pin 3 and the negative output
plastic itself can be measured in timer, as a buffer.
terminal of the circuit, because
pence and grams. You're paying Buffers are also produced in
these will increase the ripple.
for the tooling depreciation and dual-in-line packages which
Layout apart, inadequate copper
maintenance costs as much as contain several Darlington-type
track or wiring thickness could
anything, and even I hesitate to transistor drivers that can be
also introduce resistance which
purchase nice plastic boxes used for interfacing a low-power
adds to the ripple.
because of the one-off costs. circuit to the outside world.
For (5), using another
I wish you continued succ- Examples include the ULN2001
device as an example, National
ess with your wooden boxes! range, which contain seven npn
Semiconductor warns against
introducing resistance between
the output and the “adjustment”
pin of its LM317K, because this
can affect load regulation. Visit our Web site at www.epemag.com
Ripple rejection is specified in
dB, and is often expressed in
In our library you will find:
the form of a series of graphs. ♦ Alan Winstanley’s internationally aclaimedand
National Semiconductor defines invaluable guide to soldering
load regulation as the line
regulation (change in [regulator] ♦ An amazingly useful article on LCD displays
output voltage for a change in
♦ And much, much more!
input voltage) for a.c. signals at
Copyright © 1998 Wimborne Publishing Ltd and EPE Online, November 1998 - www.epemag.com - 62
Maxfield & Montrose Interactive Inc
Darlington transistors (these protection circuitry which we
have a very high gain, which now all take for granted.
means they require negligible It can be treated like a blow-
input current to turn each trans- out proof power transistor - it
istor hard on) and can supply even looks like one. As always
roughly 500mA to the load. with this type of protected
You often see Darlington device, you should check any
“arrays” like these used for heat-sinking arrangements
powering external solenoids, closely to ensure the device
indicator lamps and light- doesn't shut down prematurely
emitting diode displays. Some due to thermal overload.
types are classed as TTL/ The National LP395Z is a
CMOS compatible, which scaled-down version made in a
means that they are guaranteed small plastic transistor package
to work with ordinary 5V logic, which handles up to 100mA.
remembering that a 5V TTL They are relatively cheap and
logic “high” can be as low as form ideal buffer transistors
2·7V: a logic-compatible device especially if a “fail safe”
knows how to cope with this. protection is needed to protect
the load or the circuit. You can
One in a Million fetch details of these devices
from the National web site
If you don't actually need
seven buffers in one package, (www.national.com) and the
then you could use any of a LM395T or LP395Z are listed by
number of MOSFET transistors Farnell, RS Components, and
to interface a low-power circuit others.
to a load. MOSFET transistors You can, incidentally, buy a
are voltage-driven switches CD ROM directly from National
which use so little gate current Semiconductor in the USA. It
you can ignore it. contains tons of data sheets and
However, manufacturers application notes for National's
such as Zetex (famed for their devices. It costs $4.00 US,
small but spirited bipolar E-line takes about a week to arrive,
ZTX transistors) will argue and is great value. Check their
convincingly that bipolar web site as noted above.
technology is still an excellent
choice over MOS in many
applications. Indeed, the ZTX CIRCUIT THERAPY
range of transistors combines Circuit Surgery is your
high capacity with small column.
package size and they make If you have any queries or
very good buffer/interface comments, please write to: Alan
devices. Winstanley, Circuit Surgery,
An interesting driver which Wimborne Publishing Ltd., Allen
has been around for over 20 House, East Borough,
years is the (bipolar) LM395 Wimborne, Dorset, BH21 1PF,
produced by National United Kingdom. E-mail
Semiconductor. At the time, it alan@epemag.demon.co.uk.
was marketed as a power
Please indicate if your
transistor with a gain of a
query is not for publication. A
million, and was produced in a
personal reply cannot always be
TO-3 can as the LM395K. The
guaran-teed, but we will try to
present version LM395T has a
publish representative answers
TO-220 package (hence the “T”)
in this column.
but still incorporates the
Copyright © 1998 Wimborne Publishing Ltd and EPE Online, November 1998 - www.epemag.com - 63
Maxfield & Montrose Interactive Inc
