| |
|
| |
Everything
you should know about Power Supply Dips & Spikes but were
too afraid to ask.
By Les Rose |
 |
|
| |
Supply
Authorities such as ETSA (Electricity Trust of South Australia)
recommend appropriate power conditioners as a normal part of
an industrial or commercial computer installation. |
|
|
| |
Why
would the Electricity Trust of South Australia make this recommendation
in their booklet "Computer and Electricity"? Because
they acknowledge there is a problem. But do you? |
|
|
| |
As
computer technology advances it is becoming increasingly apparent
that the mains distribution system is very inadequate for modem
electronic equipment. It is important for every hardware and
software supplier to have a basic understanding of the limitations
of the power system and the way in which it interacts with modem
electronics. Hopefully this will increase professionalism in
supplying 'total" computer solutions and eliminate much
of the wasted time, energy, money and confidence that occurs
due to power related problems. |
|
|
| |
The
previous decade has seen dramatic changes in computer technology
and the direction of computing as a whole. In their infancy,
computers were basically used for data storage and retrieval,
and number crunching. In today's hi-tech society they are employed
as on-line devices in a real time capacity. |
|
|
| |
Therefore
any downtime caused by software failures tend to have immediate
and often catastrophic effects on industry productivity and
profitability. |
|
|
| |
Technical
advances in integrated circuitry have produced phenomenal processing
power and memory capacity in what seems to be ever-diminishing
package sizes. However, there has been a corresponding trade-off
in that much of the needed protection circuitry has been eliminated
to reduce device real estate. As a result these devices have
now become more susceptible to damage and degradation due to
operating conditions such as excess temperature and power supply
disturbances and fluctuations. |
|
|
| |
If
we look at the basics of our power distribution system we discover
some interesting facts. |
|
|
| |
First
of all the system was originally designed for applications such
as lighting, heating and motors; none of which employ predominantly
sensitive electronics. |
|
|
| |
The
electrical supply authorities regularly catty out certain procedures
to ensure reliable, quality and cost effective supply for these
applications. These measures include generator switching, power
factor correction, load redistribution and fault clearing. All
of these procedures produce some form of power disturbance or
interruption to supply, none of which have any significant effect
on lights, heaters or motors, which, in spite of this, accept
the electrical supply as a reasonable stable source of power. |
|
|
| |
However,
sensitive electronic equipment such as computers perceive these
power line disturbances as potentially hostile to their reliable
and effective operation. |
|
|
| |
While
this is a problem and an area of concern it has been discovered
that the greatest and more common threat to computers and the
like stems from lights, heaters and motors, and a wide range
of other electric and electronic products. |
|
|
| |
In
the majority of cases this equipment is located in the vicinity
of the computer installation, either in the same office, building
or similar geographical location. |
|
|
| |
Most
devices connected to the power cause some form of disturbance
or interruption even if only for a fraction of a second. These
may not be discemable to the human eye, but depending on the
timing, magnitude and rate of accurance, they can cause havoc
with the computer system. One of the most crucial periods occurs
the instant a device is switched on. Many pieces of equipment
act almost like a short circuit for a fraction of a second and
others create extremely high voltages and noise. |
|
|
| |
It
is a practical and physical impossibility for the electricity
supply authority to supply infinite amounts and power and protection
solely to accommodate the power requirements of sensitive electronic
equipment, which only constitutes a very small percentage of
its supply. Hence the need for the consumer to provide suitable
protection. |
|
|
| |
There
are a multitude of interruptions and disturbances, which occur
on the mains supply. Here are some of the more common problems
encountered. |
|
|
| |
Voltage
Fluctuations:
Any gradual change above or below the prescribed 240 volt supply
is classified as a fluctuation. They are caused by heavy load
variations on the system. |
|
|
| |
Voltage
Sags/Surges:
These are rapid changes in voltage that occur for a fraction
of a second and are generally caused by local loads such as
air conditioners, compressors, system overloads; etc. |
|
|
| |
Noise:
This can be defined as any undesired signal superimposed on
the mains supply. It is often referred to as EMI or RH interference.
Noise can be applied directly to the power lines by appliances
or electrical devices. It can also be picked up by the lines
acting as antennae from sources such as radio/N transmitters,
medical equipment, welders, lightning, etc |
|
|
| |
Transients/Spikes/Glitches:
These are very high voltage short term disturbances superimposed
on the AC waveform. They can rise to several times the normal
mains voltage and are caused by motors, SCR controllers, welders,
fluorescent lights, lightning, etc. |
|
|
| |
Loss
of Supply:
This can occur either as a momentary interruption lasting from
a fraction of a second to a few seconds or as a total power
failure. It can be caused by situations such as system faults,
overloads, generator switching, accidents, storms, etc. |
|
|
| |
It
may come as quite a revelation to know that our computers are
subjected to such a hostile power environment. |
|
|
| |
Before
the power can be used to energize logic and memory circuits,
and storage devices within the computer, it must first be processed
by the systems power supply known as the switched mode power
supply, (SMPS). |
|
|
| |
Very
simply this device converts the 50Hz 240 volt mains to a high
frequency in the order of 20-50khz, reduces the voltage and
converts it to the 5 and 12 volt DC levels necessary for computer
operations. The SMPS is the interface and buffer between the
mains and computer hardware and therefore its operating environment
must be kept stable in order to maintain satisfactory operation
of the system. |
|
|
| |
Overseas
research has revealed that the majority of hardware and software
problems can be directly related to mains disturbances which
cannot be tolerated by the SMPA. |
|
|
| |
SO
WHAT HAPPENS TO THE COMPUTER?
All electronic circuits used in computers comprise of digital
and analogue devices that simply process electrical signal voltages.
The 5 volt facility in the switch mode is used for these devices
for the control of all logic functions. The characters which
computers produce are represented by a combination of binary
digits 0 & I which in turn are generated by certain voltage
levels within the 5 volt range. It is therefore not hard to
comprehend that a noise burst, high voltage spike/transient
severe fluctuation or momentary power loss could cause a voltage
change thus confusing the logic, which of course interpret the
change as data. |
|
|
| |
In
most cases disturbances tend to affect software rather than
hardware. Noise can introduce an unwanted bit anywhere in the
processing train or commands could be modified; intermediate
computations may be affected or incorrect memory addresses may
be acknowledged and non-programmed jumps m~, be activated.
Where voltage spikes are significantly high in level and energy
they can cause immediate damage to sensitive electronics or
cause progressive degradation to the pointwhere failure can
occur at any time. Because the switches mode operates at high
frequency it is relatively easy for high frequency noise to
penetrate and cause problems. |
|
|
| |
Although
the switch mode design enables it to cope with a reasonably
wide input voltage range, it nevertheless relies heavily on
a continuous noise free supply of power for trouble free operation.
The unit does not consume a constant level like heaters or lights.
Instead, it draws current in a series of bursts. The peak value
of these current burst are quite high and are used for charging
the power supply capacitors which in tum supply voltage to the
computer. In order for the required charging current to be available,
the mains voltage must suffer no significant distortion. Should
a hea'vy load be switched on, the resultant voltage from on
the mains can prevent the capacitors from receiving their necessary
charge voltge resulting in incorrect voltage levels at the logic
boards and disk drives. |
|
|
| |
This
scenario would be one of the major contributory factors towards
data loss, corruption and hard disk failure in industry today.
Typical loads which cause the problem often exist in or close
to the computers own environment such as industrial machinery,
air conditioners, photocopiers, heaters, cookers, etc. Whilst
a dedicated line can minimise these problems, it is not the
total solution as the switchboard still provides a point of
common coupling thereby enabling disturbances to be reflected
back down the line to the computer. |
|
|
| |
Power
disturbances are a fact of life and we must acknowledge its
existance and take steps to protect both our hardware and software. |
|
|
| |
Electricity
supply authorities across the country acknowledge the problems
and publish brochures and booklets on the subject recommending
that suitable precautions are taken. |
|
|
| |
WHAT
DO WE DO ABOUT IT?
In spite of the problems that haunt the personal computer user,
there are a number of solutions available on the market which
can be used in conjunction with the computer system to eliminate
or minimise the effects caused by power disturbances. |
|
|
| |
Power
Filters:
Filters are designed to provide attenuation of undesirable noise
on the 240 volt waveform. The most effective filters available
incorporate devices which provide protection against high voltage
spikes/transients and lightning surges in addition to RFI/EMI
noise. |
|
|
| |
Line
Conditioners:
These devices are designed to compensate for heavily fluctuating
mains line voltages by providing a well regulated voltage supply
to the load. A well designed line conditioner will also incorporate
a good power filter. The majority of computer equipment today
is very tolerant to mains fluctuations and therefore these devices
are not used to the degree they used to be. |
|
|
| |
Unintentiptibie
Power Supplies (UPS):
There are basically two types of UPS -On-Line and Standby. |
|
|
| |
On-Line
UPS:
With this system the computer is normally energised from the
mains power supply via filter. When a voltage variation occurs,
the load is switched to the integral battery back-up supply.
When the main supply returns to normal the computer load reverts
to mains. |
|
|
| |
Standby
UPS:
With this system the UPS generates its own regulated supply
and ensures the computer is totally isolated from the mains
supply. The battery maintains the output supply without any
switching |
|
|
