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Tale of interest no.3
The computer monitor that burnt down a school:
| The following story should
users something to think about.
Late last year (2005), a local government department was burnt to the ground in the early hours of the morning.
The local "hoon" element was at first thought to be responsible.
The subsequent investigation revealed that a computer monitor was to blame.
It was left on overnight.
Who admits to leaving their monitor on?
I know I sometimes leave mine on!
Why should it matter if it is left on?
Well, it shouldn't in theory, but in reality, it is a very risky practice,, especially with older CRT type monitors.
There are lots of them about still and indeed, some people prefer them to the LCD types.
The vast majority are made using the cheapest assembly techniques and components.
As they operate with high voltages, many major components eventually break down.
Even in "standby" mode, they are partially active.
It seems that proper fire retardant materials are not generally used in monitor construction either.
I have often had to repair these beasts, with their large burnt holes in the circuit boards from dry joints arcing or from overheated components.
Some of them looked disturbingly close to catching fire!
The circuit boards are usually made from phenolic resin which will burn quite readily.
What is the solution?
Switch them off at the power point when not in use!
Seems simple enough, but it is easily forgotten.
When the screen appears blank it is easy to fool oneself that the monitor is off.
I go further than that and switch everything off at the mains when I'm finished with the computer for the day.....
Well, most times!
Most computers have an IEC mains power outlet on the back, specifically intended for the monitor to be plugged into.
In the old days, computers had proper power switches on them which would remove the power from this outlet when switched off.
Not so today!
All the modern "power switch" does now is put the computer into standby mode, or reactivate the circuits into full power mode when needed.
The power outlet at the back of the computer is "live" all the time the mains power point switch is left on!
After forgetting to switch my computer off at the power point one day, I finally decided to do something about this problem!
A simple circuit was designed to control the power to the monitor.
I based the design on a meaty triac with an isolated tab, with a zero crossing triac-optocoupler to drive it.
This allows plenty of isolation between the "mains" and the 5 Volt logic circuitry controlling it.
There are plenty of leads within the computer power supply with 5 Volts on them.
The red ones for the disk drives, etc. are the easiest to access.
If you don't want to void the warranty on a newer computer, then the control unit can be powered and assembled external to the computer case using a spare USB port.
When the computer is active, the 5 Volt supply to the disk drives, etc. will be present, as will the power to the USB "green" ports.
This is the ideal way to control the monitor's power supply.
The idea can even be taken further and used to switch on all of the other peripheral devices connected to the computer.
There seems to be a disturbing lack of proper power switches in modern equipment.
I found that the power supplies in my computers pulsed on for a fraction of a second when first switched on at the mains.
This was an annoying complication but easily overcome.
The good old 555 timer IC was put to good use to provide a slight delay when the computer is first switched on.
(please see editors note 1)
The "Power Down" as I've called it, worked like a charm and has given me peace of mind.
I have installed it within the power supply on one computer which only has 2 USB ports.
It is an old computer so the power supply is well and truly out of warranty!
The other computer is newer (many thanks to AES Computers) and has 4 USB ports, so I built an external unit for it.
The external unit is housed in a UB3 "zippy" box 130mm x 67mm x 44mm .... Black of course!
Anyone interested in one? ...... Now if I could only remember to always switch the computer off at the mains!
Till next time.
this article belong to "Rick's
Workbench" in Tasmania
|(editors note 1) :
Some computer power supplies have long delays from proper mains turn on, till being able to turn on from their normal
front power switches. This delay in some circumstances can be as long as 30 seconds.
Rare, but not unknown.
During this time they sometimes turn themselves off again, (and sometimes on again).
We do not recommend attempting mains powered projects without
The articles from "Rick's Workbench" are for entertaining reading only.
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Perhaps planets should be at least 4000 Kilometres in diameter (that would include all the known inner planets of our solar system),
and should have been molten or fluid at some time in their lifetime to reshape into something more spherical.
Whether their orbit planes are close to the average for that system should not really matter, as captured planets and those that have
had their paths heavily disrupted by collisions or near collisions can still be planets nevertheless.
The furthest outer "planets" of our solar system have diameters smaller than 4000 Kilometres and are more rock like.
This would remove Pluto and possibly any other later discovered smaller objects from being classified as planets.
Planetoids? Now that's another subject altogether.