Saturday, November 4, 2006

Switch switch switch

the wall socket connected to the ...extension board and extension board connected to the.... monitor....
and then there are switches!!
theres one on the wall, theres one on the extension board and then theres one on the monitor...
(the switch on the monitor is a simple microswitch which turns on the high voltage section via a bigger (more power handling capacity!) solid state switch - DP904C (POWER IC PWM Samsung Power Switch ))

So whats the big deal about the switches??... yeah , yeah you have heard many people tell you that they are the most under respected components.. and they are the most important... but simply said.. they have LIFE!...

Or in more technical words, "Mean Time Between Failure" ... The datasheets (the technical documents provided by the guys who make 'em) specify these numbers in number of cycles... ie they specify the number of time you can switch them on and off before they fail... of course these numbers are just a rough estimates...
and they probably don't factor the varied load conditions that the switch might experience in its life time..
so some switches give way earlier than others... while some last longer...
Common wall switches in humid places (Mumbai) get screwed beause of the corrosion of their terminals.. while the same ones in a dry place with extreme climate (Delhi) get screwed due to daily cycles of extreme temperatures (the plastic starts to crack)...
So what the hell do you do to avoid any inconvenience?.. simple:
1) connect many switches in series, some of these switches will be the ones inside and accessible from the front panels of many instruments.... use those ones in the chain most frequently which can be easily replaced... keep the other switches in the chain always on..
2) if there are more than one easily replaceable switches in the chain... use both equally to get double the MTBF..

so heres an example.. consider my computer system.. or rather just the monitor... there is a switch on the wall and then one on the power strip and then one on the monitor..
I used to use the switch on the power strip as the "main" switch... ie this was the first one to be switch on or off and was responsible for cutting of the flow of power to any equipment. What i mean to say is that i used to tun on the power to the system using only this switch while all other switches (one on the wall and computer) remained on. Consequently the electric arc that formed when i used the switch to cut the power eventually got the best of it and it gave way.. i had to open the power strip and i soldered thick wires across the switch effectly isolating it and shorting it out. The power strip was now permanently on! Now I had to use some other switch in the series to turn on and off the power to the system... The wall socket switch was it next!... but during all this time i used to use the monitors front panel switch to turn it on and off whenever i had to leave my computer un attended (while downloading, burning dvds or just going to the loo).. its a good idea to turn off the monitor while away to cut the power consumption.. monitors, especially CRTs are the main power consumers in the computer system. But turning off the monitor physically is not a good idea, because you are using up the precious number of cycles between failures of a switch which is in accessible for you to replace easily.. and on top of that its a semiconductor switch...not easy to obtain and solder back. So what do you do?? just set your monitor to blank out after 1 minute through the power options page in control panel. I suspect that virtually turns off the semiconductor switch; but not fully... its a PWM switch so probably the duty cycle goes down and it doesn't have to switch large currents instantaneously to do so. So in this way we probably preserve the life of those inaccessible switches. (I screwed my DP904C inside my Samsung Syncmaster 793s before i learnt this lesson fully!)

As for using the wall switch, well I will use that one to switch the mains power on and off to my system... arcing during turning off and humidity will probably get it long before its estimated MTBF .. but then what else have i got two spares for?... and plus i can easily replace those myself... heck ! even my grandma can replace those!.. you dont need to be an engineer for that!

so all contacts arc when seperated under loaded conditions (ie you see a spark when you turn off a switch)
and its this sparking that can cause whole load of damage,
Go thru the following video to see how bad and mean an electric arc could be. (go thru the rest of the page too, its quite informative!)

Talking about life and switches, all electronic components have the following characteristic life curves:

taken from:

These are called the bathtub curves... because they resemble one!
I believe humans and many other living/non-living things follow this curve.. When we are young there many factors which can cause our lives to terminate. Barring attacks by predators, disease or non-natural accidents, SIDS (Sudden Infant Death Syndrome)
seems to be a major cause of child death. It seems SIDS is like the having your brand new computer failing with the first week of its purchase... (I have heard the story of brand new computers failing the very next day many a times.. i have one of such experiences myself!)
In case of humans, there isnt much you can do, atleast not yet, but in case of electronic components, there is something called a burn in test, wherein a manufacturer operates each products that rolls out of its assembly line for a few hours at a testing facility and catch as many infant mortalities as it can. The thought is that since the device failure probability is very high during the start of its life, its better to have the device failed at the manufacturers end than at the customers end so as to save time and money wasted in transo=porting a "to be" defective piece of equipment to the customer and back; not to forget the lost goodwill of the company!

See page 909-910 of Digital Design - Principles and Practices by John F Wakerly (3rd Edition updated) for more on this bathtub curve!