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Unread 12-02-2009, 11:12
Unsung FIRST Hero
Al Skierkiewicz Al Skierkiewicz is offline
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AKA: Big Al WFFA 2005
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Re: Solving static electricity problems by grounding the frame

OK,
Let's hit these one at a time.
Computer cases and most home appliances are at ground/earth not power supply common. No current flows in the case unless there has been a catastrophic failure and then the power breaker or fuse trips. This is an attempt to protect the user from death, not prevent failure in the components.
Cars do have static buildup but they are huge and produce a rather nice capacitively coupled connection to earth through that nice plate and conductive additives in the tires. Static sensitive components in your car are designed to operate in this environment through the use of shielding, isolated power supplies and internal protection. Ask any car electrical engineer and they will tell you that the electrical environment on a modern day vehicle is one of the worst environments they have to contend with.
Auto body common power supplies are not ground nor are they earth they are power supply common. No current flows between the car battery and the ground beneath the car. If the addition of one wire would add $.025 cents to the cost of a car, the manufacturer making 1 million cars would be spending an additional $250,000 a year per wire.
Metal bumpers are the exception rather than the norm these days. You will find them on some vehicles, mostly SUV or truck designs. Again you have to visualize the current flow. Where does it go. Does current flow if the positive lead of one battery is connected to the body of another car? No, there is no complete circuit. If the two bodies come together than the path is through the positive lead of the first battery, through the second car body, then through the common body contact and then through the first car body and back to the first battery. Where the current flows is anybody's guess and it is only limited by the resistance of the connections in between. There is a real reason why firefighters cut the battery leads when they first appear on an accident scene. High curent translates to sparks, heat, and explosive batteries.
Grounding the case of the Crio to the robot frame will not prevent ESD. Again the issue is with the discharge path, huge currents develop huge voltage potentials across resistance. Think about this...how much voltage is developed by 1000 amps flowing through a one ohm resistor? Ohm's Law says V=I*R. V=1000*1=1000 volts.
As I have discussed earlier, First wants it's teams to be able to play, as long as they want or can. Do you want to be dead during your finals match on Einstein because of an electrical mishap in a simple robot collision? Do you want your Crio to smoke when you have no money left to buy a new one? Do you want your students to watch in horror as smoke and flame rises from their robot or that of one of their alliance partners or both partners?
__________________
Good Luck All. Learn something new, everyday!
Al
WB9UVJ
www.wildstang.org
________________________
Storming the Tower since 1996.
 


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