|
Re: Voltage on Frame
In an eariler post, I had said:
Quote:
If your frame is indeed floating (as it should be) the voltage is UNDEFINED and not "zero" as it has no reference. It could just as easily be +100V or - 500V... Is is just not defined... In general, it will usually come to rest between 0V and 12V due to leakage resistance of the wires.
Take Ken's first suggestion to heart and use a 100K resistor to ground and measure. Then 100K to +12 and measure. If you get 0V in the first case and 12V in the second case, you have no problem.
We had an inspector at NE about 4 years ago who knew his stuff. He asked the students to make that exact measurement (frame to ground) and then asked them to explain where the voltage came from. It got them to thinking about Ohm's Law. I later used an extra 100K pot to prove to the students that there was no short.
|
A former alumnus from that year's team responded via private message:
Quote:
|
...You explained it to me after the inspector drilled me for an answer... but all i can remember is induction.... maybe you can shed some light on this issue for them (and me!)...
|
To which I replied:
Quote:
Let me pose this in the form of a question. What is the voltage potential between a fragment of Eta Carinae and Haley's comet?
Don't know the answer? Neither do I. If the systems are truly isolated, the answer is undefined. In the real world, nothing is undefined but the coupling is so weak that the voltage could be trillions of volts. The probability that the voltage is zero is, for all purposes, zero.
In our robot, the DC "coupling" due to the leakage resistance of the system is quite weak (on the order of 10s to 100s of megohms so I would expect the DC voltage to be between 0V and 12V. Note also that the act of measuring with a voltmeter which is not ideal (less than infinite ohms) will change what you read (Heisenberg is alive and well).
Induction would likely have a more measurable effect on the AC coupling. If you measure AC volts, I can almost guarantee that the voltage will not be zero either.
Now to the 100K resistor: Its effect is much stronger than the weak coupling of the leakage resistance or of inducted fields. When I connect a 100K between ground and frame, I expect near zero volts unless there is indeed a short in the system. Likewise, a 100K between frame and +12 will raise the frame to a 12V potential unless there is indeed a short in the system.
The 100K resistor “defines” the undefined system and has a impedance low enough such that the non-ideal aspects of the meter’s input impedance is reduced dramatically.
Does this make more sense?
|
I hope this helps others understand the phenomenon better.
__________________
Mike Betts
Alumnus, Team 3518, Panthrobots, 2011
Alumnus, Team 177, Bobcat Robotics, 1995 - 2010
LRI, Connecticut Regional, 2007-2010
LRI, WPI Regional, 2009 - 2010
RI, South Florida Regional, 2012 - 2013
As easy as 355/113...
|