Quote:
Originally Posted by WizenedEE
As I said, our frame has a little over 1 megaohm to both leads -- it is not completely isolated, nor does it have to be. I was trying to point out that voltage means nothing, it's about resistance.
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Opinion from a 10 year 3 events/yr RI & 15 year FIRST Mentor:
NOT so! the voltage on the frame is stored in the frames capacitance.
Capacitance (Joules) is like a charged battery with a pretty low internal resistance ready to dump its energy to anything that will take it independent of resistance that charged it. (R affects time to charge it to 7v or whatever)
The charge time is ~3xRC. your R is 2x1meg in parallel= 500K C= TBD but will be significant charge on such large metal structure so as to blow out or damage (cause indeterminate intermittent) to CMOS & TTL inputs and maybe even outputs (even protected TBD)!
there is surely potential to upset digital logic:
Future problem scenario: robot reset via intermittent pinched wire contact to 7V chassis! Of course it happens near beginning of playoff match or crucial qual. Murphy is mean!
How many resets, raspy control scenarios can your team endure and still win?
>10K per se means little; LRI likely to give provisional pass but wise to suggest team keep looking for the offending continuity and clear it, for team & alliance performance benefit.
More importantly teams should view this as a symptom thus give it priority to avoid future unexplainables: painful resets, jerky response, unhappy partners
I am sure I could justify Risolation being >500K or even 1M due to chassis charge stored and its ability to upset logic under many circumstances of vibration, impact, etc.
If R isn't infinity, something is wrong. There are no insulators used on the robot wherein it is normal for chassis to either battery terminal to be <infinity
Some coatings & anodization are normally insulators may exhibit diode behavior polarity sensitive R one direction high, other very low (Copper oxide rectifiers = diode used 50 yrs ago)
(so depending on your meter..infinity = >1m, >10M >20M >40M is normal)
see my earlier post this thread for 2011 team 207 Rbatt-to-chassis= 200k-300K offender RS775 <1ohm measured at motor
Good luck.. Go for infinity!!!