[FTC]: [FTC] Samantha and batteries

[Hagrid]

Originally Posted by pyroslev

To me, the Samantha should be as early in the power chain as possible.

I would need to check, but I think this is exactly the configuration that would be of concern with respect to high ground current via NXT in the case a damaged ground connection.

Have I got this wrong?

[pyroslev]

Could you pose it in a situation on the Field, a scenario. I think I'm following you but I think better with a detailed scenario. (Mostly because it's not a week into the season and I need to retrain my FTA brain cells.)

[Hagrid]

[Sorry for the funkiness with the pictures - trying to transplant drawings from a smartphone]

Consider this arrangement...


The black lines are the ground connections between the components. The yellow approximates the current flowing in the wire. The wider the yellow, the higher the current.

If the ground connection ONLY is broken between the battery and the motor controller, you will have this...



The NXT cable and USB cable (and Samantha power cable) have to handle the return current for all the DC motors/servos powered from the battery.

If the Samantha is powered from the end of the Tetrix controller chain, it will be less of an issue as a broken connection is likely to take out the Samantha as well. But this creates the possibility for differences in the ground potential that *could* produce other problems.

With Matrix there is no power-out connection for daisy chaining: one controller, one battery.

I found the Samantha alternate wiring for Matrix document which highlighted the issue back in 2012.

[pyroslev]

If I were wiring up a Tetrix Robot, I'd still use the Samantha Alternate Method. I would put it after the Switch so it's the first item powered before any Controllers or a Power Distro Board. Instead of crimps, I would use Anderson Powerpoles for quick connect swapping. Teams using this setup have been able to drive with 4.018 volts.

I've seen many Samantha's in this configuration. The only issue I've seen related to electrical was where a First Gen Samantha's ESD protection was broken and led to power cycling. If you use the correct guage wire, proper connections and at your discretion and legality per the rules a fuse, then your Samantha should have power, be protected and be ok.

If the grounds get that broken up, then you're involved in a match were everything has gone wrong. I've only seen this happen once where a team burnt out a motor controller and two motors. Extreme ESD played a factor here. In fact, If you have not already, I would say read the document Addressing NXT Lockups. Once you combine all the information in the Wiring, Samantha and Lockup Documents, you should be able go ahead and wire up the robot.