Quote:
Originally Posted by Kevin Sevcik
Alan,
I personally think the actual likelihood of a good modern bench supply generating entrainment with the PD board is very low, but I felt I should recount what I'd heard.
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Kevin -
You are correct, using a benchtop power supply is not recommended. However, it is safe. Some revisions that never saw the light of day would become unhappy when fed with a wimpy power supply. The revs that are shipping can survive with any input supply with a nominal voltage between -20 and 20V with any supply impedence, and can survive any load impedence that does not generate power. Don't expect it to work over this range: the range of ~3V to -20V is guaranteed not to work.
It is a "feel free to do it, it just might not work" situation. The reason for this is that the switching power supplies effectively have a "negative resistance". For a given power out, lower voltage in requires higher amperage in. This can interact with the power supply's internal protection, creating two stable points:
1) The good one, in which you are getting 12V and low current from the bench.
2) The bad one, in which you have hit the current limit of the supply, and have constant current at a low voltage. If the power out of this isn't beefy enough, it will latch in this state and won't work.
Nothing will be damaged.
A kit battery doesn't really have a state 2, because it can push *way* more current that the power supplies could ever want or use.
Also, the momentary loads of starting a CIM or other big motor can easily brownout most bench top supplies. You will (briefly) take 10-100 amps from the supply. A supply that can handle that is impressive. I'm willing to bet that most supplies available to teams won't handle it. You will trip the brownout protection in the PD and DSC, which will disable the drive motors momentarily (in order to protect the cRIO and Wireless). When they are disabled, the gargantu-load of the motors will go away, which will allow the voltage to rise, which will enable the motors, which will repeat the cycle. Again,
nothing is damaged, but it isn't working.
Neither of these situations occur with charged a kit battery. However, a battery in its death throes will exhibit the limit-cycle behavior described in the preceding paragraph. Don't worry, this happens after the battery has been drained beyond usability. In my testing, this occurred after the battery was unable to provide enough power to move a kitbot.
Long story short:
Don't use a bench top supply.
If you do use a bench top supply, don't complain that it doesn't work.
Quote:
Originally Posted by Kevin Sevcik
Ribbon cables work well as the digital sidecar cables. It's possible to use ribbon cables to move the analog breakouts off the top of the cRIO, though they're obviously unshielded and more noise prone.
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On the other hand, the cables that connect the sensor to the breakout aren't shielded either. Yes, adding a ribbon cable adds more opportunity for noise to get in. However, it might also reduce the length of the 3 pin connectors.
6 of one, half dozen of the other. For some teams, keeping it attached the cRIO makes more sense. For other teams, remoting it with a cable will make more sense. Some teams might even want to use a shielded cable. You (generic You meaning each team) won't know until you are designing the specifics of your robot. I won't pretend to know what a team needs better than they do, so I'm happy that these connectors provide more options.
However, you probably don't want to use a ribbon cable (unless you do).
Lastly, please review the rules before selecting an alternative cable. I am expecting that the connection to the Side Cars will be required to have all 37 pins connected straight thru. We'll find out soon!