2005 Electricals or 2006?

[Al Skierkiewicz]

Originally Posted by Gary Dillard

We used the 2005 at my request - clean, compact, light. I loved it when it came out. Didn't use the diagnostics.

I have a couple questions, however:

2) Likewise, isn't ground ground?

Gary, Gary, Gary. This isn't ground, although many people call it that (ground is what you walk on). This is the return side of the battery and should be treated just like the supply side.
I would modify the IFI statement by saying power to the RC should be from the 1 or 2 position and then follow the IFI recomendations for any additional outputs. You can visualize the current path within the box from where the input terminals are located. Since everything has some resistance, high current items will draw current through this resistance causing a voltage drop across the resistance. The high current outputs (40 amp) are above the input terminal and will drop voltage from the terminal to each output while minimally affecting the lower current outputs. Anything that draws current from the lower current outputs will also drop voltage from the input terminal to the output. That is why I recommend that the RC be powered from the position that will have the least voltage drop/closest to the input terminal. If you look at the board you will see that the return terminals are also numbered so that they roughly correspond to the current path the supply side takes to minimize loss. The RC is the most voltage sensitive device in our system, cutting out when the input voltage falls below about 8 volts. By putting the power closest to the battery inputs allows a minimal drop during current spikes and lets you play longer and harder.
As to the wire length question, the breaker panel has all robot current concentrated in one device while wiring is carrying current to individual devices. The amount of current that flows through the panel is the same current flowing through the battery and at full charge that could be well over 400 amps. Ohm's Law cuts in where V=I*R, so if you keep R constant and increase I the voltage also increases. #10 wire is .001 ohms per foot but I bet the panel is about one tenth of that or less.

[Gary Dillard]

Originally Posted by Al Skierkiewicz

Gary, Gary, Gary.....

Dang, a mechanical engineer trying to be an electrical engineer gets busted for his ignorance. V=I*R? I vaguely remember that from college; must have been that circuits class I slept through.

Better to keep mouth closed and be thought a fool.....

Thanks for the info guys

<note to self - have electrical team rewire robot at Palmetto>

[Gary Dillard]

Originally Posted by Gary Dillard

Better to keep mouth closed and be thought a fool.....

<I need a bigger mouth or a smaller foot; arguing electricity with Al Skierkiewicz - Dillard's a moron (please don't make that a spotlight quote>

OK Al, I've been thinking about this and I disagree with you - I'll have to draw up a circuit diagram tomorrow if I didn't explain it well here - although your point about current spikes may be correct (remember M.E.'s think in steady state).

First item is a nit - If you look at the IFI panel, AND all of their documentation, they refer to the return side of the battery as Ground. It's marked ground 2 places on the panel, and each terminal is G-something.

Breaker Panel Reference Guide Section 3, paragraph 2: Connect Negative(-) side of the battery or Ground to the center post contact labeled GND.

Section 6, paragraph 1: The return path or Ground for these outputs are located at the bottom of the unit and are labeled G1 through G22.

Second, your point about the resistance of the panel being one tenth that of the wiring is exactly what I was referring to in my first question. If my load (motor/controller) is located on the ground side of the board (bottom left), here are the 2 extreme cases for routing:

(1) Big honking wire from battery terminal negative to ground lug, 1 inch of board from ground lug to ground blade#1, 6 inches of wire from ground blade #1 to Victor, N feet of wire to load, N feet of wire back to Victor, 4 inches of wire from Victor to Output blade #21, 6 inches of board from blade #21 to positive lug, big honking wire from positive lug to battery terminal positive.

(2) Big honking wire from battery terminal negative to ground lug, 1 inch of board from ground lug to ground blade#1, 6 inches of wire from ground blade #1 to Victor, N feet of wire to load, N feet of wire back to Victor, 8 inches of wire from Victor to Output blade #1, 2 inches of board from blade #1 to positive lug, big honking wire from positive lug to battery terminal positive.

Both are series circuits, so the total circuit resistance is simple addition. The only difference in the circuits between the 2 is trading 4 inches of wire in (2) for 4 inches of board in (1). By your numbers that's a difference of .3 milliohms, with circuit (1) - my circuit - having lower resistance. So, for a fixed voltage source (which we have) I will have more current (more power) to my load, or looking at it another way I will have an additional .027 mV in available voltage for a 30A circuit to drive my load that is taken up in by loss in your circuit.

Third - you said there are 400 Amps of current running through the board, same as the battery? What is the 120 Amp breaker for then?

Or should I just take my multimeter and go home?

[steven114]

While the main circuit breaker is rated for 120A, it takes a significant amount of time for it to trip at that current. I seem to remember someone quoting 8 seconds at 120A, 4 at 240A, etc. (although that is from memory, I'm sure there's a curve somewhere that has the exact speficiations)

[Al Skierkiewicz]

Gary,
You are right on in both of your illustrations. The one thing you are leaving out is that fact that there is other current flowing through the conductors (supply and return) on the circuit breaker panel. If you were to add two FP motors at position 1 and 3, then add a Globe to 5 and all of the solenoids to 7 and finally connected the RC power to 9, (extreme case but I have seen teams do this.) then you have the ability to max out current in stall to close to 200 amps for these loads. Taking the current through the resistance of the board on just the supply side, then the loss becomes 200 x .0005 ohms (guessing on the actual resistance) for a voltage drop of 0.1 volts. The drop is very small I know. But add to that the internal resistance of the battery (.011 ohms at full charge) current draw through the #6 (.0005ohm/ft.), current in a few Chalupa motors and the resistance in the connectors, terminals, breakers, etc. and the required 8 volts to the RC becomes an issue. After a minute or so of competition pushing, it is easy to drop the battery and added losses to the point where the RC is seeing only 8-9 volts. At that point I want every 0.1 volt of margin I can get my hands on. I make this recommendation because I want all my alliance partners to have the best power margin available.
As to the term "ground" it is unfortunate to use this term on any of our electrical system. It's use in other areas implies the the return side current flows through a nearly lossless conductor, i.e. chassis or metal common. Since it is not legal for any electrical tie to robot frame, "supply return" just seems more appropriate and descriptive. The use of "ground" has unfortunately caused many teams to ingnore the losses when troubleshooting electrical problems such as the inability to reach free speed or output power on kit motors.

[Al Skierkiewicz]

Originally Posted by steven114

While the main circuit breaker is rated for 120A, it takes a significant amount of time for it to trip at that current. I seem to remember someone quoting 8 seconds at 120A, 4 at 240A, etc. (although that is from memory, I'm sure there's a curve somewhere that has the exact speficiations)

Steven,
The circuit breaker has an infinite trip time at 120 amps. It can sustain a 600% overcurrent for a few seconds and increasing times for current less than that. However, since this is a temperature sensitive device, combinations of current that increase the temperature also reduce the breaker trip point. A hot breaker may not be able to handle even the 120 amps, going down to 50% at 200 degrees. The data sheet is available in the FIRST documentation under Section 5 Robot specs.