Weird issue with the power distribution block

[Al Skierkiewicz]

When you push a robot, the motors begin to act like a generator. The speed controllers (both Victors and Jaguars) use power FETs that have a diode internal to the devices. As you generate current in the motors, that current forward biases the diodes in the FETs and since they are in a bridge configuration, power is back fed to the electrical system. There is no way you can generate enough current to back feed a full 12 volts to the system, but you can generate enough to light some LEDs and make it look like you have an electrical problem. All of this is normal.

[EricVanWyk]

Originally Posted by Al Skierkiewicz

When you push a robot, the motors begin to act like a generator. The speed controllers (both Victors and Jaguars) use power FETs that have a diode internal to the devices. As you generate current in the motors, that current forward biases the diodes in the FETs and since they are in a bridge configuration, power is back fed to the electrical system. There is no way you can generate enough current to back feed a full 12 volts to the system, but you can generate enough to light some LEDs and make it look like you have an electrical problem. All of this is normal.

Actually, it is possible to generate more than 12V - you just have to back drive the motors faster than the 12V speed. It is hard to do this with drive motors, but possible if you have a manipulator that is heavily geared down or a bunch of students.

Another thing to consider is that when the speed controllers are being back driven through their diodes, their ability to handle current goes way down. These diodes are usually about a volt (they are worse than intentional diodes), where as when they are turned on they are usually a couple milliohms. Consider that at 10A you are dissipating 10 W instead of .1W in those FETs, and that the fan isn't on.

I have seen components destroyed this way, but it took a lot of physical effort - several students pushing a robot geared for torque (not speed) at running speed to see what happens. They got sweaty, the electronics got unhappy.

In conclusion, it is normal but not recommended.

[Russ Beavis]

Simple physics - where does the energy go? It's certainly not recharging the battery since the 120A main breaker is open. So... something is going to get warm and/or generate light (kinda like a different type of "warm" actually). At some power level (energy per unit time), that extra heat will cause a failure.

Try pushing a robot that's powered on (with actuators disabled for obvious safety reasons). Then try pushing a robot with the main breaker open. Any difference? There probably will be since more of the energy will be used to recharge the battery.

Good lessons to learn. But, to Eric's point, be gentle to avoid harming your robot. And never play around with enabled robots.

Russ

[Al Skierkiewicz]

Just so that we don't confuse anyone, the diodes are speced at 1V@20 amps and 1.25V@40 amps each when forward biased. So even if you could drive the motors at full RPM (highly doubtful) then the diodes would drop at least 2 volts in the speed controller. Remember that the bridge would have two diodes in series with the power supply input pins plus the 0.0005 ohm resistor in the Jaguar negative lead plus the wiring resistance. It is unlikely that you could provide enough power to light up the power convertors for the Crio but you might. Just not long enough for any of the system to boot.

[sparkscarz]

nice
!

[ZInventor]

Originally Posted by Al Skierkiewicz

When you push a robot, the motors begin to act like a generator. The speed controllers (both Victors and Jaguars) use power FETs that have a diode internal to the devices. As you generate current in the motors, that current forward biases the diodes in the FETs and since they are in a bridge configuration, power is back fed to the electrical system. There is no way you can generate enough current to back feed a full 12 volts to the system, but you can generate enough to light some LEDs and make it look like you have an electrical problem. All of this is normal.

odd, we've gotten ALL SYSTEMS to boot up while pushing the bot!

by that i mean, we had connection between the bot and DS without the battery being plugged in! the station read 12.24 volts at peak (which lasted about a minute)

needless to say, i was exhausted after pushing the robot that long.

-Z