Re: Back-up battery charging circuit from IFI
 

We have determined that there are too many things wrong with the circuit as given from IFI to use on our robot at competition. We have a modified circuit that works as it should with all the proper protection, but since it is not the IFI circuit, we won't be using it in Atlanta. It will just be used on our practice field here...

Re: Back-up battery charging circuit from IFI
 

You are completely right. The LM317T is set to provide 8.26V. D1 has a forward voltage of at least 0.66V, That means it will only conduct when the voltage is 8.92V, but the zenner diode will limit that to 8.7V. The entire LM317T part of the circuit does nothing.

The way the LM317T is used in the schematic, it should pull down the voltage on the output, but it has to do that through the 1.24KOhm resistor. It wasn't designed for that.

The only way to use this circuit is to exclude R1, R4,D1, and D2, and connect the "+" output to VOUT. This should provide approximately 8.26V to the backup battery.

Re: Back-up battery charging circuit from IFI
 

Actually, better would be to put the diode at the 12 volt input. The diode exhibits an approximately 0.7 volt drop across the PN junction, which will reduce the circuit's output voltage if put in the output. The input is sufficiently high that the diode drop will have no impact.

On the other hand, I agree with Don Wright and others, I just do not like the circuit, and although we used it during testing, it never went into the crate.

Don

Re: Back-up battery charging circuit from IFI
 

I'm not sure if this statement is correct. I'm going to attempt to describe what I believe is going on and the CD community can let me know if I'm off in left field. :)

When the backup battery is drained (< 8.3 volts) the very low impedance of the battery will pull the output voltage down (and the 8.7v zener is not active). At this time the lowest impedance path would be through the 22 ohm resistor from the 8.3 volt regulated supply. This will charge the battery (and supply the RC).

Once the battery is charged to a voltage higher than 8.3 volts the voltage potential will no longer draw from the regulated supply and then current will flow from the 12 volt source with the current limited by the 43 ohm resistor.

This should create a staged charger for the backup battery as well as supply the RC.

Edit: I would like to see IFI add some protection to prevent any back feeding from the backup battery when the 12 volt supply is disconnected. (Or just diode OR the 12 volt and battery sources internally to the RC).

Re: Back-up battery charging circuit from IFI
 

We used this circuit (with minor modifications) in our robot, and never once had a voltage-related issue with the camera, nor did we change the backup battery. It was a nice reprieve from previous years. The circuit does look a bit... odd... and pieces seem to be redundant, but there is a reason for that: FIRST likes redundancy a lot. If the programmable regulator ever blows, or if there is a current spike, or a voltage spike, or any one of thousands of possibilities, the circuit won't cause a catastrophic failure. In short, the circuit works as advertised, and I'll be advocating its return for next year.
Oh, and no heat problems surfaced here either. We did, however, place a rather large resistor in the line between the charger and the circuit breaker - the darn backup battery kept the Vics on with no 12v source. Having a resistor also kept the circuit from blowing itself out in case of a heat-related issue.
That's all. It's got my vote.

Sparks