Last minute problems…

I noticed something unexpected and unfortunate recently. If the voltage level is above ~ 13.5 Volts suddenly the current ramps up to over 3 Amps! After spending some time probing around I’ve come to conclude that what’s happening is that once the ground to the circuit is cut I am left with what seems to be a short circuit between the +8 V battery terminal and the +13.5 V power cord. Both of these flow into the same path (the battery through a diode), and when there’s a ground reference, this is not a problem and the circuit performs as expected. But when the circuit is turned off, then the short circuit ensues. Anyways, this doesn’t kill the project but what it does mean is that I will leave the overvoltage mosfet switch off for the time being because although it does it’s job, it makes things awkward for the moment. Why? Because as I explained in the last post it already limits the band in which I can recharge, now secondly if I go much above the desired cutoff voltage of 12 Volts then the short circuit occurs. As a result, I must keep the power on whenever the circuit is charging. This isn’t a problem, and in fact is the way that things are supposed to operate. But this unfortunately is a state that I have to ensure with my faulty brain, as opposed to make concrete in the circuit. Who knows when I forget that rule and blow shit up (glad to have added the fuse). One relief is that as long as I have even one module on the bus turned on then all the attached circuits see ground and the problem is avoided. Mostly, it’s a bit of disappointment in what I was hoping was a very elegant and final circuit! The solution I believe is to move the on/off switch to the high side so that no + voltage is on the circuit at all. But what about the mosfet which works better on the low side?…maybe use the n-fet (vs. p-fet) on the high side??.

Another annoyance is the current drive of the Xbees. They are demanding little beasts, and are affecting the circuit in ways I haven’t yet figured out. For example, when the xbee is present, there is a 5 second delay before the device becomes operational after it’s turned on…Whithout it, the circuit becomes operational immediately…why? I don’t know. Maybe because the xbee is tied to the reset pin through a transistor. It also pulls a lot of damn current. And when the device is charging, the little regulators heat up a whole lot because it’s dropping about 9V (from 12V down to 3.3V). Will I pull these at the last minute?

Finally, these damned knock-off motors are tricky buggers. They aren’t retaining anything in eeprom, and reset after power is cut. This could be a tricky issue because all the motors share the same bus, so if they all reset to an ID of 1, then how do I differentiate between them?? Only way would be to keep the xbees and cut the direct communication link between the motors.

Taiya is doing great work. She’s putting together libraries which we can quickly call on functions we always reuse from program to program. Kerry too has been very helpful with assembling the modules. Thanks to both of you. It’s stressful I have to say…there really is too much to do, and too little time. But we just keep on moving.

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