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Originally Posted by dtengineering
Good point. I didn't really explain my circuit well. Putting the capacitors downstream of a 7805 would definitely present the problems you have illustrated. Rather what I meant was that I'd have a diode (a fairly skookum one) coming off +12, leading to a capacitor bank. Upon start-up the caps would charge rapidly. The diode might exceed it's rated specs for constant current flow, but could likely handle the brief burst of current needed to charge the caps. It would also keep the capacitor bank from being "drawn down" when the main battery voltage slipped below the capacitor bank voltage.
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That works fine as long as the capacitors in question are capable of withstanding 15V. The super capacitors I linked earlier would have to have at least 7 in series to make that work. Assuming a voltage rating of 2.5V to 2.7V.
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I'd have the voltage regulator (either switched or linear) run off the 12V capacitor bank. The capacitor bank would often be drawn down below 12V, but every time you stopped the motors and the battery voltage went back up again it would recharge. So long as the caps stayed above about six volts, the Pi would be fine.
I'd also put a capacitor across the 5V leads on the output of this circuit, of course, just to deal with any ripple from the regulator.
The main reason I'd tend to not use the caps downstream of the regulator as my "backup supply" is that they would drop below 5V very rapidly as they would only be charged to 5V to begin with!
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Ideally I agree you'd want to regulate post storage as well but the issue with that is the Raspberry Pi in particular out of the box have RG1/RG2 post regulators on the board. I just used a 7805 as an example it could have been a 7809 or a 7812 or an LM317. With just a linear regulator (7805) outputing 5V 2 2.7V max super capacitors is possible but of course there is the in-rush current to consider so it would be a bad idea. One of the supplies on the Raspberry Pi produces 3.3V and the other 5V. The on-board 3.3V linear regulator is by default using up the extra energy as heat anyway (some people replace that for just that reason). You can operate a Raspberry Pi down to 4.75V usually but that's pushing it.
If your capacitors are not super capacitors or have really high capacitance then this becomes no issue. There are plenty of capacitors in the 25V range (just in case) you could put on the robot like that. If the robot is not really drained a short ride through should be fine. However because you have an upper limit (cost wise / design wise) on the amount of capacitance that can be used like this there is a limit to how much reserve you can really store. So if there's a serious protracted draw down on the robot battery this might not be enough.
Plus as I said I've heard people complain they were asked to remove largish capacitors from robots during inspection.
So I am in the camp that if there's an issue here let FIRST officially settle it.