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Unread 08-08-2013, 15:58
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Teaching Teachers to Teach Tech
AKA: Jason Brett
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Re: Battery powered raspberry pi

Quote:
Originally Posted by techhelpbb View Post
...Well outside of the rules I can think of several reasons to limit the capacitance between the power supply and something like the Raspberry Pi.

The current that a high value capacitor will draw when deeply discharged needs to be limited. If that current is drawn through a 7805 regulator as discussed earlier in the topic it could cause some damage unless there is at least a series resistance between it and the regulator output. ....
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.

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!

The main point that I wanted to make, however, was that a capacitor bank would be legal, so long as it was charged by the robot battery.



Jason