Software
So finally I’ve been working on the schematic for the power supply. In an earlier post I mentioned that I will be using Eagle PCB design software from Cadsoft USA. They offer a free version that is board size limited to 4 inches by 3.2 inches and limited to 2 layers. You are also limited to 1 schematic sheet. Other than that you can make complex designs. I have the full professional version, so I can make bigger boards. Since this is designed for a Mini Sumo robot and is meant to attach to the Parallax SumoBot chassis, the board dimensions are going to be 3 inches wide by 3.3 inches long. This will fit within the limits of the free version of Eagle. The one issue will be that the schematic is spread out over 3 pages. Making the schematic compatible with the free version of Eagle will take some work, but when the files are available from my download page they will work. Now on to the power supply.
Part 1: Design
Starting at the beginning, after applying power to the board, we will need some reverse power protection. Several designs will show a 1N5817 or 1N5819 Schottky diode in series with the input terminal of the voltage regulator. The anode side of the diode faces power coming in, and the cathode faces towards the unregulated input of the regulator. This works well, but for battery applications we run into an issue. The diode will drop about 0.35 volts across it, this is on top of the regulator’s minimum input voltage requirements. So the total minimum input voltage could be as high as 7.5 volts. A 6 cell NIMH battery pack might have enough initial voltage to power the circuit, but don’t rely on it. To use Alkaline batteries you would need 5 cells or more. How about Li Po? You might get away with a 2 cell Li Po pack. You could always use larger battery packs to meet the minimum voltage requirements of the diode-regulator circuit. I want to save space and weight, not to mention that if I am using hobby servos they don’t like more than 7.2 volts. What we want is the power supply circuit to handle any input from 6 volts to 12 volts.
What I am getting at is we can lower the minimum voltage required to run our circuits. It requires a change to the standard voltage regulator circuit and a change in components. We are going to replace the reverse protection diode with a P channel MOSFET. Then the good old LM7805 voltage regulator is getting replaced with a low drop out type regulator. So let us get to the schematic…
Schematic
Battery connection and reverse protection MOSFET
Here is the battery input, power switch, and the reverse protection MOSFET. The battery connection is the standard screw type terminal block with 2 inputs. This allows you wire up any type or configuration of batteries and does not rely on a difficult to find (read purchase) battery plug. The power switch shown is a single pole, dual throw slide switch. The real switch will be a double pole, dual throw. I am looking at using the second pole of the switch to discharge the capacitors when the circuit is turned off. Now the P channel MOSFET is being used in the reverse battery protection mode. How this works is the gate of the MOSFET must be a lower voltage than the drain otherwise the device will not turn on. So if you wire up the battery backwards, the rest of the circuit will not see any power. Your electronics are saved.
What’s Next
This has been a long enough post so I am going to split up the power supply into a few posts. So come back soon for part 2.
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