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Robot Power Distribution Board

Provides stable 5V and 3.3V rails, a direct power output, and I2C expansion.

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While building my robot, I decided that I needed a way to split out a single power source into direct power (for the motors), and both 5V and 3.3V for peripherals. I figured while I was at it, I'd add an I2C expansion, too.

The basic problem is this: my controllers and peripherals are 3.3V or 5V, but the motors need as much voltage as I can throw at them.

The board is split into several sections.

The first part is the power intake. It's a screw terminal input for the most flexibility, and adds in a kill switch and large decoupling capacitor. This splits out to a direct power output to the motor controller with a motor kill switch to let me test the other parts of the robot without worrying about the motors. I put the cap before the switches so that the capacitor discharge doesn't affect the time to kill the system.

There's a pair of voltage regulators: an LD1117 to provide 3.3V and an LM317 to provide 5V. These have decoupling caps placed after them. They go to a set of headers that provide power connections for the relevant systems.

Finally, there's an isolated I2C expansion that lets me add up to seven I2C peripherals via standard header.

Revision history:

2.0: fixed capacitors (the caps were in-line with the circuit instead of acting as decoupled caps).

1.0: First pass.

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  • Testing on a Raspberry Pi 3B+

    Kyle Isom08/02/2019 at 03:56 0 comments

    It mostly works. Building ROS packages causes it to power off, which I suspect is due to not being able to source enough current.

  • V2 testing results

    Kyle Isom08/01/2019 at 04:53 0 comments

    I hooked up some power resistors (1-8R 10W) to the various outputs and ran it through its paces.

    First up was supplying 8V @ 1A to an 8R resistor hooked up to the motor controller (8V / 8R should be roughly 1A). 

    Next, I attached a 4.5R load to the 3.3V output (2x1R in parallel with a 4R in series). The LM111733V is limited to 800mA sources, so that should give 733mA that's around 2.5W. After about 30s, it starts to short circuit; I suspect this is due either to the traces not being able to supply the current or the resistors overheating and causing problems; at 10W, I wouldn't suspect that but they did feel pretty warm afterwards.

    Finally, I tried supplying 8V @ 1.1A with 4.5R load on the 5V outputs (which should be about 1.1A). There's a 0.2V drop across the load, which I suspect is due to the load.

    I did try switching the motor kill switch while the load was applied and it didn't fuse, which is a good sign.

    The tests indicate that I should aim for improved thermal dissipation and widen up my traces.

  • v3 / v4 prototypes

    Kyle Isom07/31/2019 at 19:00 0 comments

    I wasn't happy with the parts I'd used in Eagle and the resulting silkscreen, so I updated the board... which meant having to rearrange things. So that's v3. I also added some indicator LEDs to a v4 board with SMD parts; I'm keeping v3 and v4 separate so as to keep the option of not doing SMD open.

    Something that I've been thinking about is whether the copper trace widths I used (10 mil) is enough for the power I want to pull. I think the traces are short enough that it's fine, but think v5 is going to feature wider traces.

  • Prototype arrived

    Kyle Isom07/31/2019 at 01:07 0 comments

    The prototypes arrived! I hastily assembled one to test it...

    The capacitor footprints are a little small, so I'm going to have to update the board to fix that. I think I'll add LED indicators, too.

    It's testing working, though only under a bench power supply and not under load.

  • Rev 2.0 assigned to panel

    Kyle Isom07/23/2019 at 23:45 0 comments

    Just got the email from OSHPark that the boards have been panelized - they should be sent to the fab in the next couple days.

  • First boards ordered

    Kyle Isom07/23/2019 at 04:38 0 comments

    I ordered the first set of boards from OSHPark - we'll see how they turn out. I'm a little concerned that the power switch is only rated for 200mA...

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Kyle Isom wrote 07/28/2019 at 23:21 point

I'm leaving the input power flexible; I have some 2x18650 battery packs (~8V), some 4xAA battery packs (6V), some 5xAA battery packs (7.5V), and 6xAA battery packs (9V).

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Humpelstilzchen wrote 07/28/2019 at 19:06 point

This problem sounds familiar...

I wonder what power source you plan to use with this board?

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