I Reworked My FB Express Power

Part I - Removing DC -> AC -> DC

I have a solar panel connected to a LiFePO4 battery with an in-built inverter to run the Farmbot. The battery is just at the edge capacity-wise. I was using the stock AC->DC block that came with the FB Express XL but it would never last through the night even with X and Y unpowered while idle. There was incredible conversion loss.
DC Power source 12V DC to 24V DC assuming 100W peak
First I had to deal with unidentifiable 2-conductor on the FB 24V input. I have an aversion to proprietary or uncommon connectors so I adapted it to a TE AMP Superseal 1.5 (typical automotive power connector) with a 16AWG cable. Next it the matter of stepping up the voltage. As I have a power box (actually an old mailbox) to house the battery, I decided to use a step-up converter with a display so I can also measure power use.


Step-up with controls

Part II - Camera and Accessory Power.

I have had continued problems with the camera. Sometimes I can get it to latch but most of the time the USB camera is “not found.” I have another boroscope camera with similar cable length and that seems to be always recognized. After finally getting a decent multimeter setup outside it is very clear that power to the camera peaks at 5.05V but is only reliable at 4.8V and will dip to 4.5V when other systems are running. I assume the camera is not found because that dip happens at boot time when the system is likely querying the bus for devices. I also noticed that the cable for the camera is not shielded, which seems like a significant oversight.
Additionally, I plan to connect all manners of devices at the tool end so I need more than the RPi zero offers anyway.
Run a shielded USB extension out to an amplifying hub at the tool end and provide higher capacity 5V.
I have started with a USB hub with a FE1.1 chip that is intended for downstream power distribution. The model selected also has a 15ft shielded cable. The hub is housed in a waterproof box with cable glands on the Y-axis at the Z-axis junction. For more reliable power I have also dragged 24V (from the FB system input) out to a 5V regulator in that junction box. The regulator feeds the hub but can also supply power separately. This also gives me the option to add regulators for other voltages as needed in the future.
Junction Box

USB Hub and Cable

Power Cable

Step-down(buck) from 24VDC to 5VDC

As a related matter, @roryaronson, are you using passive breaking in the motor drivers? The motor coil hum I hear seems to imply you are doing active braking when “always power motors” is on or no braking when “always power motors” is off.

@aronrubin This looks very interesting. Everyone on the team appreciates your openness in sharing design modifications. I look forward to hearing more updates.

That is correct. I usually keep always power motors on for Z, but off for X and Y.

Would it be possible to request the ability to use passive braking instead?

Or getting away from something that consumes power all of the time (braking the motor), a solenoid and spring operated (low powered) brake could be employed. I know that it would add complexity, but it could be made with a fairly simple subset of 3d printed components, that would be optional for those who have a mostly continuous power source.

@dmbgo and @roryaronson, Passive braking does not consume any significant power. If you short the coils of a stepper (no power, just shorting the cable) you will find it is very hard to turn. The stepper driver can do this by latching the coils together which is referred to as passive braking. The TMC has such a mode under the “freewheel” settings.

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@aronrubin You are right of course, I’m sorry I lost my temper :slight_smile: (joke)

If you’ve lost your temper you can just heat up until cherry red and then quench yourself quickly.

Looking for this explanation long time ago!! nice

@aronrubin thanks for the great suggestion! I had no idea the drivers were capable of this. @Tim, what do you think?

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So everything was working but the camera again. So I voided my warranty to find a very odd notion of USB color scheme inside:

The red and yellow are swapped from the normal and someone’s fingers are probably stuck together with all the glue in here. The top two pads on the board are D+ and D- and the bottom should be +5V. A 3V3 regulator would be typical but that looks like a voltage divider to me. In any case, it is all fixed now and works a treat with the new power setup.

on the 24V to 5v step-down converter, I went simple https://www.amazon.com/gp/product/B086PRHKC1/ref=ppx_yo_dt_b_asin_title_o02_s00?ie=UTF8&psc=1and got a voltage meter and a fuse in the process.

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I was fooling myself. That board is gone. When I thought it was working, it was actually the other endoscope that I saw.

That is a good way of sourcing the converter if you don’t need the USB to pass through.

Of course, this would only be valid if you were made of copper or brass etc, if you were iron with a high carbon content you might shatter. :slight_smile:

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It’s nice when a software change can improve the mechanical side.