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.

@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.

@roryaronson and @Gabriel I am letting you know that both parts of this modification have been a total success. I would like you to consider standardizing/adopting the the “Camera and Accessory Power” modification.

Also in case you want it the fritzing file is here:
Farmduino Express.fzpz (570.2 KB)


@aronrubin Can you lay out exactly what this helps?

Disclaimer: I am not an EE but I can read schematics and datasheets so please do not consider my statements authoritative.

There are two issues that are addressed:

Downstream Power

The power that exists

  • Available budget
    The Farmduino Express runs on 24V input with a budget of 150W (6.25A). The 24V feeds to a TPS5430 to power 5V logic and peripherals with a budget of 3A. The Raspberry Pi (RPi) Zero W, included in that budget, has a downstream USB connector with 5V with no regulator or limiting circuitry but it’s generally understood that over 1.2A can be risky. The RPi does have a 3V3 regulator and output.
  • Power instability
    It appears that high consumers on the 24V side of the house can cause significant drops (brownouts) on the 5V side of the house. Unlike the RPi 3B+, the RPi Zero has no power sense circuitry so it cannot report these issues. Additionally, there is a normal power drop over long distances of wire. Any downstream peripherals, including the stock camera (with no regulator), probably want a stable voltage source for measurements and communication.
  • Noise
    The USB camera line, which serves as the only downstream communication line, is unshielded and running parallel to high frequency, high power motor lines over its entire length.

Proposed power

Run a 24V line to a junction box at the Y-Z axis mount point. In the junction box regulate power down to required voltages especially 5V. This works around voltage drop and creates a tap for peripherals.

Downstream Communication

State of affairs

It seems that FB did envision downstream peripherals in its current incarnation. For the wider configurations of FB, running 15ft cables for GPIO pins seems dubious. Not to be down on it, it is a great setup for accessories mounted at the main box. I would like to see a more robust communications design for FB in general.

Proposed downstream comms

Putting aside grand visions, I will settle for USB 2.0. Run a shielded USB extension cable out to an amplifying hub in a junction box at the Y-Z axis mount. Power this hub from the aforementioned regulated 5V in the same junction box.

P.S. all of this is working in my current setup.


How about looking at a Servo to force down a rubber stop on the track server would only have to be powered when engaging or disengaging the break. so the code is basically have to read just moved X Y or Z
PowerUp servo
engage break
unpowered brake servo and motor

when you would want to move it again
PowerUp Servo and motor
disengage brake
move around freely.