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POWER UP! (DC-DC)

A project log for Wheelchair EZ-Motion

Hacking wheelchairs / mobility scooters to make them more accessible to folks with more than 1 disability

criptastichackerCriptasticHacker 04/15/2023 at 06:510 Comments

Having proof of concept from the working firmware test, it's time to dive into designing some PCBs.

We need have a very robust and reliable power system.  This is a medical device, and something that I rely on heavily, so it can't be prone to errors and issues.

I made the mistake in my previous design years ago by not allocating enough attention toward a large power PCB.  It's time to fix that.

Since this is a battery-powered system, we will be saving power by using a DC-DC converter.  The biggest challenge in the system comes with the high-voltage requirement - it utilizeds 2x 12V lead acid batteries in series, giving us a max fvoltage of 29.4V(!) if you count the theoretical perfect float voltage charge state.  In reality, it's going to be more around 27-28V.

For this we need a bulky step-down converter, and nothing fits the bill more affordably or easier to come by than the LM2596 module:

specs (from this link):

Input voltage: 4V-35V
Output voltage: 1.23V - 30V
Input current: 3A (maximum)
DC-DC Buck Converter Step Down Module LM2596 Power Supply
Specifications:
Conversion efficiency: 92%(highest)
Switching frequency: 150KHz
Output ripple: 30mA9maxmum)

These modules are provide enough power but are stupid-cheap, so we'll need to make out own modifcations to improve the reliability.  It's a great starting point.

I'll be replacing the capacitors with more reliable solid polymer ones, reflowing some of the poor quality solder, and (most importantly) adding improved heatsink.

I also need to create reliable working voltages for the MCU, digipot, and other add-ons.  +3.3V and +5V are standard for that.  I also want things to be well regulated, with low voltage ripple and...fused!  Lot's of fuses.  Additionally, I want a schottky diode in series with the input, to prevent any potentional problems from affecting the $300 wheelchair CPU module.

For these reasons, I designed 2x add-on PCBs for the LM2596 module.  One for input, one for output:

The input PCB incorporates a protection fuse, in-line Shottky diode, an additional filter capacitor, an additional PCB heatsink, a JST-XH-2 power input, and a JST-PH-2 output for a battery gauge. I think this will prove to be very useful...

The bottom PCB Has a little more going on. It incorporates 2x LDO (low dropout) regulators; an AMS1117-3.3 and AMS1117-5.0.  These are popular low-cost LDOs that are very affordable and do a decent job of creating smooth power rails for microcontrollers.  There are also fuses for each output and LED indicators.  Note, the 3D model actually doesn't show the solder mask removals along the power sections. I kept heat dissipation in mind for this, as we'll see when the PCBs arrive in a couple weeks.  Also, disregard the floating drill there, that is something that I kept in my LM2596 module footprint for alignment purposes. (It has no effect on fabrication).


Here are the two PCBs + module combined, just begging for a 3D enclosure to be modeled! :D

(disregard the lack of detail on the Input PCB, KiCAD doesn't make it the easiest to export and re-import .step models.  But this will do the job for the enclosure design and as a general mockup.).

With this set up, we have a respectable power module. It's a good place to start, so now it's on to more exciting things...

[coming soon...]

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