14.2vDC -> 5.1vDC linear supply with features


I’m sharing this because this email reflector has been a great source of ideas.  
Also, the more I tell people about it the more egg I get in my face if I don’t follow through, and I want to follow through. 
This is vaporware of the thinnest kind but at least I’ve had some success in kit building projects in the past.  See http://tarpn.net/t/nino-tnc/nino-tnc.html

Larry WN8P and I have been working on a project to build a 5.1v USB-A source for 5.1v power to the Raspberry PI 4B. 
The creeping feature device we have ended up with takes 14.2vDC input and has the USB-A connector as part of the design.
It’s a linear supply so no more noise to the ham-shack.
It’s also a through-hole kit which should be easy enough for a new kit builder to start with.  Assembly instructions are coming after we test the first production ready boards.
Our intention is to sell the PCB on ETSY for cost + shipping + tax.  It should be under $10 total though the first run will no doubt be a little more due to small quantity. 
We’ll give away the Bill Of Materials in a ready-for-upload-to-Digikey form.  Also will have instructions on how to do that. 

The supply wants 14.2v input because that is the ideal voltage to trickle charge a plain old Gel Cel.  The voltage will go through a diode and a 47 ohm resistor before making it to the Gel Cel so we needed to start with 14.2v. If you put 13.8v into the supply, the only bad part will be that the Gel Cel won’t ever reach full charge.  That may shorten it’s life by a little.  We’ve found some $9 each (in quantity 10) 2.5amp 12v gel cels but any old left-over cell from a UPS cell swap will do. 

The supply’s features include
All linear supply so no extra hamshack noise
3 amps current capacity
5.1v output to a USB A socket so it’s ready to get connected to your PI 4B
12v Gel Cel for backup 
Instant switchover using a diode bridge so no glitches
Float charges the Gel Cel
Low voltage detect drives a GPIO to the Raspberry PI warning of imminent failure.  
Low voltage protect RELAY so the Gel Cel won’t get drawn down by the charger, or the Raspberry PI
LEDs to show many elements of functionality to make it easier to get working. 

The schematic is attached.  
The board and photos of the board will be in our assembly instructions.  See http://tarpn.net/t/nino-tnc/n9600a/n9600a3/n9600a3-assembly.html for an example of my kit-assembly instructions.  

Beware that this is pre-prototype.  Please kibitz because it’s not too late to change things.  

The disadvantages of using a linear circuit are: 
It gets hot and needs high current parts and a heat sync. 
It isn’t efficient, and turns more than half the power into heat.  A 12v 2amp gel-cel is only as good as a 5v 2amp battery as a back-up device. 

but… it’s easy to assemble as a kit, and has no high speed clocks to cause RFI.  

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