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builders ➜ Power Supply and Backup ➜ Robust power for Raspberry PI
See also: Control Panel/Raspberry PI Shelf
See also: 5.2 V Regulator
See also: Purchasing and wiring a Meanwell RS-15-15
See also: Assembling the Switchover Circuit

Robust Power for Raspberry PI

This page discusses a very inexpensive system for using a GelCel to provide stable power to the Digital Section of a TARPN node. This involves creating a UPS (Uninteruptable Power Supply) for the Raspberry PI. Having this diode-switched UPS increases the mean-time between SDcard corruption-caused-app-fauilres, from months to years.

The Control Panel/Raspberry PI Shelf page goes through a step by step instruction for building the Raspberry PI and Control panel shelf, including the robust power supply. Before deciding to build your system, please review the pages in the TARPN Raspberry-Pi-Shelf series linked from this page.

Low cost power switchover device

Total parts cost is about $50 at retail if you have to buy a $20 Gel Cel. I have been able to buy the gel-cel for $120 for 10 of them using Amazon. Perhaps you can replace the gel-cel in one of your UPSs and use it's old gel-cel for this circuit.

This schematic at the bottom of this page shows a simple battery floater circuit and switchover to power a USB power supply. The purpose of this is to keep the Raspberry PI up through power drop-outs and while moving the network cabinet. A 2.5 amp gel cell should run the raspberry PI for several hours. Most of the drop-outs that hurt us are very short, and possibly not noticed.

With the trickle charge circuit described here, it would take a day or more to recover the battery after a couple of hours of outage. That's ok because we don't expect to be on battery for very long, or very often.

The resistor should be such that even if the battery was a dead short, it wouldn't draw enough power to hurt anything. The 50 ohme resistor used in this circuit, with a 10 watts rating is excellent.

Important: 14.2 V is required as the feed into the diode switch so the float voltage onto the SLA Gel-Cel is idealized. We had been using an HP server supply modified to serve 14.2 VDC, and that was ideal. But a 13.8 V or lower supply, to feed the radios, is now cheaper and much more common, so instead of using a 14.2 V supply to the entire system, we add a cheap, readily available off-the-shelf Meanwell brand, 14.2 V supply to feed just the Raspberry PI/digital section (including backup battery) of the node.

The schematic needed 3 diodes. The nice thing about the bridge rectifier is that it gave us the diodes we needed in a single package and it had a screw mounting hole. If you get a bridge rectifier with 1 amp diodes or larger, this is fine.

Above:
example fully assembled node boxes.
Top is a KM4EP cabinet.
Bottom is an NC4FG cabinet.

Parts for the switchover/trickle charge circuit

Parts list:
  1. Gel Cel battery
  2. Meanwell RS-15-15 Specifications [meanwell.com]
  3. DC-DC regulator device with 5.2 V adjustable output
  4. 6 or 7 inch short USB cable - Amazon has some 7 inch and .5 foot cables - any length cut down to 6" or so if home-brewing the regulator-output connection
  5. some scheme to mount the regulator (I use nylon #8 hex nuts McMaster-Carr 90089A305 (100) for $2.62)
  6. double face tape for permanently attaching the Meanwell to the top of a shelf within or on the top of your node cabinet.
  7. 50 Ohm (+/- 10% on value) 5 or 10 Watt resistor -- the photos below show a 56 Ohm 5watt resistor.
  8. Bridge Rectifier
  9. (2) female Crimp-on lugs for the Gel Cel
  10. 3-foot of hookup wire, 16 to 20 AWG. You'll want 4 different colors, not including white and green. This wire passes at most 1 Amp at 14.2 VDC
    Jameco 18 gauge hookup wire
  11. TARPN Control Panel or other switch
The Bridge rectifiers come from Parts-Express and are 400V 6A Bridge Rectifier -- Part # 050-030
bridge_rectifier

The resistors (I usually buy them in quantity and keep some in stock) come from Parts-Express and are 47 Ohm 10W Resistor Wire Wound 5% Tolerance -- Part # 016-47
resistor

The gel cells I've used so far run from 2 to 4.5 amp-hour. Here is my favorite model because it fits in the housings we've been using:
Parts Express has Power-Sonic PS-1220 Sealed Lead Acid Battery 12V 2.5Ah -- Part # 140-360
Amazon has also had these in quantity 10 for lower cost per unit.
PS_1220_gelcel
The variable output 14.2 V power supply is a Meanwell RS-15-15
See also: Purchasing and wiring a Meanwell RS-15-15
Adjustable output regulator to power the Raspberry PI.
See also 5.2 V Regulator
drok-terminals-output-led-regulator

Schematic of the robust power supply

Note that the AC to DC supply should have 14.2v output in order to float the GelCel propertly.

robust-power-supply-005
Note: The - side of the Meanwell, the - side of the battery, and the -in for the LM2596 variable regulator can all be tied to the same ground bus used for the radio power supply and the radios.

Photo showing method of constructing the robust power supply.

Here's the bridge rectifier and trickle-charge resistor.

Step by Step Instructions.

    1. Before proceeding:
    • Unplug the Raspberry PI power from the Raspberry PI.
    • Remove the memory SDcard from the Raspberry PI.
    • turn off the battery switch and
    • disconnect the positive lead from the battery.
  2. Hook the Meanwell to an AC power cord. See detail on AC wiring, below.
  3. Tin and screw the red wire from your bridge rectifier circuit to the +V screw terminal.
  4. Tin and screw a black wire, 12" long should be enough, to the -V screw terminal.
  5. Carefully clip lead a Volt Ohm Meter to the black wire and to the orange wire from the bridge rectifier circuit.
  6. Set the V-O-M to measure 20V or more DC.
  7. Plug the AC cord into a switched outlet and turn it on.
  8. Observe the V-O-M reading, and adjust it to 13.8V DC.
  9. Turn off the Meanwell and unplug the power cord.
  10. Secure the Meanwell supply from the bottom (no holes) side against the flat surface of your power shelf (top of nodebox shelf shelf or top of node box)
  11. Make the AC cord mechanically rigid with respect to the Meanwell supply by securely attaching it to the shelf.
  12. Secure the bridge rectifier circuit and the DROK regulator device.
  13. Power on the Meanwell.
  14. Observe that the DROK supply has lit its LEDs.
  15. click the right and left buttons on the DROK until tbe numeric display shows the 13.8 V and the right red LED (near the button closest to the output terminals) is on telling you that you are measuring the output voltage.
  16. Adjust the multi-turn pot closer to the INPUT terminals counterclockwise about 8 turns, maybe. The voltage won't drop for several turns and then it moves quickly. Adjust the regulator output voltage to 5.2 V (as measured using the USB Voltage Tester or using a VOM at the OUT screw terminals) or so before plugging in the Raspberry PI.
  17. Test the battery with the positive lead disconnected and make sure the is at around 12 Volts.
  18. Turn on the battery connect switch.
  19. Test the voltages at the battery leads (without the battery in circuit) and adjust the Meanwell so the battery leads are at 13.8 V.
  20. Turn off the battery connect switch.
  21. Attach the battery to its leads.
  22. Turn on the battery connect switch.
  23. Unplug the Meanwell and verify that the DROK stays lit.
  24. Replug the Meanwell.
  25. Attach the USB to the DROK supply and plug in the Raspberry PI.
  26. Observe the power light on the Raspberry PI.
© Tadd Torborg, 2014↝2023 -- all rights reserved