Electronic – Two 5V Raspberry Pi UPS Idea

5vraspberry piups

I need a cheap, reliable and easy to build UPS for my raspberry pi server.
The UPS must maintain power for a maximum of 1 hour.
Based on what I found on the internet,I came with two solution. (see diagrams below).

Please rate both and tell me, which one has fewer flaws.

The first solution uses a relay that switches the 5V output from power supply to 20000mAh bank power. In order not to reset the raspberry when relay switching power source, the voltage for 2 seconds is maintained by the super capacitors. When blackout end, the relay should switch back to the power supply

Idea number 1
The second solution uses BMS with three 18650 li-ion batteries, generic 12V power supply and step down converter. In the event of a power failure, the batteries support the power on buck converter. When blackout end, PSU charge batteries and buck converter.

Idea number 2
For the first solution i have every part except supercapacitor but i found 3$ deal on Ali. SuperCapacitor
Relay is Omron G6BU-1114P-US-DC12

For second solution i need more parts:
XL4005 5A Max DC-DC Step Down – 1$
XL4005
30A 3S Polymer Lithium Battery Charger Protection Board 3 Serial 12V – 2$
BMS
Barrel jack, 12V PSU, 6 old 18650 battery from laptop, some usb cable lying in my house

In your opinion which solution is beter for 24/7/365 UPS?

Best Answer

I'll just comment on the first proposal.

The first solution uses a relay that switches the 5 V output from power supply to 20000 mAh bank power. In order not to reset the raspberry when relay switching power source, the voltage for 2 seconds is maintained by the super capacitors. When blackout end, the relay should switch back to the power supply.

Let's draw a simplified circuit with the mains power on.

schematic

simulate this circuit – Schematic created using CircuitLab

Figure 1. A simplified version of the mains-powered switching.

  • Mains is switched on. U1 generates DC output.
  • RLY1 is energised. A great big spark occurs on the relay contact as C1 initially appears as a super-short circuit and has to be charged.
  • C1 charges up at a rate determined by the current limit of the PSU.
  • Mains power is lost.
  • Current now flows backwards from C1 to the relay coil through the NO contact. The relay remains energised until C1 discharges below the relay hold-on voltage. This is typically 40 to 60% of the pick-up voltage. It then slowly drops out until the contacts open and the rest of the contact transfer motion occurs quickly.
  • Meanwhile the Pi has lost power and will reboot when the battery connects.

Option 2 has a better chance of success.

Related Topic