Foreknowledge: This post is the extension of a previous post (DIY uninterruptable power supply (UPS) for Raspberry Pi Zero), but due to the community rules I ask it as a new separate question. It is good to read the previous post.
I have several pi-zero boards and utilize them to monitor environmental conditions (such as humidity, temperature and intrusion) of each room at home.
For continuous operation, I want to eliminate power blackouts. So, I decided to make a basic UPS for pi-zero boards. (need to be as small as possible)
I can not use combo boards of battery charger and booster, because they are expensive and hard to collect (for me).
Hence my new design: The relay selects the power supply dynamically depending on the status of the main power supply (which is AC/DC adapter).
- AC/DC adapter powers the battery charger and the relay coil.
- Also the adapter's (+) output pin (named as V1) is connected to the relay's normally open (NO) pin.
- The battery charger charges the 3.7V battery.
- Also the charger's (+) output pin (named as V2) is connected to the relay's normally closed (NC) pin.
- The relay's common (COM) pin is connected to the step-up converter's (+) input pin.
- The pi-zero is powered with the output of the step-up converter.
My questions are:
- Is there any unsafe point similar to the issue stated in the previous post?
- Even if the relay is not driven by any NPN or PNP transistor, is it required to connect a diode to the relay coil in parallel?
- What kind of capacitor should be connected to the load in parallel to prevent short power cuts at power supply changes?