Supply voltage for DC backup switch/relay

[pwnell]

Considering this device on Amazon: https://a.co/d/8RwpMJl

It specifically mentions it works with 12 - 48V IN/OUT.  They also mention that the IN1, IN2 and OUT voltages must be the same - i.e. battery, DC supply and load must share the same voltage level.  I cannot identify the small IC on the circuit board to understand what they use for voltage regulation / detection, but my question is - if I have 24V DC supply, 12V battery and a load that accept 12V and 24V, can I use this in principle?  What is the danger of using different battery and DC supply voltage if the load can deal with it just fine?  If there is a real issue, would a strategically placed diode fix this?

[Whales]

What they're probably trying to say is that this board does not do voltage conversion.  If you input 17V then it will output 17V.  They have probably had customers misinterpret the 12-48V rating to mean they can wire any combination of things within that range to it at the same time.

The SOIC8 chip has an inductor next to it, so it's probably a switchmode power supply (to provide 12V for the relay coil).  At a guess they'd be using the ENABLE pin on this chip to detect whether or not the relay needs tripping.


> if I have 24V DC supply, 12V battery and a load that accept 12V and 24V, can I use this in principle?

Maybe.  I'd have to reverse engineer the whole circuit on that PCB and get the datasheet for that chip.  With certain wiring arrangements it might cause the switchmode chip to fail.

[pwnell]

That is what I thought. I will post the part number of the IC once the unit arrives.

[calzap]

It’s basically a cheap UPS.  Most of the onboard circuitry is probably there to provide 12 VDC to the relay coil from the non-battery power source via the power adapter terminals.  When adequate power from that source is available, the relay directs it to the output terminals. When adequate power from the non-battery power source is not present, the relay connects the battery input to the output terminals.

Shouldn’t matter (but no guarantee) if the battery and power adapter are different voltages.  Even if the adapter and battery started at equal voltages, circumstances could easily cause them to become different.  For example, the battery could lose charge over time or the adapter could slowly fail.

Mike