For example, I have an antique portable television made by the Singer manufacturing company that plugs into US mains. However, it also has a 12v DC input and can run off of that. I also have a retro CCTV monitor that uses the same 120v mains plug and has a placard on the back that says it can use a 12V 12W battery. How is this possible? If an electronic device is designed to take 12v, wouldn’t 120v destroy the device? On the other hand, if a device is designed to accept 120v, shouldn’t 12v not be enough to operate it?
Edit: added a clarifying question
In: Physics
I assume the inputs are separate plugs.
Usually the case is that the 120V in is transformed down with an SMPS to 12V.
The 12V input then just bypasses the SMPS. (Or runs a completely separate power supply system that converts the 12V in to whatever voltage it wants to work with internally.
That and a couple diodes or power switching to keep power from flowing between the two power supplies.
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