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
These devices are typically designed to run on the lower of the two voltages, so in this case 12V DC.
Inside the device you’re going to find a 120V AC to 12V DC power supply. So if you plug it into a 120V outlet, it runs on 12V internally, and if you connect it to a 12V battery, it also runs on 12V internally.
It’s also worth noting that for many electronic devices, 12V is likely going to be an intermediate voltage that only very few parts of the circuitry would actually run on. Something like the audio amplifier or LCD backlight may use 12V directly, but most of the electronics are going to run on something like 1.2V, 2.5V or 3.3V.
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