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One example is a CD boombox. Battery-powered.

During operation, if I pull or insert a cable, it restarts, if the cable is connected to the wall power supply.

If I pull/insert fast enough, it even stays on, but the screen backlight of the JVC RC-EX30 goes dark for the fraction of a second.

If I insert a cable which is not connected to the wall on the other side, the device refuses to run from batteries, unlike smartphones, that also run while the USB cable is connected but not powered by any power source or by an empty powerbank.

What is the technical explaination behind this?.

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    \$\begingroup\$ Sometimes the switch built into the power socket is break-before-make, so it's unpowered for a fraction of a second ,on insertion. And if you switch power to external, but external isn't powered ... \$\endgroup\$
    – user16324
    Commented Feb 22, 2018 at 15:42
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    \$\begingroup\$ Smartphones are designed to be hot-plugged. The CD thing is not. And there are many ways to design it, so there is no single explanation. \$\endgroup\$
    – Eugene Sh.
    Commented Feb 22, 2018 at 15:42

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The cable input is wired in parallel with the battery input. If they were allowed to be active at the same time, the batteries would be charged from the cable input.

The device is designed to be used with non-rechargable batteries. Non-rechargable batteries can leak or explode if you try to charge them.

Therefore the manufacturer has implemented a mechanical switch inside the power jack, which disconnects the batteries when you plug in the cable.

As Brian mentioned in the comments, these are often break-before-make. In other words the battery circuit is disconnected before the wall power is connected. And when you pull the plug, the battery circuit is not connected until after the wall power is disconnected. This is simply to prevent the user from partially plugging in the power and charging the non-rechargable batteries until something catches fire and the manufacturer gets sued. It's a failsafe.

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