USB as a charging source.

[catgate]

We recently got a cheapo MP3 player. It is so cheapo that there is no name on it and the instruction "manual" was a single sheet of paper about 2" x 3 1/2". It charges from a USB port. There was no indication "in the manual" as to whether the thing should be switched on to allow the charging to take place or not. (I am presuming that as long as the computer is on there will be a reasonable supply of electrons available at the USB port.)

So my question is what is the norm with these things? I can see reason for them to have a switched charging circuit,.... but then who knows.

[Alan2273]

A long read but interesting.

http://www.powermanagementdesignline.com/s...cleId=196902337

[-pops-]

The players I'm (slightly) familiar with switch themselves on when the charger plug is inserted. They then go into a sort of charging mode with a progress bar/battery indicator on the screen or LED.

[catgate]

Thanks for the link Alan2273. Certainly very interesting, but without a circuit diagram for our little beastie I fear that taking it to bits might be the only way to find how it fits into Mr Papparazzi's scheme. He does indicate that Ohms law is still alive and well though. I think one of his closing sentences sums it up :-

However, using the USB port as a power source for battery charging does present significant challenges to the design.

As to our little beastie, pops, it has no screen, but it does have a green led which lights up when it is switched on for boogying.

To try to charge it, prior to reading your post, I was plugging in and then switching on, whereupon a red led lit up. Since reading your post I find that if it is switched on without the USB connection the green led shows but when the USB connection is made the green goes off and a red led at the other side of the thing comes on (though this will not come on without the switch being on).

This all seems to suggest that the charging circuit is switched, but that switching is done electronically.

Thank you both.