USB puzzling me

[GB]

I have made a little robot car, controlled by a Rpi. The Rpi runs off a USB battery pack. The power for the motors comes from a separate pack with 4 AA batteries in it.

I thought the motors might run happily off the 5v available from the other USB port on the USB battery pack, so I cut an old USB lead, and stripped the insulation off the wires. However, on testing that, I only get 1.5v off the red and black wires, not 5v.

Things are obviously much more complicated than I hoped. Can you put me right, please?

[alan_m]

1.5V when open circuit or under load?

[Andy Burns]

The powerpack probably expects to see one (or more?) resistors, e.g. under 200 ohms between the D+/D- lines (white/green) before supplying power

[GB]

Open circuit.

[GB]

So, I just need to join the white and green wires, and it will fire up?

[Theo]

I think the powerpack is smarter than that. It lets some small voltage through at high impedance, but doesn't start up the SMPSU to begin with (it may also do the same with pulses). If it sees current being taken then it'll start up the SMPSU to provide however many amps it can offer. If no current is taken it'll revert to this idle state.

This saves power because it'll automatically turn off the SMPSU when the phone it's connected to becomes fully charged and stops taking current (or is unplugged), rather than running the SMPSU with no load and draining the power bank battery when nothing is connected.

So I would just try putting some load (eg the motors) across the power pack output and you may find 5V appears.

Theo

[Andy Burns]

should be ok, the USB BC1.2 spec says

"A Dedicated Charging Port (DCP) is a downstream port on a device that outputs power through a USB connector, but is not capable of enumerating a downstream device. A DCP shall source IDCP at an average voltage of VCHG.

A DCP shall short the D+ line to the D- line."

[The Natural Philosopher]

the USB spec is not designed to deliver massive current You want to use a proper set of batteries for the motor

[Brian Gaff]

Well most usbs protect themselves so one suspects the two supplies are too much for the usb to cope with. On the other hand of course, if the circuits are connected to each other somewhere, unexpected commonalities might make things do strange things.

I am guessing the power taken from the usb already is only done because the usb itself is where the control signals come from, so it seems daft not to reduce battery drain by not using all you can from it. Nothing to stop you using rechargeable cells to drive the dry battery part and then making a composite plug and socket for both charging circuits through the one multicore cable to two chargers. Brian

[Brian Gaff]

Well the first socket which no doubt powers the 'brain' is probably taking near the max you can take, hence the other problem. Brian

[Brian Gaff]

Yet you get USB fans on sale, I notice. Brian

[SteveW]

They are less than 5W and all but the early USB ports are capable of an Amp or more at 5V.

The latest (USB 4) ports (with negotiation), can provide up to 240W - 5A at 48V.