Bus Power Only
October 9, 2012 3:01 PM   Subscribe

What does "bus power only" mean as regards to power requirements?

I want to use the below device with a iphone charger as a stand-alone cd player but i cannot supply power to it. According to specs, it need "bus power only" DC+5V. How can I power this player externally?


posted by raphael19 to Technology (14 answers total)
Does it not work if you plug it into a wall charger with a full sized USB port? It reads to me as if that device is built to work conform within the USB specs for power supply - IE it is powered by the USB bus only. So, for example, a typical wall-charger with a USB port would work to power it, as (I would think) that your iphone charger if it has a a full sized USB port would as well.
posted by McSwaggers at 3:09 PM on October 9, 2012

In this context, bus power means that it is powered completely by the USB connection. You will be able to power it with any USB cable plugged in to a stand-alone charger.
posted by AndrewStephens at 3:11 PM on October 9, 2012

Bus power here means that the CD-ROM drive takes power through a USB (Universal Serial BUS) connection. Virtually all devices with a USB jack that the CD-ROM drive can plug into can provide DC+5V 500mA bus power.

The iPhone charger provides DC+5V 1000mA - which should be adequate to power the player. This hints at a problem with the device, a problem with the charger, or that the device was not designed to be used as a standalone audio player even when provided with power over USB.
posted by eschatfische at 3:14 PM on October 9, 2012

10W Apple Wall charger with standard USB port cannot power this device.

When I get the power from a macbook the device works as a stand-alone cd-rom player, provided that it stays connected to the notebook.
posted by raphael19 at 3:17 PM on October 9, 2012

Does the Apple charger work otherwise (charge other devices)? Maybe the power supply is broken. Do you have another charger you can try?
posted by AndrewStephens at 3:23 PM on October 9, 2012

Are you sure that the wall charger works? 10 watts is way more than this device requires.

peak draw: 450 milliamps on seek = 0.450 amps
5 volts * 0.450 amps = 2.25 watts
posted by scose at 3:26 PM on October 9, 2012

Of course. Apple charger works fine. It just does not power this device. Maybe, Apple is restricting it?
posted by raphael19 at 3:27 PM on October 9, 2012

@scose I think the device will draw current as much as it needs. So a greater power output should not harm...
posted by raphael19 at 3:28 PM on October 9, 2012

Are you sure that the device is capable of standalone functionality? Even though it has a headphone plug it is not clear to me from that page that it has the electronic smarts to do all the decoding/processing necessary for playback. Does the player have stand alone button controls that would suggest as much?

Edit: Thanks google, I see that it does.
posted by McSwaggers at 3:30 PM on October 9, 2012

It could be that the CD drive doesn't know how to talk to the Apple charger to request power in the way the charger expects.

Have you tried using a generic USB power adapter?
posted by chengjih at 3:30 PM on October 9, 2012 [1 favorite]

@McSwaggers Yes, it has standalone functionality. As I said, when connected to a macbook, it works as a cd player without using Itunes or other software, but only with the control buttons present on the unit.

@chengjih No, I did not try to use other generic stuff, maybe I should.. Thanks!
posted by raphael19 at 3:34 PM on October 9, 2012

Apparently the drive has buttons to play and stop for "playback when not used with the CD player software." It's possible it still wants to communicate with a computer before it will spin up. The iPhone charger would not interface with it in the same way; it would only provide power.
posted by WasabiFlux at 3:34 PM on October 9, 2012 [1 favorite]

Assuming USB2, the drive will need up to 500mA to work properly (highest current draw @ spin-up). The USB spec is only good for 100mA without negotiation, which won't happen with most chargers (they don't have a proper USB host chipset to do any negotiating).

Note that this is different to the USB Battery Charging specification, which allows smart devices (e.g. phones) to see that a dumb charger (i.e. iDevice charger) can supply the necessary current.

So what's happening is that the drive interface powers up on the initial 100mA, then immediately requests that it be allowed to draw 500mA. Unfortunately, when plugged into a charger, it's requesting that from a dumb device which - while it may be capable of providing 500mA or more - isn't smart enough to answer the drive.

You could try using a powered hub as a supply - they have host(-like) chipsets, and may be capable of doing the negotiation when not connected to a PC, although I've never tried it.
posted by Pinback at 3:57 PM on October 9, 2012 [4 favorites]

Pinback : You could try using a powered hub as a supply - they have host(-like) chipsets, and may be capable of doing the negotiation when not connected to a PC, although I've never tried it.

FWIW, I use this trick with a handheld GPS - It won't run off a "dumb" USB power supply, but will run off a USB hub with no upstream connection just fine.
posted by pla at 5:43 PM on October 9, 2012 [1 favorite]

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