External USB Case: DVD Overcurrent

Well, it turns out that the DVD drive I stuffed into that case really does require a whole bunch of current. I tried playing a DVD and got erratic results, including weird keyboard (!) failures. Finally, I hitched a bench supply to the coaxial power jack on the case and caught it in the act:

Laptop DVD - current display
Laptop DVD - current display

That jack normally connects to the power-only USB cable, which implies an upper limit of 100 mA. A bit of poking around inside shows that the coaxial power jack simply parallels the USB jack’s VCC line, so there’s no fancy negotiation or current sharing going on.

When the keyboard went nuts it was sharing an unpowered USB hub with this thing, which means that the overcurrent dragged down the hub’s supply. I was permuting all the choices to see if the failures suggested anything; eventually it did.

A bit of rummaging in the Basement Laboratory Warehouse Wing uncovered a 5.0 V 3.7 A wall wart switching power supply that is grossly in excess of the drive’s 1.5 A rating. Amazingly, it even had the correct coaxial power plug on the end of the cable, which never happens.

Alas, because the external supply back-powers the USB data cable, it lights up the Q150’s power button when the PC is turned off. I think I can insert an isolation diode into the USB power trace to isolate it from the jack, somewhat along the lines of that hack. However, that seems to require removing the USB connector to uncover a very well protected top trace. For now, I’ll just unplug the drive.

8 thoughts on “External USB Case: DVD Overcurrent

    1. USB should be good for 500mA.

      That’s after the device negotiates with the host, which the bare power cable certainly doesn’t. Before negotiation, the device must draw just enough juice to wake itself up.

      The new standards for USB battery charging allow for dedicated ports with more current, but the old junk around here knows naught about that!

    1. You mean the notion of leaving your laptop powered up for 10 hours just to recharge a handful of batteries doesn’t make sense to you? [grin]

      Almost as bad as those solar chargers…

      1. If folks suppose they can have an Las Vegas in the middle of nowhere powered by a car battery, so a USB-charger got to be magic.

  1. I think that most desktop/hub host USB implementations will source 500mA even before negotiations. It is easier to design for the worst case 500mA than try to limit it to 100mA.

    1. That adapter obviously depends on it!

      In all fairness, I realized that I’m stuffing a laptop SATA drive into a USB adapter, so I’m getting exactly what I deserve. The drive has a 1.5 A maximum rating: far more than it should draw from two negotiated links.

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