Originally posted by FunkyLaptop I have never even heard, until now, of a PC Card that needed its own power supply... wow.
I have an Adaptec Firewire PCMCIA card that has a power jack on it, but for the $20 it cost me, they didn't even include the power adapter.
I can understand the concern about drawing extra power from the laptop, though. They're getting down to fleapower motherboards and are getting more battery time out of smaller batteries (users like lighter) but there isn't a lot of reserve there.
But speaking of bus powering of outboard stuff, switching on the phantom power on a TASCAM US-122 when connected to my laptop computer would cause the supply voltage to to the US-122 (through the USB port) to drop low enough so it would shut down, and I had to unplug and re-plug it to get it initialized again. If the phantom switch was on when the US-122 was plugged in, it would come up OK,, but I guess the initial inrush current for the DC-DC converter was too much for the little laptop. I didn't have the same problelm when plugging it into a desktop computer.
Out of curiosity, I opened up a USB cable, put a 0.1 ohm resistor in series with the power lead and connected an oscilloscope across it to measure the transient current. It peaked at over 3 amps for about 10 milliseconds. I belive the USB spec allows for 500 mA.