Electronic – Why does this USB device draw different currents from different sources

current measurementhdmiusb deviceusb-pd

What would cause a USB device to draw drastically different amounts of current, but still function properly?

Background & Research:

  • I purchased a Playstation 2 Graphics port->HDMI converter (like this one) that requires external power via micro-USB port, claiming to require a 1A rated source.
  • Using a USB Tester, I found the typical current draw to power the HDMI converter from a USB wall adapter to be .25A (250mA) (~1/4 of the manufacturer recommendation).
  • The USB specs for my Playstation 2 (Model SCPH-30001) is supposed to be USB 1.1 with a maximum output current of .1A (100mA).
  • I tried plugging in the HDMI converter to the Playstation 2 for a few seconds at a time to see if it would function correctly, and it did, though it only drew about 0.01A (10mA). So…

Questions:

1. What is going on here?

The HDMI Converter pulls 250mA at 5.05V from the power brick, but only 10mA at 5.05V from the 100mA-rated USB 1.1 socket. It seems to work just fine from either, displaying HDMI signal on the TV.

The HDMI Converter does not seems like it would require a lot of power, as it should only need to provide power for HDMI communication (rated minimum: .055A) and powering the tiny chip. Why would it pull so much more from the wall adapter?

2. Is this safe?

Am I damaging one or both of the devices by powering the converter from the Playstation 2?

I DO NOT want to harm the Playstation 2, as I have burnt out USB ports by overloading them before. (But I CAN afford to fry the HDMI converter for experimentation's sake.)

Best Answer

  1. I suspect the device you have will takes power from both the 5V line in the playstation graphics port and the usb input (possibly fed through a diode). The output from your wall adapter is likely slightly above 5V but the playstation will have a well regulated 5V supply. Because the voltage of the wall adapter is slightly higher that PS2, the device will draw most of its power from the wall adapter rather than the graphics port (or you may even be injecting powering into the PS2 via the graphics port), when you use the PS2 usb it draws most of its power from the graphics port.

    You wonder why the adaptor needs so much power, firstly the adaptor has to do quite a lot of work - it takes the 3 analog video signals and digitizes them at something like 30MHz before sending digitized data out over HDMI. Secondly an HDMI source provides a 5V line for powering electronics downstream, and your adaptor has to provide that power as well. It sounds like the manufacturer has been conservative and states that you need at least an amp to cover both things.

  2. If the converter has been designed to run from the PS2 then I would do that. The dangers are:

    a) using the PS2 sockets, that you overload the power supply/sockets in the PS2

    b) or using the power adaptor, that whoever designed the device never expected it to be powered when the PS2 wasn't and didn't take any steps to prevent it feeding power back into the PS2, or forcing whatever drives the video output into SCR latchup.

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