Thread: USB power limits

Bluetooth, WiFi, and GPS are all big power users. I dont know about the auto-on, but a relay can take 400mA easily depending on the size (or it could take 50mA again depends).

I think you have actually surpassed the cutoff for getting a powered hub.

Since it is a laptop, you cant even distribute the usb to not overload any root since the laptop only has 1 root (driver, not physical ports).

I say get a powered hub. Or at the least, some of those car chargers for mp3 players/cell phones that have a USB port on the other end so you can splice power off of that, and data through the unpowered hub as normal. And keep in mind the hub itself does use a little power.

Why is most hubs 4 ports?

It's because every device is allowed to comsume 100mA and request 500mA from its hub (if passive). So a hub (in itself a device) request 500mA, uses 100mA itself and remainder goes to its four ports.

A 7 port is like two 4 port hubs in chain, so there are no passive 7 port hubs. If you want to be within spec, and I think you do, considering all sorts of problems you might have with USB in the car anyway, you'd better get a powersupply.

One more thing if you are wiring USB all over the car and have active hubs: You can cut the USB cable open, leaving just the data wires. That will ensure no ground loops and data traffic is less subseptive to noise.

I run into an active hub that returned 5V from its supply to the PC, which is a no-no. Cutting the power will for sure prevent that.

5V DCDC is not cheap, and you need to be able to turn them off or you will drain your battery. There is a clever device that will be turned on and off depending on presens of 5V on its USB input.