High current draw from USB

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High current draw from USB

Post by Trekmeister » Wed Nov 21, 2007 6:10 pm

Does anyone have experience in how much current you can safely draw from an USB connector?

I am aware that the specs says 100mA from the start and then one can negotiate for 500mA. But when I plugged in my little project here and measured the current draw I used more than 1A, 10 times more than I should be allowed to!

I was expecting the motherboard to limit the current but obiously not. So has anyone got any experience in pulling large currents from USB connectors?

(I might just have to put a 5V plug in the front of my computer direct from the PSU instead, you never know when you might need it.)

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Post by |Romeo| » Thu Nov 22, 2007 12:24 pm

I think (it's a long while since I looked at the spec) that you must provide 100mA without negotiation and must be able to provide 500mA if the device requests it, but the spec does not prohibit supplying more, it merely states that a device should (must?) not attempt to draw more than 500mA at any time.

My understanding of contempary motherboard implementations is that they simply connect Vcc to the +5V rail on the PSU, so the current you could draw if you tried is not really limited (obviously motherboard pin headers for front USB are not designed for high current and will probably be the weak link).

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