Is it bad to plug 5V 900mA speakers with USB cable, into USB2?

(I read here What is the power output of a USB port? that USB2 is max 150mA)

I know that with a device the amps at the supply should be >= the amps of the device. And i've heard that if you plug a device is high ampage, into a supply that doesn't provide enough, then it's bad for the supply.

So if it's not bad then I wonder why it wouldn't be? How many amps/mA would a device have to be to damage a USB2 port(and potentially a motherboard)?

And if it wouldn't damage a USB2 port, would that imply that if you plugged a low ampage supply into a high ampage device, the supply would be fine?

  • No, USB2 is max 500mA and USB3 max 900mA. I guess it wouldn't be a problem for the USB port in your scenario but the speakers likely wouldn't work. – user931000 May 14 '19 at 17:08
  • You say "No, USB2 is max 500mA and USB3 max 900mA. " <-- I'm aware of those figures and that's my point. And i'm not askng if they would work, i'm asking if it's bad for the USB port. – barlop May 14 '19 at 22:38
  • As I said in the first comment, I guess it won't be harmful but I'm not qualified to provide an authoritative answer. – user931000 May 15 '19 at 0:15
  • You're kinda in the same boat as older USB hard drives with power requirements exceeding USB 2.0 limits. The actual official limit is 500 mA. It's not a sanctioned solution, but you could use the same type of Y cable they used to supply with some USB hard drives. – fixer1234 May 20 '19 at 0:27

System designers usually protect systems from overcurrent to avoid liabilities from house-fires. So you normally can't damage USB ports - there is either a polyswitch (resettable fuse), or electronically-limited "high-side" switch that would self-disconnect if the load current exceeds the set limit.

Regarding how much current a port on a mainboard can supply, USB specifications call for "at least 5 units of load", each unit being 100 mA (150mA and six units for USB3.x). Typically the overcurrent threshold is 40-50% higher than the minimum from specs, to accommodate for circuits production variability. So an average USB 2.0 port may deliver 900 mA without causing any USB issues, and the connector won't melt down (typical current rating of USB connectors is at least 1.5A). However, some product samples might be marginal and work "half-way", randomly disconnecting and recovering back. So it is up to your definitions of what is "bad" and what is not.

Regarding "low ampage supply" to plug into "high ampage device", it is an entirely different story, and this kind of mismatch will likely cause some power starvation somewhere in the device and a malfunction of it. Typical example would be SATA 2.5" HDD USB enclosures that can consume something like 1.8A peak currents from cable during spin-up or active track search, and sagging voltage would prevent the HDD to be recognized by USB-SATA bridge, and this mass storage device would fail enumeration.

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