How much current can USB source ?

[things]

Ive got a few things I wanna try powering off USb port, but i'm unsure of how much current a USB port can provide. Ive read about 500ma somewhere. is this true ?

 

[tresca]

typically its about 150-170mA. I believe that you can achieve 500mA but you have to request it from the host controller that you need that much.

 

[things]

How would it tell the host controller that it needs more than <200ma ?

 

[Gayan Soyza]

I always use 500mA fuses when taking currents from USB.

 

[tresca]

im going entirely off memory, because i had to do a usb project a few years ago but the project got scrapped for whatever reasons...

anyways...

you would need a usb device to send such a signal...
if you have a USB enabled microcontroller, you send the appropiate command through it, im not to sure if you can bit-bang. It is serially transmitted so I'm going to say you can, but seems complicated with timings.

Thats what I know anyways.

 

[things]

Well I just connected it to the laser driver, and the lasers are getting 420ma, which means the port must be capable of 500ma atleast, as the laser driver has been set to output 420ma.

 

[blueroomelectronics]

Some computers will shutdown the USB power if it's over 200mA unless the firmware on the USB device request more (upto 500mA)

 

[Leftyretro]

How would it tell the host controller that it needs more than <200ma ?

The USB standard is quite complex and power consumtion is part of the standard. As stated the first hundred or so MA is avalible at all times. The USB device, it it's load requires more is suppose to ask for it via communicating with the USB device controller per the standard and can have up to 500ma, and can then apply the load when the USB controller acknowleges the higher power request.

However I have read that some PC USB internal controllers make up to 500MA avalible even without using the USB protocal method and I've read that indeed some laptops will not supply 500ma. External USB hubs are probably all over the map, depending if they are externally powered or not.

I guess a lesson here is if you are developing something commerically or somthing you wish to work on all possible computers then you should reseach the USB power standards and always try and use as little power as possible.

I really kind of cringe at the latest types of electronics devices that plug into a PC USB port just for power, or recharging their internal batteries, and without actually talking to the PC, just using the 2 power pins from the USB connector. It sure seems like asking for trouble at the savings of just a cheap wall wart transformer. Why risk damage to the USB port or even the PC motherboard for just such a simple 2 1/2 watt power source

 

[pixnum]

Generally current varies from 100ma-200ma in USB port !