Simple Question: can USB be used to power a picaxe project drawing about 400mA

[Alex Leech]

Hi all !

Nice easy one for you !!

Can USB be used to power a picaxe project drawing about 400mA ??

And can i just wire it straight to the ground and +5V on my board if the answer is yes??

Just thought i better check !!!

Charz !!!

Alex

 

[lanternfish]

Hi all !

Nice easy one for you !!

Can USB be used to power a picaxe project drawing about 400mA ??

And can i just wire it straight to the ground and +5V on my board if the answer is yes??

Just thought i better check !!!

Charz !!!

Alex

The USB standard specifies a single unit load of 100ma with an optional maximum 5 unit current draw of 500mA.That does not necessarily mean that your USB port can supply that 500mA current. Externally powered USB ports may.

Only one way to find out

 

[womai]

Short answer - basically yes.

Longer answer:

The USB spec requires the device to draw not more than 100 mA during startup (until it is recognized by the PC). After that it can draw up to 500 mA. If you are using a USB-to-serial-Cconverter (e.g. FTDI's FT232R) the maximum consumption can be programmed into the flash memory of that converter.

Second, while USB nominally supplies 5V, the actual voltage can be significantly lower. In my experience, if the device is connected through an unpowered hub (i.e. not plugged into the computer directy, or plugged into a hub that has its own power supply), the voltage can be as low as 4V.

That said, my latest design (a USB oscilloscope) gets powered through USB using the FT232 chip, and draws almost 300mA with no problems whatsoever.

 

[kewakl]

Short answer - basically yes.

Longer answer:

The USB spec requires the device to draw not more than 100 mA during startup (until it is recognized by the PC). After that it can draw up to 500 mA. If you are using a USB-to-serial-Cconverter (e.g. FTDI's FT232R) the maximum consumption can be programmed into the flash memory of that converter.

Second, while USB nominally supplies 5V, the actual voltage can be significantly lower. In my experience, if the device is connected through an unpowered hub (i.e. not plugged into the computer directy, or plugged into a hub that has its own power supply), the voltage can be as low as 4V.

That said, my latest design (a USB oscilloscope) gets powered through USB using the FT232 chip, and draws almost 300mA with no problems whatsoever.

This response is along the lines of my posting about the SparkFun Programmer
I want a programming adapter that ALSO provides the supply voltage(s) for projects in development.
Currently I am making an adapter for the Axe027 stereo to *whatever* to a header that supplies the programming pins and the folowing voltages (5.0, 3.3, 3.0)
I would like to get the 5V from the laptop KYBD port(or USB), with (LDO) regs for 3.3V and 3.0V
I am completely happy with my axe027, just want to get rid of the batteries/wallwarts for development!

The last spec that I remember was ~1Amp from the KYBD port. Maybe not so much on a laptop port.

 

[hippy]

Note that the 500mA USB supply maximum is 'per host controller' not 'per USB socket', so if you have one host controller, a PICAXE drawing 400mA, the USB-to-Serial, keyboard and a mouse ech drawing 100mA you will be well over the allowed limit.

While it seems one can often get away with some 'abuse' of the USB specification one wouldn't have much cause for complaint if that did permanently damage a host controller or motherboard.

Don't forget that circuits taking power from USB ports can often have in-rush and instantaneous currents in excess of 500mA. Any faults in the circuit can cause permanent damage to any of the components in the power supply chain. It would be recommended not to use USB ports to supply power in this way, and if doing so to connect via a powered USB hub.

 

[manuka]

I've never had fatal mother board crippling USB port failures - has anyone else? My experiences with USB ports has shown overloads indeed will shut down ports, but (when corrected) these have always come right again after a reboot. Stan.

 

[hippy]

Plenty of examples of such on Google though it's not always clear what the failure was caused by. The outcome probably depends on failure mode. Data lines sit at 3V3 or lower depending on mode and shorting those to 0V or 5V, connecting any signal to higher than 5V or negative voltages may be much more damaging than shorting the 5V supply to 0V or drawing too much current. A PICAXE can often recover from short term fault conditions but be damaged by longer term faults and be instantly destoyed by others; I can see no reason to believe other components cannot behave in a similar way. Some motherboards have SMD fuses, some faults may blow them, some may not.

The difficulty is that one cannot guarantee what will happen when a fault occurs. One may get lucky, maybe not.

 

[kewakl]

I've never had fatal mother board crippling USB port failures - has anyone else? My experiences with USB ports has shown overloads indeed will shut down ports, but (when corrected) these have always come right again after a reboot. Stan.

Yes, but not from overloading. I suspect static discharge, but cannot prove this.
I had a Gigabyte motherboard with a cell phone attached via the phone maker's USB cable.
I moved the phone, while connected, across my desk. The PC speakers made a popping noise, the PC stopped responding. I tried to reboot... Nothing.

I opened the case, and there it was, the faint smell of Dippy's magic smoke.
On further investigation, I noticed the slightest bump on the Southbridge chip. Bye motherboard.

 

[inglewoodpete]

I've not had any fatal problems due to overloaded USB. I've had a group of 4 USB ports stop (included mouse and keyboard).

Fortunately everything came good when I rebooted the PC. However, a tense couple of minutes until the ports came up after the reboot!

 

[westaust55]

Power from USB ports

And here is a thought (untested by me) . . .

ASUS, and possibly other makes of, motherboard do not turn off the USB power when Windows shuts the PC down.

Ostensibly this is to provide power for various wake events.

Thus the USB power is always available to charge various devices and operate others when the operating system is shut down.

So possibly, for the brave PICAXE users, you may be able to power your PICAXE from the PC USB port even with the PC turned off using a software shutdown.

 

[gengis]

USB from a motherboard is limited to ~120 ma, but an externally supplied (wall wart style) USB expander may. I have one that works without a wall wart at ~100 ma and with a wall wart at ~1 amp total between one or four output ports.

 

[LizzieB]

There are also USB Y cables available to power things like USB disk drives that take more power than one port can deliver. I think there must be some current limiting on each port otherwise those cables wouldn't make much sense since the ports are likely to be on the same host/controller.

 

[westaust55]

USB Ports

For USB ports, (from: http://www.usb.org/developers/docs/ )

A unit load is defined as 100 mA in USB 2.0, and was raised to 150 mA in USB 3.0.

A maximum of 5 unit loads (500 mA) can be drawn from a port in USB 2.0, which was raised to 6 (900 mA) in USB 3.0.

There are two types of devices: low-power and high-power:
- Low-power devices draw at most 1 unit load.
- High-power devices draw the maximum number of unit loads supported by the standard.

All devices default as low-power but the device's software may request high-power as long as the power is available on the providing bus.


Also an article from early 2008:
http://www.usb-core.co.uk/03-04-2008-toshiba-announces-sleep-and-charge-usb-ports.html

 

[Alex Leech]

And here is a thought (untested by me) . . .

ASUS, and possibly other makes of, motherboard do not turn off the USB power when Windows shuts the PC down.

Ostensibly this is to provide power for various wake events.

Thus the USB power is always available to charge various devices and operate others when the operating system is shut down.

So possibly, for the brave PICAXE users, you may be able to power your PICAXE from the PC USB port even with the PC turned off using a software shutdown.

I can back up this one. With my gigabyte motherboard as long as at least one USB device is plugged in upone shut down, all the usb ports will stay powered.