How to get power from USB cable?

[vishmi]

I can't get any voltage out my USB cable. I measured the voltage between the red and black wires of a USB cable while it was plugged into my laptop and the multimeter read 0 volts. When I connected these wires with a resistor it was still 0 volts. Does any one know why I'm not getting any voltage?

In case you want to know why I'm doing this, I plan on attaching a voltage divider to my USB cable so that it will match the required input voltage of my shaver, so I can charge it.

 

[farhada]

That is very strange, the power should be +5V, I have been using this for years.
Check out these links:
USB As A Power Source
Universal Serial Bus - Wikipedia, the free encyclopedia
**broken link removed**
**broken link removed**

 

[RCinFLA]

It has to be a USB mastering capable device. Many cellphones, MP3 player, external disk drives, and similar devices are not mastering capable (they can not initiate USB communications) and will not source 5 vdc.

Some computers save power by strobing 5v supply. You may have to tie a 1.5k from D- pin to Vbus to get source to maintain 5v supply.

 

[vishmi]

Thanks for your help. I have the voltage working now. I tried using another cable and it worked. Now I have a problem with getting enough current. I might be able to figure out the circuit if I knew the output impedance of the usb cable and input impedance of the shaver. i was getting strange results from my multimeter and so maybe i'm measuring impedance wrong. for the usb cable i got 0 ohms and for the shaver input it was sometimes 0 ohms and sometimes infinity. to get these results i just connected a multimeter lead with each of the two parts of the power wire (Vcc and ground). i tested the multimeter for known resistances and it tested normal. so basically, I need help figuring out the impedances.

 

[Enlightenment]

You might be running into the default maximum current, which is set to a low-power device of 100mA. The device, your end of the cable, is suppose to send a USB request to become a high-power device of 500mA (which I don't know how to do). Some USB interfaces will supply a lot more current, but don't expect it from all USB interfaces. If you need a bunch of current, then plug into a USB battery charger, because many will source up to 1 Amp, and the new ones that support iPad will source up to 2.1 Amp.

 

[john blue]

Dont draw over 500mA if not you might have chances of getting your USB burnt

 

[pranam77]

Dont draw over 500mA if not you might have chances of getting your USB burnt

Yes...this point is to really considered to protect the port from excess current drain, and damage. Afterall you get a USB charger/ adaptor with 5 volts output for just 1$ usable for all these devices which would be preferably safe instead using the PC/ laptop for special applications.

 

[vishmi]

Thanks again everyone. I figured the device input impedance is about 50 ohms because it has 4.9 V and 96mA. However, the device is rated for 3.6V and 100mA. Do you think the voltage will cause any problems? I was going to add a resistor in series to reduce the voltage across the load but any more resistance would make the current too low.

 

[vishmi]

also, my device has a different impedance when it has a different power source connected to it. (63 Ohms) is it normal for devices to have impedance that changes?