On the RPi usb power cable - Revision history

Salamander: Correction (devices that have their own PSU also have their own cables, so it does not make sense using another (cheap) cable)

2012-07-04T06:56:08Z

Correction (devices that have their own PSU also have their own cables, so it does not make sense using another (cheap) cable)

Rew at 07:35, 19 June 2012

2012-06-19T07:35:57Z

Ghans at 18:35, 18 June 2012

2012-06-18T18:35:05Z

Rew at 09:17, 9 March 2012

2012-03-09T09:17:49Z

Rew: Created page with "We have tested several USB cables. As many USB devices do not use much power, the resistance of the conductors in the cable is irrelevant. At 5 ohms and 50mA of current the volta..."

2012-03-09T09:16:21Z

Created page with "We have tested several USB cables. As many USB devices do not use much power, the resistance of the conductors in the cable is irrelevant. At 5 ohms and 50mA of current the volta..."

New page

We have tested several USB cables. As many USB devices do not use much power, the resistance of the conductors in the cable is irrelevant. At 5 ohms and 50mA of current the voltage drop will be about 250mV. This is acceptable for most devices (and that's how it should be: the USB specifications require tolerance of such voltage drops).

On the other hand, some devices require more power. Those often have their own powersupply so they too will work with the cheap cables. And thirdly, phones that charge off USB will simply not charge as fast as they might with a good cable. In short, you won't often notice a higher resistance in USB cables.

However at 500mA a 5 ohm resistance will cause a 2.5V voltage drop. As the Model B will draw about 700mA, the voltage drop over such a cable will be quite unacceptable.

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 We have tested several USB cables. As many USB devices do not use much power, the resistance of the conductors in the cable is irrelevant. At 5 ohms and 50mA of current the voltage drop will be about 250mV. This is acceptable for most devices (and that's how it should be: the USB specifications require tolerance of such voltage drops).
-On the other hand, some devices require more power. Those often have their own powersupply  so they too will work with the cheap cables. And thirdly, phones that charge off USB will simply not charge as fast as they might with a good cable. In short, you won't often notice a higher resistance in USB cables.
+On the other hand, some devices require more power. Those often have their own power supply so they too will work with the provided cables. And thirdly, phones that charge off USB will simply not charge as fast as they might with a good cable. In short, you won't often notice a higher resistance in USB cables.
 However at 500mA a 5 ohm resistance will cause a 2.5V voltage drop. As the Model B will draw a peak current of 700mA, the voltage drop over such a cable will be quite unacceptable.
 The cable that comes with a modern phone that charges using the micro USB port will probably be fine.

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 On the other hand, some devices require more power. Those often have their own powersupply  so they too will work with the cheap cables. And thirdly, phones that charge off USB will simply not charge as fast as they might with a good cable. In short, you won't often notice a higher resistance in USB cables.
-However at 500mA a 5 ohm resistance will cause a 2.5V voltage drop. As the Model B will draw about 700mA, the voltage drop over such a cable will be quite unacceptable.
+However at 500mA a 5 ohm resistance will cause a 2.5V voltage drop. As the Model B will draw a peak current of 700mA, the voltage drop over such a cable will be quite unacceptable.
 The cable that comes with a modern phone that charges using the micro USB port will probably be fine.

← Older revision		Revision as of 18:35, 18 June 2012
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	We have tested several USB cables. As many USB devices do not use much power, the resistance of the conductors in the cable is irrelevant. At 5 ohms and 50mA of current the voltage drop will be about 250mV. This is acceptable for most devices (and that's how it should be: the USB specifications require tolerance of such voltage drops).		We have tested several USB cables. As many USB devices do not use much power, the resistance of the conductors in the cable is irrelevant. At 5 ohms and 50mA of current the voltage drop will be about 250mV. This is acceptable for most devices (and that's how it should be: the USB specifications require tolerance of such voltage drops).

← Older revision		Revision as of 09:17, 9 March 2012
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	However at 500mA a 5 ohm resistance will cause a 2.5V voltage drop. As the Model B will draw about 700mA, the voltage drop over such a cable will be quite unacceptable.		However at 500mA a 5 ohm resistance will cause a 2.5V voltage drop. As the Model B will draw about 700mA, the voltage drop over such a cable will be quite unacceptable.
		+
		+	The cable that comes with a modern phone that charges using the micro USB port will probably be fine.