# How do I measure the voltage coming out of a USB port?

If I attach a USB cable to a USB port, and then strip the wire at the end, then how do I connect a multimeter to it to measure the voltage?

• Remember that drawing any more than 50 mA before asking for more (if you do at all) is a violation of the spec and potentially damaging to the bus (and all other devices on it). With the USB Charging Specification, devices are also able to draw up to 1.8 A from a USB port (if allowed). – Nick T Oct 21 '10 at 19:18
• it is 100mA without asking, not 50. – Kortuk Oct 22 '10 at 3:01
• Just to make sure awakeFromNib is clear, the 100mA limit has nothing to do with measuring the voltage with a multimeter since the multimeter will not load the circuit much. The only case that this does come into play is if he connects a large load, such as a 10 ohm resistor, across the vbus and ground pins. Doesn't sound like this is what he is asking about though. – Kellenjb Oct 22 '10 at 5:49
• How do you get past the 100mA limit? – 200ok404notfound Oct 23 '10 at 5:24
• As part of the enumeration, you negotiate how much power is needed. – rsaxvc Aug 28 '15 at 15:31

The standard pin numbers and colours in a USB cable are shown below.

Pin Number   Cable Colour   Function
1          Red         VBUS (5 volts)
2          White       D-
3          Green       D+
4          Black       Ground


If there is no device connected to the USB port then the maximum current that the port can support may be quite low. Once a device has enumerated itself on the port then it can negotiate for up to 500mA from the host controller.

To measure the voltage then you either touch the meter probes onto the stripped ends of the wires or, if your cable end is a large connector, then touch the probes (without shorting them together) onto pins 1 and 4 of the connector. The power pins on the USB connector are easy to identify as they are longer than the data pins. This is so that they mate first and ensures that the power is connected before the data.

• Ok, so how do I hook up my multimeter to the wires to verify the voltage that I am getting. – 200ok404notfound Oct 21 '10 at 9:16
• Added a measurement technique to the answer. – uɐɪ Oct 21 '10 at 9:28
• I had a bad experience with this approach. I checked one of the USB ports of a computer at work. I probed pins 1 and 4 with a multimeter in DC mode. The PC's fan started spinning fast, then blue screen of death. Fortunately the mobo was not damaged. But I still don't understand what went wrong. Never repeated the experiment again. – Mister Smith Jul 26 '13 at 8:58
• (Years later, hey, I'm here) I wonder if "Mister Smith" shorted something out. I repurposed a dead phone charging cable by cutting off the phone end and stripping the wires back to make my USB measurements using my multimeter. – user1683793 Feb 21 at 18:22
• Downvoting, becuase the pins on a "large" USB connector (ie, USB A) of either gender are quite close to an exposed and often grounded shield; it's really not easy to probe them both while being sure you cannot slip and short to the shield. – Chris Stratton Sep 7 at 23:41