Safely Drawing 5V Power from USB Port: Current Limits, Wiring, and Considerations

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#1 21659340 02 Aug 2011 12:45

Bob Casiano Bob Casiano

Anonymous

Post #1
21659340 02 Aug 2011 12:45

I'd like to utilize the 5 V from a USB port. Are there any considerations, requirements, for taping into the USB port?
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#2 21659341 02 Aug 2011 12:58

Chris Roth Chris Roth

Anonymous

Post #2
21659341 02 Aug 2011 12:58

The current limit on the usb port is 500 mA.
#3 21659342 02 Aug 2011 12:59

Cody Miller Cody Miller

Anonymous

Post #3
21659342 02 Aug 2011 12:59

I am not sure if there is a protocol for being a power only device on a USB hub. It is a good question. Here an image of the pinout from "source":http://www.connections-usa.com/usb_pinout.html
#4 21659343 02 Aug 2011 12:59

Dan Oh Dan Oh

Anonymous

Post #4
21659343 02 Aug 2011 12:59

I also have been looking for this type of information. Appreciate if you share the informtion once you have obtained. Thanks.
#5 21659344 02 Aug 2011 13:05

Bob Casiano Bob Casiano

Anonymous

Post #5
21659344 02 Aug 2011 13:05

Sure thing.
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#6 21659345 02 Aug 2011 13:10

David Figueroa David Figueroa

Anonymous

Post #6
21659345 02 Aug 2011 13:10

Well, First you have to consider the load and the current draw from the USB port. You first have to have to find out the specs on the corrects on the deices you are going to be running.second are you making a interface cable??..No, yes..?
#7 21659346 02 Aug 2011 13:15

Bob Casiano Bob Casiano

Anonymous

Post #7
21659346 02 Aug 2011 13:15

My project will use up to 250mA max. I plan on using a mini-usb jack on the board, so the USB cable will be off the shelf.
#8 21659347 02 Aug 2011 13:20

Bob Casiano Bob Casiano

Anonymous

Post #8
21659347 02 Aug 2011 13:20

Thank you for the pin out. This will help.
#9 21659348 02 Aug 2011 13:28

David Figueroa David Figueroa

Anonymous

Post #9
21659348 02 Aug 2011 13:28

The USB configure should be
pin 1-- 5 Volts
pin 2-- data-
pin 3--data+
pin 4--ground
shield ..Think this should be of help..
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#10 21659349 02 Aug 2011 13:43

Jeff Evemy Jeff Evemy

Anonymous

Post #10
21659349 02 Aug 2011 13:43

It is true that the limiting power is 500mA, although many ports will expect a lower power (typically 90mA) as standard and will give the familiar 'USB power surge' message if yiu try to draw more - although it'll probably still work - it's just a bit messy.

If you are adding any sort of interface to the USB cable then the current required can be specified during the enumeration process (up to 50mmA - not sure what happens if you ask for more).

At first introduction, USB interface design can seem pretty overwhelming, but a really good (and cheap) entry method is to use one of the communication management chips (such as the FT232) from manufacturers like FTDI (http://www.ftdichip.com/index.html q.v.).

I've used these in several commercial products involving PC interfaces and have found them both easy to use and precise in their performance.

Best of luck
#11 21659350 02 Aug 2011 14:05

Alec Alec

Anonymous

Post #11
21659350 02 Aug 2011 14:05

I agree with Jeff on the FTDI chips. It's surprising how many semiconductor manufacturers use them on their demo and eval boards anytime they need to accommodate a USB interface to a PC.
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#12 21659351 02 Aug 2011 14:32

Olin Lathrop Olin Lathrop

Anonymous

Post #12
21659351 02 Aug 2011 14:32

You're not borrowing power from USB since there's no mechanism for giving it back. The USB power line is not a storage device.

If you are just looking for a little power at 5V, then simply connecting up to the USB power will work on most desktop machines. Most desktop motherboards simply have a polyfuse between the USB power out and the internal 5V supply. The maximum a USB device can draw is 500mA, so if there is just a polyfuse it must be set to somewhat above that.

Laptops and other mobile devices, with their need to manage power more carefully, can be different. A USB device is guaranteed 100mA in normal operation. It can ask for more during enumeration, but the OS could decide it's asking for too much and shut the device down. There are also special standby modes where the device must draw considerably less than 100mA. So if you're thinking of stealing power from a laptop via USB, it might be compilcated or you don't get much without enumerating. It will also vary between laptops.
#13 21659352 02 Aug 2011 14:34

Olin Lathrop Olin Lathrop

Anonymous

Post #13
21659352 02 Aug 2011 14:34

True, but by itself that statement is a bit misleading. A device is only guaranteed 100mA during normal operation unless it requests and is granted more, for example.
#14 21659353 02 Aug 2011 14:58

Dorin Dragan Dorin Dragan

Anonymous

Post #14
21659353 02 Aug 2011 14:58

The USB 2.0 port is limited (as has been said) to 500mA. I had a project during university that required power over USB and I think depending on host it can provide more or less units of power, one unit being 100mA.

I suggest like Jeff did, if it also involves data transfer over USB you should use FTDI chips; their well documented and easy to use.

If you want to test how well it works with the port USB and if it's compliant I guess there are some developer tools to help with that over here (haven't used them and don't know if they're still valid but I did stumble upon them while I was researching USB myself):

http://www.usb.org/developers/tools/
#15 21659354 02 Aug 2011 15:34

Bob Casiano Bob Casiano

Anonymous

Post #15
21659354 02 Aug 2011 15:34

Thank you for all your responses.

This project will only involve the +5v/Gnd pins on the USB port. So, as long as the system under use does not decide to shutdown the USB port under use, then all is good. Of course, the device will be within the USB parameters.

Thank you all again!
#16 21659355 02 Aug 2011 15:43

Alec Alec

Anonymous

Post #16
21659355 02 Aug 2011 15:43

I looked up the official USB 2.0 specification, and Olin is very much correct about the 100mA limit without requesting more.

I was surprised, I didn't know that. I'm glad he mentioned it or I might have had to find out the hard way someday, thanks Olin.
#17 21659356 02 Aug 2011 16:13

Alec Alec

Anonymous

Post #17
21659356 02 Aug 2011 16:13

A picture of the spec...
#18 21659357 05 Aug 2011 22:57

Bob Casiano Bob Casiano

Anonymous

Post #18
21659357 05 Aug 2011 22:57

This is good to know. Although, I was hoping for a simple passive solution to "steal" a little more power from the USB port or rather trick the OS to allocate more. Maybe version 2...
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Topic summary

✨ Drawing 5 V power from a USB port requires consideration of the USB power specifications and port behavior. Standard USB 2.0 ports provide up to 500 mA current, but devices are initially guaranteed only 100 mA unless they request and are granted more power during enumeration. Drawing more than the default current without proper negotiation may trigger power surge warnings or cause the port to shut down, especially on laptops and mobile devices with stricter power management. The USB pinout for power-only connections uses pin 1 for +5 V and pin 4 for ground, with pins 2 and 3 reserved for data lines. Using off-the-shelf USB cables with mini-USB connectors is common for such projects. For devices involving data transfer, USB interface chips like the FTDI FT232 series are recommended due to their ease of use and reliable performance. Developers can refer to official USB specifications and tools for compliance and enumeration protocols. Passive methods to draw more power without enumeration are generally unreliable and not recommended.

FAQ

TL;DR: You can draw 5V from USB; USB 2.0 allows up to 500 mA, but “A USB device is guaranteed 100mA” unless it enumerates for more. [Elektroda, Olin Lathrop, post #21659351]

Why it matters: This FAQ helps makers and engineers wire USB power safely, avoid port shutdowns, and size loads correctly.

Quick Facts

USB 2.0 default is 100 mA; devices may request more during enumeration. [Elektroda, Olin Lathrop, post #21659351]
Practical USB 2.0 ceiling is 500 mA per port under spec-compliant conditions. [Elektroda, Olin Lathrop, post #21659351]
Typical pinout: 1=+5V, 2=D−, 3=D+, 4=GND, shield to chassis. [Elektroda, David Figueroa, post #21659348]
Some hosts flag a “USB power surge” if draw exceeds default power (seen near ~90 mA). [Elektroda, Jeff Evemy, post #21659349]
Laptops may cut or limit power if you skip enumeration or exceed policy. [Elektroda, Olin Lathrop, post #21659351]

Can I power my project from a USB port safely?

Yes, if you respect limits and wiring. USB 2.0 guarantees 100 mA without negotiation. You can request more during enumeration. Desktops often protect ports with a polyfuse tied to the 5 V rail. Laptops may enforce stricter policies and shut ports. [Elektroda, Olin Lathrop, post #21659351]

What is the maximum current without enumeration?

Without enumeration, design for 100 mA. That is the guaranteed current in normal operation. To exceed it, the device must ask and be granted more. “A USB device is guaranteed 100mA in normal operation.” [Elektroda, Alec, post #21659355]

How do I get more than 100 mA from USB?

Implement USB enumeration and request additional current. The host operating system may approve or deny the request. If it denies, it can shut the device down. Plan for policy differences across hosts, especially laptops. [Elektroda, Olin Lathrop, post #21659351]

What happens if I try to pull about 250 mA without asking?

You may trigger a “USB power surge” warning. Some hosts default to lower allowed current, around 90 mA. The device might still run but this is messy and unreliable. Avoid this approach in products. [Elektroda, Jeff Evemy, post #21659349]

What’s the correct USB 2.0 pinout for power-only wiring?

Use: Pin 1 = +5 V (VBUS), Pin 2 = D−, Pin 3 = D+, Pin 4 = GND, and connect the shield appropriately to chassis. For power-only, route Pin 1 and Pin 4. Verify cable and connector ratings. [Elektroda, David Figueroa, post #21659348]

Do laptops and desktops behave differently for USB power?

Yes. Desktops often use a polyfuse from the 5 V rail and are tolerant within spec. Laptops enforce tighter power management. They can limit, require enumeration, or shut the port if current is excessive. [Elektroda, Olin Lathrop, post #21659351]

What is USB enumeration in this context?

Enumeration is the process where a device identifies itself and requests resources. For power, a device can request more than 100 mA. The OS decides whether to grant that budget. Without it, only 100 mA is guaranteed. [Elektroda, Olin Lathrop, post #21659351]

Can I trick the OS to get more power without data lines?

No reliable method exists. Drawing extra current without enumeration risks shutdowns or warnings. Hosts, especially laptops, may cut the port. Design a proper USB interface if you need more current. [Elektroda, Olin Lathrop, post #21659351]

Should I use an FTDI bridge if I also need data?

Yes. FTDI parts like the FT232 simplify USB communication and current budgeting during enumeration. They are well documented and proven in commercial products. “Easy to use and precise in performance.” [Elektroda, Jeff Evemy, post #21659349]

Any tools to check USB power compliance?

Yes. The USB-IF host and device tools help validate behavior and compliance. Use them to observe negotiation and current draw during development. This reduces field failures and returns. [Elektroda, Dorin Dragan, post #21659353]

What is a polyfuse and why does it matter on USB?

A polyfuse is a resettable fuse. Many desktop motherboards place it between the internal 5 V rail and the USB VBUS line. It limits fault current and self-resets, protecting the host during overloads. [Elektroda, Olin Lathrop, post #21659351]

How do I wire a mini‑USB connector for 5 V power only?

Connect Pin 1 to your 5 V input through appropriate filtering.
Connect Pin 4 to system ground; tie shield to chassis as needed.
Leave D+ and D− unconnected if no data is required; respect current limits. [Elektroda, David Figueroa, post #21659348]

What are USB power units in the spec?

Power is discussed in units of 100 mA. Hosts may provide more or fewer units depending on policy and negotiation. Design conservatively around these units for portability across hosts. [Elektroda, Dorin Dragan, post #21659353]

Will a simple power-only dongle always work?

It can work on many desktops if you stay within 100 mA. Exceeding that without enumeration risks port disable on laptops. Build for enumeration when loads vary or exceed 100 mA. [Elektroda, Olin Lathrop, post #21659351]

Edge case: what if the host is in a low-power or standby mode?

Some modes require devices to draw well under 100 mA. Hosts may cut VBUS or enforce strict limits. Budget your standby current accordingly to avoid disconnects. [Elektroda, Olin Lathrop, post #21659351]