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]
