The bus-powered hub carries no brick – its whole power budget is the host port’s 900 mA, and the budget’s arithmetic explains every success and brown-out the class experiences. This guide does the arithmetic.

The 900 mA Arithmetic

USB 3.0’s unit load: 900 mA per port at the host. The bus hub consumes one unit load upstream (its own silicon and LEDs: ~100-150 mA), then distributes the remainder across downstream ports – each port guaranteed 100 mA (the enumeration floor that lets devices announce themselves), with negotiation for more up to the pool’s remainder. The arithmetic’s consequences: four flash drives (each content at 100-200 mA) fit comfortably; one 2.5-inch HDD (900 mA running, 2A surge) cannot – the drive alone exceeds the hub’s whole downstream budget, and the spin-up surge browns out the hub. The math explains the class’s entire reputation: flawless with peripherals, dramatic with drives.

The Enumeration Floor

Every device gets 100 mA just for showing up – the floor that makes mice and keys universal. The devices that need more negotiate; the pool’s remainder is the ceiling on the negotiation.

What Works, What Brown-Outs

Served confidently: mice (50-100 mA), keyboards (100-150 mA with backlight variance), flash drives (100-200 mA), one bus-powered SSD at a time (500 mA-class, fits the remainder), card readers (100-300 mA during bursts). Brown-out territory: 2.5-inch HDD spin-ups (the 2A surge), 3.5-inch drives (12V never arrives anyway), two SSDs at once (the remainder splits and each throttles), phone charging (the 1.5-2A ask exceeds the pool), and RGB everything (the lighting tax compounds). The symptom ladder: the plugged device that works alone but not with friends is the budget’s arithmetic, not the device’s defect.

The One-SSD Rule

The bus hub serves one external SSD well – the remainder fits. The second SSD splits the pool: both run, both slower. The rule keeps the desk honest.

The Duty Mapping and the Upgrade Line

The bus hub’s habitat: the peripheral desk (mice, keys, dongles, flash media), the laptop bag (portability’s argument – no brick to pack), the fixed single-drive duty (one SSD as the only big talker). The upgrade trigger: any HDD, any charging plan, any second SSD – the powered tier’s brick dissolves the budget. The buying lines within the class: silicon named, the enumeration behaviour clean (devices recognize on hot-plug), and cable integrity (the 3-foot rule). The class’s price ($10-20) prices its arithmetic: the 900 mA budget is the product, and the buyer who knows the budget buys once, correctly.

The Bag Argument

The bus hub’s honest advantage is the bag: no brick, no outlet dependency, grams of weight. The travel kit’s hub is bus-powered by design – the peripherals travel light.

Bus-Powered Budget Table

Real-World Application Scenarios

A writer’s travel kit: bus hub in the bag serves mouse, dongle and flash media at every cafe – the brick-free kit’s whole point. An office’s peripheral desks run bus hubs under the monitors; the drives live on the powered unit across the desk. A student learned the arithmetic empirically: the mouse that died when the HDD spun up was the 900 mA lesson.

Recommended Internal Resources

(1) Product cluster – https://zh-zong.com/productlist/ (anchor: bus-powered hubs for peripheral desks, powered tier for drives); (2) FAQ – https://zh-zong.com/faq/ (anchor: budget and brown-out questions); (3) Contact – https://zh-zong.com/contact/ (anchor: describe the device mix, get the tier answer).

• bus-powered hubs for peripheral desks, powered tier for drives
• budget and brown-out questions
• describe the device mix, get the tier answer

Related Reading in This Series

• 4-Port USB Hub: The Desk Standard, Sized and Spec’d
• 2-Port USB Hub: The Minimal Multiplier and Its Real Jobs
• 6-Port USB Hub with Switches: Per-Port Control, Done Right

Frequently Asked Questions (FAQ)