A polyfuse is a type of resettable fuse that automatically disconnects a power supply when too much power is drawn through the fuse.
The UD-3000 properly cut power to the offending port, and as a result, the DisplayLink device and other USB ports would continue to function in this situation.Įach USB port of the UD-3000 is equipped with a device called a polyfuse. When the next step after the last one on this chart was turned on with the current tester, the port was disabled, and no longer delivered power until the load was removed. The UD-3000 has a 4A power adapter, and actively tries to prevent attached devices from affecting the core functions of the dock (graphics, network, audio). While it claims 7.5 amps, on every test we did, the actual current was less than 7.5 amps because the draw had caused the supply voltage to drop, reducing the current. Once connected to the device under test, the power draw was increased from 0 all the way to 7.5 amps, and the voltage recorded at each stop. Everyone here was really hoping for a fire (for better or worse, we weren’t able to create that much excitement even at the highest load). We wanted to be able to take a hub and its power supply to the maximum, then a little further. Remember that the limit for USB 2.0 is only 0.5 amps–so we can pull up to 15 times more current than a host should be able to deliver. By selecting different combinations of resistors, it can draw from 0.5 A all the way to 7.5 A in 500 mA steps. Each set of resistors is chosen so that they draw 500 mA, 1 A, 2 A, and 4 A from a 5 volt supply. Each of the switches at the top turns on one bank of resistors below.
The red and black connectors are connected to the ground and +5V lines of the USB bus, and connected to a meter so that the bus voltage can be monitored. The box itself was designed in SketchUp and printed on a Printrbot 3D printer. So what happens when a device pulls more power than a hub or charger expects? What does a graceful or non-graceful failure look like? We wanted to know. The USB-IF has a standard for doing this called the Battery Charging class, but the big guys like Apple, Samsung, and others don’t follow it universally. There are many proprietary methods devices use to detect “their charger” and start pulling higher amperages. Hubs and chargers are supposed to fail gracefully if a device pulls too much.Īnd it’s all gotten more confusing as USB has become a primary way of charging phones and tablets.
This is to avoid voltage drops which might knock other devices off the bus. Devices are supposed to follow certain rules, like pulling no more than 100mA unless they configure up to a higher amperage (500mA for USB 2.0 and 900mA for USB 3.0). Devices can pull as much power as they want, but there are consequences. It’s natural to assume that hubs “deliver” a certain amount of power to a device. USB power and charging issues are surprisingly complex.
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