I was planning to use the Teensy attached to an self-powered USB hub and use VUSB to power and control some
relays (4x 20mA).
The relay board says 10-20 mA, but from the size of those relays, my guess is the 10 mA is only to turn on the LED inside each optocoupler, which appears to turn on a little transistor that then turns on the relay. My guess is the board will draw quite a lot more current from the VCC pin when those relays are on.
Of course, that's all guesswork on my part, from only those photos. Like most cheap Chinese products, there's little reliable info. You'll probably only find out the real current using a multimeter.
Does the placement of the fuse mean that the hub is safe when the relays play havoc but the Teensy might get killed?
It's always difficult to predict exactly how things will fail, especially when using 2 other non-PJRC products, each made as cheaply as possible in China and sold without good technical specs or schematics.
But I'd guess too much current will simple drag to the 5V power down to a level where things don't work, or fail mysteriously. I'd recommend monitoring the 5V power with a multimeter.
And that I am restricted to 500mA total (Teensy + relays + stuff)?
TL;DR = yes, you're limited to 500 mA, if the hub is "self powered" (which in the USB specs means it has a power adaptor).
I usually use the terms "powered" and "unpowered", which aren't as formal as the USB spec, but people see to find less confusing in conversation. If the hub is unpowered, it may not be able to deliver 500 mA.
Also, you should be aware the PTC "fuses" are crude devices. They're merely a resistor that increases value. They work by having metal particles mixed into a ceramic that thermally expands, adding more gaps between the metal as it gets hot. Before it gets hot, a pretty incredibly large current can flow. Of course the larger the current, the faster is heats. Because it's only a resistor, the current is never shut off. The PTC only adds more resistance in series with Teensy, until the circuit reaches some equilibrium based on the input voltage, PTC resistance and whatever is trying to draw so much current. That's a lot better than a dozen amps flowing through the thin PCB traces and burning them, but it's certainly not a highly precise process. It's only a little chunk of thermally expanding ceramic with little metal bits mixed in.
