Okay to use USB Vbus to power TTL devices?

In the past I’ve burned up more than my share of semiconductors, including a few microcontrollers, by powering them directly from an unregulated 5V power supply. Well I’ve learned my lesson, and now I almost always use a regulator…

I say almost because in the past few weeks I’ve been connecting TTL devices to my computer via USB to Serial adapters (both Pololu and other brands) and there’s +5V, right there, with a dedicated labeled pin, begging to be directly connected.

I’ve opened up a couple of powered USB hubs, and some do have regulators between the AC Adapter and the bus. Others pass the power right through, so I wouldn’t try doing this with any of those. What about the 5V coming right out of my computer? The USB spec on Wikipedia says bus voltage should be 4.75–5.25 volts, which is similar to most TTL device ratings (i.e. 78LXX logic chips).

Is this a good idea? Does anyone know a reason this should or shouldn’t be done? Has anyone had bad (or good) experiences doing this? It’s been fine for me, so far…



The USB 5V is supposed to be regulated and good for up to 500 mA. On the hubs you’ve seen that pass the input voltage straight through, they probably use regulated power adapters. You should be safe using it for 5V projects.

- Jan