Electronic – USB Power – The basics

You can't count on what a device will do when it doesn't receive valid USB power. For example, it might not even turn on its pull up/down resistors to indicate the speed until it thinks it's plugged into a USB, which it might detect by looking at the bus power voltage.

If you can tolerate a little lag when a device is plugged in, wake up periodically and turn on the USB power for a little while. One problem is that devices might themselves take a little time to come up once power is applied. Some of my PIC programmers, for example, have deliberate waits of a few 100 ms after they run before turning on the USB. This is to make sure everything is truly stable before talking to the host. There is no guarantee how long you need to wait for a device to want to talk to the host after being powered by the USB. A half second is probably good enough for most cases, but you can't know that for sure.

A more reliable strategy would be to make a converter that takes very little power when no current is drawn from its 5V output. That shouldn't be too hard. If done properly, the drain on a 5V output with nothing connected can be a few microamps or less.

USB is optimized for light weight devices at the expense of more burden on the host. There is no free lunch.

You have a bench supply and USB supply with AC mains feed.
These may have two wire or 3 wire (phase + neutral + ground) leads.
The outlets used may or may not have grounds.
If both outlets and leads do not provide grounding so that both power supplies have third wire grounded input grounds then the following is likely (based on what you describe and my experiences):

- You are very probably getting coupling from mains via a Y capacitor in each lead of mains to input ground.

• If you measure the 0.4 VAC with no load and a decent AC meter you will probably see about Vmains/2 or 55 VAC or so in USA.

  - This capacitor coupled mains voltage can destroy things.

• Y caps may be 0.001 or 0.01 uF.

"More data needed" including certainly whether PSU ground is connected to mains ground, what a "Bus Pirate" is (Gargoyle will for sure know but I/we ought not need to ask it) etc.

BUT some supplies have isolated in and out DC wise and Y capacitors from phase & neutral connected to input ground. If you use a 2 wire power feed and connect in-gnd to out-gnd you get mains/2 capacitively coupled. If you are getting bitten you have >>> 0.4 VAC and probably half mains via Y caps.