ProDigit wrote: ↑
Mon Aug 19, 2019 11:55 am
If you've got to power it externally anyway, a Y splitter is your only option.
No it isn't. As you say so yourself below.
If it could damage the pi, then go for option 2:
A powered USB hub.
Because it doesn't matter if you use a Y splitter cable onna secondary PSU, or use the same PSU as the Pi's PSU. The voltage on the 5v output will always be higher than the voltage on the Pi's USB 5v output.
It's by no means a given that the external PSU will have a higher voltage than the Pi's USB port. A PSU can supply less than 5v and still be in the USB spec (5v +/- 5% I think) and it depends on how good (well regulated) both PSUs are
If the two PSU aren't perfectly matched you'll hit problems.
Separate USB hub seems your best option, or, use a Y splitter cable with a Diode in series on the positive lead.
It'll cause a voltage drop from the pi, causing it to provide less power, while at the same time, preventing any over voltage from your secondary circuit to go back to the pi.
The only thing common, would be common ground, which might cause some imbalance, but ground should be ground, .. zero voltage...
Nope. The two nominal +5v lines are cross connected otherwise there's no advantage to using a Y cable. If one of those is pushing 4.9v and the other 5.1v (both values are in spec) expect problems.
Anyway, powered hub is the safest option.
Yes. Or possibly a modified Y cable with one of the +5v lines cut.
Arguing with strangers on the internet since 1993.