ORing +5V from two USB hosts

[rea5245]

Hi,

I have two USB hosts, a PC and a Sony Blu-Ray player. When either one is on, I want the +5V from its USB port to trip a solid state relay to provide power to my plasma TV (which would otherwise be a monstrous energy vampire).

My first thought is to have diodes on the +5V wires, then tie them together at the SSR. The ground wires would be tied together directly. But I'm worried whether I need to isolate the grounds from each other (and whether the diodes sufficiently isolate the +5V lines from each other).

If I do need to isolate them further, how would I go about it? I can imagine a solution with two SSRs in parallel. I can also imagine two low-power relays in parallel that turn on an independent 5V supply that then turns on the SSR. But both of those solutions seem like overkill and I'm hoping for something simpler.

Thanks,
   Bob

[SeanB]

Use the 2 diodes and common the ground and the USB shield wires. They are both connected together ground wise through the TV set.

[hlavac]

USB is usually not isolated, usb ground is usually connected to the case and mains earth.
Two diodes will work fine with SSR, don't forget current limiting resistor the SSR control input is usually an IR LED. SSR that switches on zero crossing will be best.

[rea5245]

Thank you SeanB and hlavac.

hlavac, I hadn't thought about the limiting resistor, but I just checked the SSR data sheet and it says the input impedance is 1500 ohms. Looks like I'm OK without a separate resistor. But thank you for pointing it out; it's better to check these things than to be unaware of the issue.

- Bob

[Psi]

1N5817 is pretty common and has a low vdrop.

[ptricks]

Also check that the USB ports are indeed powered off when the pc is off, some pc leave the USB power intact which would mean the relay wouldn't ever turn off.