Help: Isolating and ORing USB power - Page 1
Forum section: electronics / projects

author: rea5245
author_rank: Frequent Contributor
timestamp: September 16, 2015, 01:58:06 pm
content: Hi,I would like to build a device that has two USB inputs, which are used for power only (no data). The device should get power if either or both ports have hosts plugged into them. I assume I need to isolate the hosts' grounds and +5V from each other.There are modules available (e.g.http://www.recom-power.com/pdf/Econoline/RM.pdf) which apparently convert DC to AC, put it through a transformer, then rectify it back to DC. It looks as if I could have one on each port. My concern is, what happens if I tie the outputs of these together? The USB inputs are still isolated, so that's fine. But will +5V at the output damage something if there's no +5V at the input? Nothing in the data sheet seems to say. If the output is a typical rectifier + capacitor, everything should be fine. What if there's a regulator too? Or is it possible that the output stage isn't merely a rectifier and cap?Alternatively, is there an better way to do this?(My application is that I have a USB-controlled power strip: when there's +5V at the USB port, it turns on AC power. I want the power strip to turn on if either a Blu-ray player or a PS/3 is turned on - they both have USB ports that only go live when the device is on. So I need to "or" the +5V coming from the Blu-ray and the PS/3 into the power strip's single input.)Thank you,Bob

author: Ian.M
author_rank: Super Contributor
timestamp: September 16, 2015, 02:27:48 pm
content: If the Blu-ray player and PS/3 are connected to the same screen or audio amp, they almost certainly share a common ground. I am assuming that the power strip only senses USB Vbus and doesnt connect to the data signals.First check the power strip USB control input ground is *NOT* connected to mains earth. If its floating, you will almost certainly be able to use Schottky diodes to combine the USB Vbus and the Gnd wires from each device safely. Use two diodes, cathodes joined for Vbus and two with anodes joined for Gnd (to reduce the risk of trouble due to ground loops) and finally check that you have between 4V and 5V as the combined output (when loaded with a 1K resistor) when either or both devices are on., before making up a lead to connect it to the power strip. Do NOT connect D+ or D- at any device or to the power strip.N.B. this simple approach would be totally unsuitable for powering any real USB device (i.e. one that actually uses the data lines and enumerates)

author: rea5245
author_rank: Frequent Contributor
timestamp: September 16, 2015, 02:36:28 pm
content: Quote from: Ian.M on September 16, 2015, 02:27:48 pmThe PS3 and Blu-ray will be plugged into the same power strip. But even so, couldn't connecting their USB grounds together, even through diodes, create a ground loop?Come to think of it, the Blu-ray and PS3 would have transformers in them, so they're already isolated from each other. Is that why connecting their USBs would be OK?- Bob

author: ajb
author_rank: Super Contributor
timestamp: September 16, 2015, 04:55:47 pm
content: Presumably if you've got both the PS3 and the Blu-Ray player connected to the same TV then their signal grounds are most likely already joined via the signal ground/shield of the AV cables that connect them to the TV. But yes, unless they have grounded power cords both will be floating relative to the equipment ground unless joined some other way.If you really want to be safe, a pair of those isolated DC-DC converters with their negative outputs joined directly to the output ground and their positives joined to the output +5V via schottky diodes will certainly do the trick. Or, really, you could skip the DC-DC on one device and wire it straight through via the Schottky.