Hot USB Switched Hub (mine and others)

[MrAl]

Hello there,

Recently i decided it would be nice to have a USB hub that has switches on it so that each port can be switched 'off' or 'on' as needed. I found a number of these special hubs on the web and bought one of them. The one i bought has four switches and four ports and connects to the computer via a mini USB to USB-A connector/cord.
It didnt work too bad i guess, but with the web cam it seemed to add some interference to the picture. The main point though is that each hub could be switched on or off, and that's what i needed for my USB drives. I suspected that the switched hub was only disconnecting one wire, so i opened up the hub (some small screws) and checked inside. Sure enough, as strange as this sounds, the switches only disconnect one wire of the four USB wires, and that one wire was the *ground* wire. Strange as that seems, the only advertising they mention is that the hub is able to save power draw on the USB power from the port so by turning off your unused USB devices you can save power. So in other words, they didnt make it so that it could electrically isolate the ports once turned off. I wanted total isolation, total electrical disconnection. This led to some experiments and finally i ended up building one out of the original 4 port hub.

After removing the four switches and jumping all the grounds so that they were connected permanently, i installed switches on two of the 4 ports that would break three of the four connections on the USB device. It was a real pain because the circuit board had to be modified, but what i ended up with works really nice. It's now a four port hub with only two switched ports and two 'constant on' ports, but the two switches electrically isolate the USB devices plugged into it on those two ports.
I'm posting a pic of the near finished unit.

This also got me wondering if these kinds of ports might be made by some company already or not. I've seen lots of switched hubs now but they all advertise that they cut the power, and dont mention the electrical isolation as above. I would want one that switches all 4 lines off if possible, or at least 3 of the four lines (+5, D+, D-, leaving ground intact).
If anyone knows of these kind please let me know. Thanks.

[frhrwa]

as to being made by another company, probably not, but, by the time I finish writing this note, probably so.. someone for sure picked up on it and is in the process of building and marketing them right now... SHOULD have been you.. good idea..

[MrAl]

Hi again,


Yes, i am so surprised that i cant find any of these. Only the ones that disconnect the power, which is done with one single SPST switch. I have (or rather had) this type and although it did cut the power the data lines still stay connected to the internal circuitry. I wanted complete protection without having to pull out the drive all the time. My usage demands using for a few files, then pulling out, then later using for a few more files, then pulling out again, etc. I'd have to plug and unplug a USB drive 10 times per day sometimes which is ridiculous really.

The kind that cut the power only with one SPDT switch are not entirely bad or anything, but they dont contain the kind of protection circuitry a USB isolator circuit or protector circuit would. There is a chip between the drive and the computer port, which is better than nothing i guess, but a true port protector has diodes to clamp levels to 5v or ground, so the max any pin can see (without really extreme failure on the part of the computer port) is either ground 0v or plus 5v. That's a bit better than just a USB hub chip.

I thought about making a USB protector circuit but it's more trouble than just installing a switch. I also thought about getting a USB isolator circuit which would be really nice. These things actually isolate the power electrically even while the port is being used by your device! Nice except they are slow (12Mb/s) and also are very very expensive on the order of 100 US dollars or higher.

[Robert Reed]

MrAl
"There is a chip between the drive and the computer port,"
If these chips are C-Mos devices, would they not have the diode protection already on their inputs?

[MrAl]

Hello Robert,


Well that's a good point, but i think the diode protection circuits would use a higher current diode and also zeners i would think, rather than a clamp diode to +5v. Im not entirely sure of this yet, and i think there may be better and not as good type protection circuits too. Depending on the failure mode, i would think the CMOS diodes might work to some degree. Trouble is, i dont know all of the possible failure modes of a computer USB port and how likely each one would be compared to the others.
For example, the USB can put out 500ma and i know that the CMOS diodes are usually only rated for 20ma tops, but im not sure if a failure mode would be really 500ma or less.
I suppose that one failure mode would be that the power supply fails somehow and puts +12v on the +5v line. That would fry the USB drive and i know this for a fact because it happened to a friend of mine when he purchased a 'powered' USB hub and they sent the wrong wall wart...a 12v DC instead of 5v DC wall wart! Fired the port and all the USB drives plugged into the hub! What a mess. The web site made good on it though by sending him a 64MB USB thumb drive as well as a new hub with correct wall wart.
Anyway, if that happened i would think that we could have +12v on any of the 4 lines excluding ground maybe and that could have a 60 amp rating associated with it so it would be able to fry just about anything, even a 1 amp diode which is the type i was thinking about. So maybe the only way is complete disconnection from the port when not in use. That's where the switches would come in, but i guess that's a lot of work just to get it isolated without actually unplugging it.

My real need for this came when i realized that i am constantly plugging and unplugging the USB drive (two of them actually) and it must be wearing out the connectors because it seems to have gotten much easier to physically plug and unplug. That cant be good.

I also considered connecting two extensions with a lever arm to plug and unplug...that would only wear out the extension connectors but would be a pain to build i think. Wires would have to flex each time too.

Any more ideas appreciated.

[Robert Reed]

I always think of chips internal protection diodes more for short term static discharges. Sounds like you need something absolutely goof proof as the chips diodes have very small dies to limit shunt capacitance to a minimum. For sure they could never carry an inadvertent steady state overload. I suppose zeners and resistors would be the best approach as you mentioned, also zeners are super quick.

[MrAl]

Hi Robert,

Yes, and some of the chips have the rating right in the spec's, such as many PIC chips which rate the input diodes at 20ma max. This comes in handy sometimes.