A USB Power Controled Plug Strip. With Isolation.

Step 2 The schematic!

The concept of this circuit is pretty simple. The 5 volts provided in the USB connector is used to turn on a large Solid State Relay, which turns on the power to the power strip. All of the power control is done by the solid state relay(SSR). If you have never used and SSR for controliing power it could not be any easier, the designers have taken all of the hard engineering out of thier use. And what you get is a box with 4 terminals. Two of the terminals are for the line voltage. The other two are for the control voltage. When you supply control voltage to the control terminals, the line voltage terminals turn on. Thats it. No, really. Its just a little black box. No further engineering needed. Inductive loads, motors, lighting, resistive loads. They don't really care as long as they are within there rated current range.

The relay I chose was a Z240D10 from OPTO22. It has a maximum rated current of 10 amps @ 120VAC. This should be more then enough for my desk. The control input accepts from 3-32 VDC. So the 5 volts from the USB connector is more then enough. It was also chosen for its low cost. If you need more current capacity you can order a larger SSR.

The protection part of the circuit is three fold:

The first line of defense is the actual SSR. It uses an optical isolation between the power and control rated at a 4000 volts.

The second part of the circuit is a pair of 125mA fuses that will blow if over loaded.

The third part of the circuit is a pair of (1.5KE6.8CA) 7.14v transient voltage surge supressing diodes (TVSS) These are similar to a Zeiner diode. When the voltage across the terminals exceeds a limit. They begin conducting. Except unlike the Zeiner diode, they are bidirectional. So if for any reason the voltage in the control parts of the circuit exceeds 7.14v they act like a short and blow the fuses. The power dissapation for these parts is rated at 1500 watts for 1 millisecond. Which is more then enough to blow the fuses and protect the circuit. Circuits like this one are used in various communication devices that are subject to lightning and power surges.

Comments

[saadmanna]

i have worked on similar Project with Parallel port and serial port.
i am searching some thing like this but much more advance...so that i can control  at-least 8 switches with USB.

[bwanaaa]

for some reason that link doesnt work- search for this part at digikey: SA12CALFCT-ND

[toymotorhead]

If the supply voltage is 12vdc you should look for a rating that is higher then that. I would look for something in the 16-20 volt range. A lot of 12 volt supplies actualy run closer to 14 volts. You want a bit of headroom before the TVSS clamp down on the power supply,.

[bwanaaa]

ok, this is great, but i want to build something like this so my audio receiver can control a powerstrip. The receiver has a 12 v switched miniplug. Would the design be the same? The only thing i'd have to change is the diode so that it has a 12v rating instead of 5 (like this one http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=SA12CALFCT-ND). The failure voltage of these diodes is 13,3v

[billth87]

UL would want you to put a fuse or a circuit breaker in the hot line before any of your circuitry. If your SSR shorts to ground you could start a fire with this design. It's a 10A SSR, so a 10 Amp fuse would be appropriate.

[GHopkins25]

This is similar to the one I built. I have one point to add. There is no termination for the data lines on the USB cable. There should be a 44ohm resistor on the data line to prevent excessive line ringing. It may not be a problem, but if you get any data errors this could be the cause. See this link; http://www.usb.org/developers/usbfaq/#sig3

[jmarler]

Why not use an optical isolator like the TIL116 to truly isolate the USB line? The TVSS are a nice touch if anything shorts out, but that doesn't truly "isolate" the circuit. If the surge of electricity is beyond the diode's capacity, your system will still be fried. With an optical isolator, the two circuits are physically separated, increasing the likelihood that in the event of extremely high voltage, your computer will be safe.

[toymotorhead]

It is probably not worth it to add something like a TIL116. There is already and opto isolator built in to the SSR is good for 4Kv vs 7.5Kv in the TIL116. But.... the real "isolation" comes from the fuses. If the voltage on the USB line goes over 7 volts the TVSS diodes avalanche and short out the system which blows the fuses. Something tells me an inch of open air in a blown fuse is somehow a better insulator from a surge, then say the .4 inches of space between the sides of a DIP sized opto. Cheers Richard.