A USB Power Controled Plug Strip. With Isolation.




The whole point of this Instructable was to allow me to power on all the accessories for my computer without thinking about it. And then not power all the little power vampire wall warts when I am not using the computer. The idea is simple, you power on your CPU, all of the other parts of the system power up (monitor, laser printer, speakers, etc) When you power off your CPU, they follow suit.Now there products out there that will do this for you, and if you do not have the experience working with line voltage electricity, please stop reading and just go buy one. There are several products that do exactly what we are trying to do here, but of all the ones that I have reviewed have disadvantages over the device that we are going to build. They fall into three basic types:

There are cheap usb controlled power strips, but I have seen several that do not offer any isolation, and if you are creating a possible path for line voltage (120v here in the USA) to your mother board, and its many hundred dollars worth of over-clocked goodness. I would like some isolation.

There are current sensing power strips, One of the outlets is set up to sense current flow. When this happens the electronics in the power strip power on the other outlets. Its a good idea, but sometimes they do not sense correctly, and will not turn on the accesories. Also the electronics require yet another little power supply to be on 24/7, this we are trying to avoid.

There are well designed enterprise grade solutions with isolation, that work very well and have a very hefty price tag as well.

This circuit uses no extra power when it is not in use, and offers some hefty isolation from power surges, and does not cost a fortune to build.

Step 1: What You Will Need

First off if you do not feel confident in your ability to work with line voltage power, please stop reading. If you build this project wrong, you have the ability to destroy your motherboard on your PC. I'm not kidding.

The heart of this system is two things really, the actual switching is done by a DC controlled solid state relay, All the isolation is provided by a pair of fuses and some transient voltage surge supressing diodes (TVSS)

All of the other parts are really up to you, I used what I had kicking around. Which was mostly standard electrical fittings, and an old plug strip, and a heatsink from a junk processor, and a USB cable that was miss ordered with usb "A" connectors on both ends. Feel free to use whatever works for you.

All told the parts that I had to order (fuses and holders, TVSS, and Solid State Relay) were less then $30.00 USD from an online supplier.

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.

Step 3: Enough Talking Time to Build.

I chose to mount all of the components in a standard electrical box, for low load situations the box itself is probably enough of a heatsink for the SSR. But I had an old processor heatsink that was pretty good fit. So it was added to the box behind where the SSR mounts. Don't forget to add some heatsink compound between the SSR and the box, and some between the box and the heatsink.

The cord for the Plugstrip is cut in half and run through the electrical box. The neutral (white wire) is spliced with a crimp terminal. The grounds (green wire) are spliced and connected to the metal chassis for safety. The hot wire (black) is connected through the SSR with crimp terminals. This concludes the line voltage wiring.

The +5V and Ground from the USB cable (pins1 and 4) are are connected to one end of the fuse block, and one TVSS The TVSS diodes are simply crimped into the connectors for the fuse blocks. Simple, fast, easy. Then two wires are run from the other end of the fuses (with another TVSS) to the control terminals of the SSR. Most SSR's will have one of the control terminals marked for the positive (+) lead. Make sure to get the polarity right.

Make sure to insulate the other two wires in the USB cable from each other and the metal case. If you do not you can short out the USB buss and cause all kinds of other problems.

This concludes the wiring.

I added a small piece of plastic (recycled blister packaging) to form a voltage barrier between the high and low voltage sides of the case, as extra insurance.

Step 4: Close It Up and Test!

So with the wiring completed. and checked. Its time to close it up and test. I recommend starting with things other then your computer and all its toys for testing. Plug a light bulb into the plugstrip. Plug the power strip into the wall. If everything is correct, nothing will happen. Now DO NOT USE YOUR COMPUTER FOR THE NEXT STEP. Find yourself a USB charger from your iWatever, and plug it into the wall. When you plug the USB cable from the unit into the charger the lightbulb should light. This way if you get anything wrong, the most you should be out is the cost of the charger.

Troubleshooting: If it does not work check the switch on the power strip. Then check if you have the polarity from the USB cable to the SSR correct.

Hope you have enjoyed this. Its a fun little project. And has the side benefits of making your life easier, saving you money on electricity, and doing your part to save the planet.

Keep tinkering,



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    117 Discussions


    1 year ago

    This is a very old thread, but it is the closest to what I am trying to do. Does the lay jump whenever the USB has a draw, such as the computer being on? Or, could I programmatically turn it on and off using some sort of application installed on the computer to which it is connected?


    4 years ago on Introduction

    The best isolation would be to not use the wall outlet at all and use something else. 9first idea should be how simple can i solve this then go from there.


    5 years ago on Step 4

    Thanks for this. If you wanted to use two independent sources to control the power strip, what would be a proper way to do this? In other words, two USB inputs from two computers so that whichever computer happens to be in use controls the power strip. How would you isolate (if that's the right word here) the two 5V inputs from each other? Thanks for any help.

    1 reply

    Reply 4 years ago on Introduction

    The safest way is to use two solid-state relays, output pins parallel to each other, inputs are driven by the PCs.

    Another option is to connect USB GND from the PCs directly, and USB +5V outputs via regular diodes, the asterisk gives the 5V output to the control circuit:

    PC1 --|>|--*--|<|-- PC2

    Joel H

    5 years ago on Introduction

    I have used a similar approach which also works when my computer is in "suspend mode". So when my computer suspends my reciever also shuts down since its connected to the relay. Instead of hooking the relay directly to the usb-port its wired to a "USB To TTL / COM Converter Module" which has pins for suspend mode. The module is about 3 dollar on ebay



    5 years ago on Introduction

    A nice and safe alternative is to use a Wattstopper power unit which is UL approved, has zero voltage switching and as a bonus has DC output at about 150 mA to power your controller. One such unit (B-120 EP) is available from Amazon for about $15. - Enjoy.


    6 years ago on Introduction

    Hi lads,have read all this,i know is such of old thread,but like to ask how these tvss diodes working? How its proctecting circuit and blows the fuses? And why there are two of these on each side of fuses? Thanks

    3 replies

    Reply 6 years ago on Introduction

    Ok, TVSS diodes are are also known as avalanche diodes. Under normal operation they do not conduct. When the voltage rises to a designated amount they begin conducting. So in this case we are using 7.4 VDC diodes, in a circuit tht is powered by USB power at 5 VDC. So as long as the voltage does not rise above 7.4 VDC they do nothing. If the voltage does rise above that, they begin conducting. This will overload the fuses, and make them fail open, disconnecting the two sides of the circuit.

    Its probably a bit of overkill in this circuit as the solid state relay already has quite a bit of isolation built in. But its a second line of defense, which is good, as computers are quite expensive.


    Reply 6 years ago on Introduction

    sorry once again for my silly questions,but is this protection in case relay fail and starts give an AC power to pc usb port? is this possible? can't understand why should be another reason that usb power should rise above normal 5V?


    Reply 6 years ago on Introduction

    tell me,can be used one diode and one fuse? or this won't work? is the zener diode will do the same thing as these tvs ones?


    6 years ago on Step 2

    Hey Richard,
    This type of project is always going to draw comment from 'concerned people' and I have to admit doing a little data sheet checking myself when I first read it. Of course, people will come up with dozens of ways in which you should have made it safer or with more isolation etc, but I reckon this design is a good balance between isolation, function, safety and cost - there will always be tradeoffs and other ways to achieve the same thing or make improvements. UL, CE, EN standards, isolation transformers, circuit breakers.....hey, what about some thermal fuses in case things run away, or some fuses for the fuses, just in case they don't work?! There is always a 'whats reasonable' line and I'm sure you're well within it, especially for a non commercial, DIY project.
    Anyway - good instructible, well made and I like it. Nick


    i made this and i found a relay that was 40 amps i think but witch way do you install the diodes silver or black side facing + or -

    You can find solid state relays that will switch 30A with ease. The one chosen was selected for its low cost, and small size You can look up the OPTO22 120D45 http://www.opto22.com/site/pr_details.aspx?cid=3&item=120D4

    Its a larger form factor part, and is rated up to 45A. At those levels of power you are definitely going to need a heatsink, and you should do a little experimenting to make sure its large enough to handle the heat dissipation, or check with the manufacturer of the device you are using to make sure you have a large enough heatsink, or what they recommend. If you don't, if you are lucky, you will just make your device fail prematurely, if you are unlucky, you could cause a fire.


    7 years ago on Step 4

    I just gathered all the components to build a similar device when i realized that the +5vdc delivered to the USB port remains on when in standby mode. I have an HTPC that I connect to my TV. I rarely turn it off. Instead I use a wireless mouse and keyboard to put it in and out of standby. I would like to be able to turn off the peripherals when in standby. It seems like I might have to connect to a 5v power supply plug inside the PC instead. anyone know of a better solution?

    2 replies

    Reply 7 years ago on Step 4

    I can think a few possible fairly simple workarounds to try.

    1) Check read up on the motherboard in your HTPC. On some motherboards the USB power can be selected either with jumpers on the Mainboard, or in BIOS as to weather they have power when shutdown or in standby.

    2) If you have the room you can install a cheepo PCI USB interface, that should power down when you go to standby, you don't need anything fancy, I would suggest eBay.

    3) Check if your power supply powers down the +5v supply to the diskdrives when it goes into standby, the red wire is +5V and the Black is ground. There are usually a couple of extra connectors available.


    Reply 7 years ago on Step 4

    thanks for the suggestions.

    1) I need to have at least 1 port powered while on standby to wake it with my wireless keyboard. so this likely won't work, unless there is a separate jumper for front and rear ports.

    2) This is a possibility. If i can find a cheap one, and nothing else works, I might do this.

    3) This is another possibility, but requires a bit more work and makes it more permanent to this PC.

    It would be nice to have an outlet on the box you made where the PC could be plugged in. Then when the PC was turned on or woken up it would trigger the relay to turn on the strip. I'm not sure how the relay would be triggered though. it would have to detect the current or something,


    7 years ago on Introduction

    Wow so you made this a pretty long time ago, but I used this as a basis for my own USB activate power outlet. I needed a single outlet rather than full strip (and I don't really have room for a full strip), but it's still a really great instructable! I did forgo the extra isolation as optical isolation is pretty "fool-proof" so there's (practically) no risk of it jumping back down the USB line and my deal plugs into a surge protector so there's the AC line is also protected from surge back.

    Thanks again! Without this guide I definitely wouldn't have had the courage to set up my own system and would have ended up shelling out $30+ bucks for a pre-made "smart" power strip


    8 years ago on Step 4

    pretty good, a good use for this would be in a caravan, buy one of those usb smoke lighter thingy's and you could power a whole caravan with 5 volts of power, even one of those usb solar thingys power an entire caravan with tiny solar power