Installing CARDs in Power Strips.


Power strips with their surge arresting capability are a logical way to implement current actuated switching. At least here in Colorado Springs surge arrestors are really necessary.

APC (American Power Conversion), among others, makes good surge arresting power strips. "American" is a laugh. The parts are all labelled "made in China." A good power strip has series inductors against which typical metal oxide varistors - MOV's - can work. Strips which just have the MOV's and no inductors depend on the impedance of the household wiring and have no expectation of working for any length of time.

I picked up a few APC strips from at a good price and they have room to install the current actuated stuff I want. Some APC units provide for surge protection on telephone lines and space seems to be provided for that in their injection molded plastic parts whether they use it or not. Personally, I don't think telephone circuits should ever be in the same box with power wiring.

Unit the First:

The APC PRO8 has a semicircular appearance and comes equipped with three "wall wart friendly" sockets. Two of them are labelled "always on" while the other six sockets are connected through a switch. My price was $9.95 but the suggested retail is much higher.

The simple scheme for current actuation is to take the two always-on connections and declare them to be the master into which you will plug your controlling device. Of course one needs to wonder why there should be two sockets for that and I have no answer except that it's an easy mod. All you have to do is remove the black wires running from the surge board to the always-on and switched sockets. A close trim with a pair of wire cutters is all that is necessary.

The next step is to find room for a relay and a CARD device. The picture below shows how I did it. Starting at the unswitched printed circuit pad of the surge board, I ran two turns through the hole in the CARD and terminated on the bus bar that services the always on receptacles. Starting at the switched printed circuit pad I ran to the common terminal of a relay. From the normally open terminal I ran to the bus bar for the remaining receptacles. One side of the relay coil is connected to the common terminal where 120 volt power is present. The other side goes to one of the CARD wires. The other CARD wire is connected to the common wire on the surge board.

Click picture to enlarge.

In the picture you can see the relay at the lower left. It is in a cavity that probably houses telephone stuff in APC's other models. The CARD is just below the printed circuit board. Dress of the wires is important because of the various appendages on the plastic bottom panel which hold the receptacle parts in place after assembly. You want to be very sure that you don't squeeze a wire during reassembly. I did the power wiring with AWG 16 teflon insulated wire which is what I had available. Even though APC's wiring is AWG 14 it simply isn't necessary. The National Electric Code specifies AWG 14 for 15 amp service but there are exceptions for short runs which are what we're talking about. The real point is that you really aren't going to draw that much. AWG 22 is rated for 5 amps which will power essentially all personal computers. Typical household extension cords are AWG 18. An older laser printer with its big heater - 10 amps intermittent - might benefit from larger wire.

Unit the Second:

I had another APC unit around, a model NET7 . This one has just a single row of receptacles and a master switch. The procedure was to remove the hot bus bar and cut off one of the receptacles. There are some holes in the strip and by being a bit shrewd you need to punch only one new hole to provide for soldering two new wires to the bus.

As before you start by disconnecting the black hot wire which runs from the PC board to the receptacle bus. One new wire runs from the PC board through the CARD and then to the single receptacle we cut loose in the previous step. Other new wires run from the PC board through the relay contacts to the power bus for the rest of the receptacles. As before, one CARD wire runs to the relay coil and the other to power common on the PC board. The remaining pin from the relay coil is connected at the relay to the power wire coming from the PC board.

Click picture to enlarge.

In the picture you can see that there is space at the right for the relay and the CARD to share a common cavity. Some of the APC units use the space for telephone protection. Once again you have to be very careful about plastic appendages in the bottom panel. They hold the receptacle buses in place and must not trap a wire. I found it necessary to trim some internal plastic to make room for my wires.

Other Strips:

The two strips above have space for the relay and a receptacle structure which is easily modified. Other strips may not be so cooperative. You need the relay but you might be able to remove a power switch to make room for it. Some relays are suitable for external mounting with their connections available through holes you might drill in the surge strip. For the sensed circuit you can make up a pigtail with a female receptacle at the end which attaches to a controlling device - a computer. That way all you need is a hole in the side of the power strip for the wire which can be a standard power cord with the plug cut off.


These suggestions require a certain amount of knowledge and skill. You need soldering equipment and you need to know how to use it. You will, of course, be voiding any warranty provided with the surge strip. It's possible to burn a house down or hurt yourself by doing something stupid. Please don't do either of those. By opening the power strip you are accepting responsibility for your actions.

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