A Broken Circuit

Most of the time in electronics, we strive mightily not to build broken circuits. Sometimes, though, a broken circuit is exactly what we need.

If you’ve ever had to take the measure of current in a circuit, you’ve no doubt been irritated by the need to find a way to break the power line entering that circuit so you can insert an ammeter in series with the power flow. There are various ways to avoid the need to break the circuit, but it’s often the best way to make the measurement. When it comes to measuring the current draw of an entire wall-powered gadget of some sort, breaking the power inlet circuit is often most inconvenient. Should one get out the screwdrivers to open the device up, so you can break into the circuit at the back end of the power inlet? No need! With a broken circuit in your tool box, you can just insert it inline with the power plug just like any normal extension cable, then plug your meter into it to heal the break, supplying power to the device to be tested and getting a current measurement at the same time.

Before I go further, there’s an off-the-shelf device that does something similar, and does it very elegantly: the Kill-a-Watt. I have one, and love it, but sometimes it’s not exactly the right thing. The broken circuit has some advantages over a Kill-a-Watt, the best of which is that it lets you use an arbitrarily good DMM or VOM to get a more accurate measurement than the cheap little Kill-a-Watt can provide. (For more on selecting a good meter, see another of my articles, How to Buy a Multimeter.) A broken circuit is a bit bulkier and less elegant than a Kill-a-Watt, but it can be built for about half the cost, if you already have the meter.

Other alternatives to the broken circuit are clamp meters and shunts. I cover those in Tangent Tutorial #6: How to Use a Multimeter.

Parts

The basic parts are a power extension cable, a project box, and a pair of banana jacks. You might also want some dedicated test leads to go between the broken circuit itself and your ammeter, though the test leads you already have may suffice. There are some other bits an pieces you might want to add to this collection of parts, covered inline below.

Extension Cable

I prefer a short extension cable, perhaps 5 feet long, so that it’s just long enough to extend from a power outlet near floor level up to a nearby working surface. Anything much longer is going to be a bit bulky to keep with your other tools, not to mention being a waste of good extension cord for which you no doubt have more frequent need.

You might be tempted to go the other way, and use a very short cord, but that removes one of the advantages of the broken circuit over the standard model Kill-a-Watt, that being that you can make the cord long enough to get all the parts you want to fiddle with up onto a normal working surface, instead of hunching down around a wall outlet.

I’ve built a couple of these broken circuits, and I don’t actually go out shopping for a short extension cable, shortness being against the nature of extension cables. Instead, I either take the shortest extension cable I have on hand and hack a chunk out of the middle to shorten it — saving the chunk for some later project — or I take a section off the end of a long cable.

If you’re getting the cable for this project by shortening a long extension cable, you will also need a replacement power plug, and an inline power receptacle. These are readily available in the electrical section of hardware stores. Be sure to get ones of the same quality level or better than the one the cable came with, so as not to diminish the overall quality of the repaired cable. If you’ve never replaced a power plug or receptacle, it’s pretty easy.

The wires in your cable should be color coded, as should the contacts inside the plug and receptacle. There is some variance in these color coding standards, but the one shown in these pictures is the most common, here in the US at least: white for AC neutral, black for AC line (“hot”), and green for earth ground. The plug and receptacle will typically have a steel screw for the neutral connection (possibly plated with zinc or some other silvery metal), a brass screw for the hot connection, and a green-painted screw for ground. That is, silver to white, brass to black, and green to ground; the mnemonic value of this convention should be fairly clear, to a native English speaker, anyway.

If you’re using a cheap extension cord with no color coding, you’ll have to work out the correspondence between the plug connections and the wires yourself.

It’s easiest to use a round cable, as that makes cutting the holes in the project box easier than if you use flat cable.

You might also want some properly-sized strain relief bushings for your cable. In the broken circuits I’ve made, I just use hot glue on the inner surface of the box around the cable to prevent the cable pulling through and straining the connections inside the box.

Project Box

I used a Radio Shack 270-1801 project box, 3 in × 2 in × 1 in.

Something a little smaller could work, but beware that we’re working with AC line power here. You want enough room inside the case to be able to separate everything decently. In the pictures, you can see some exposed solder joints, and some covered by heat shrink. The heat shrink is a mere nod toward safety, and more a matter of neatness, really, not something to prevent shorts. I’m depending much more on air gaps here for insulation. Heat shrink is nowhere near as thick as AC line insulation, but air is an excellent insulator.

Speaking of insulation, I can only recommend using a plastic project box. You could use a metal box if you took care to keep the air gaps nice and big, but why run the risk? The Radio Shack case I mentioned above comes with both an aluminum cover and a plastic one; I used the plastic one.

Banana Jacks

Banana jacks are widely available. I’ve seen them at Radio Shack, Best Buy, hardware stores, etc. They’re commonly used for speaker connections, but also for test equipment, which our broken circuit is.

I prefer the sort with solder lug connections. You can also get them with various sorts of pressure fit connections, which are usually less reliable.

You can get banana jacks in pairs, exactly spaced to allow for standard dual-banana cabling, which may be convenient. A lot of test equipment uses this spacing, as does the cabling made for it.

The only thing I’d look out for is that you want to avoid the sort with an exposed metal snout. Again, we’re working with AC line wiring here. The possibility of accidental shorts is too great with exposed metal contacts.

Meter Cabling

Your meter no doubt came with test leads, typically with sharp probes on one end and banana plugs on the other. You can use these with the broken circuit, but a better sort is a pair of dual-ended banana jumpers. One end goes in the meter’s current-measurement sockets, and the other end into the broken circuit.

I made my first set of banana jumpers from Radio Shack parts, and they work fine. You need a set of replacement DMM probes, and a pair of banana plugs. The handles on the Radio Shack probe set unscrew from the tips, and then you can just heat up the solder joints on the tips to release the cable. Cut off the soldered ends of these cables, strip back to expose fresh copper, and put the banana plugs on.

Alternately, Pomona makes all kinds of weird test equipment cabling, including banana jumpers. You can get Pomona leads at many electronic parts distributors, including Digi-Key and Mouser.

Assembly

There’s really not much to surprise you in this project. The main thing is to remember that you’re working with AC line wiring here, which can cause damage or even kill you if you do it incorrectly. This project is so simple that what I’ve done should be immediately obvious to any experienced electrical or electronics DIYer just by looking at the picture at right. It’s probably clear even to many a DIY novice. Still, you should read through these steps carefully just to be safe, and because I might have figured out something that will help you to do a better job of building it:

1. Cut the extension cord about 6 in from the female/receptacle end. This will put most of the length of the cable between the broken circuit and the wall plug, so that the project box with the broken circuit connections can be conveniently placed up on your working surface.

2. If you use the sort of cable I show above, there are two layers of insulation. Strip back the outer layer half the length of your project box, or a bit more. For instance, if you have a 3 in long project box, somewhere between about 1-½ in and 2 in will serve you well.

   We’re not exposing any copper at this point. That comes later.

3. Drill four holes in your project box, two for the cable ends, and two for the banana jacks. The plan I show in the pictures works best, I find, as it reduces the risk of electrical shorts by keeping everything about as far from everything else as it can be, except where circuit connections are necessary.

4. Mount the banana jacks in their holes.

   You might want to take some pains to ensure that the jacks don’t become loose. If the screw threads are metal, I suggest LocTite. For the plastic-bodied jacks in the plastic case I used, I dribbled a bit of MEK onto the mounting surface between the case and the jack from the back side, which dissolves the plastic slightly, causing the plastic surfaces to actually meld together, creating a strong mechanical bond when the MEK evaporates away. MEK and other solvents like xylene and toluene are what’s in plastic model cements. I imagine the best choice of solvent depends on the plastics being fused, but MEK just seems to work, for me. You can find it at most any well-stocked hobby store.

5. Stick the extension cable ends far enough into the enclosure to give a secure fit, then figure out how far back to cut the AC line (“hot”) wire. (Black, typically.) You want it to be just short enough that it reaches the banana jack with enough exposed wire to make good contact. The other wires will remain long, so they can bow out and away from the banana jacks.

   Strip back the hot wire’s insulation anywhere between about ¼ in and ½ in. If your banana jacks use solder connections, you won’t need as much exposed copper as if it uses some sort of pressure connection. Make these connections now.

6. Strip back the insulation from the remaining wires, exposing about ¼ in of wire. (I’m being deliberately vague on the number of wires, as you could be using either a 2-wire or 3-wire extension cable. It really doesn’t matter which, for our purposes here.) You might need to re-trim these wire lengths a bit first, to ensure that they bow outward away from the banana jacks, but not so much that you’re going to have to find a way to stuff excess wire into the enclosure, risking shorts.

   Slide heat shrink over these wire ends, solder them together, and shrink the shrink. You could use wire nuts instead, but they’re bulky and don’t work well on the stranded wire typically used in extension cables.

7. Add some sort of strain relief to the areas where the extension cables enter the project box. I used hot glue on the inside of the enclosure around the cable. You might be a little classier than I, and use strain relief bushings.

8. Close it up, and exercise it, as below.

Using the Broken Circuit

There are two ways to do this, primarily depending on which meter cabling option you chose above.

The only common step is to set your meter on its highest AC current measurement setting, and find out which jacks on the meter correspond to this setting. It’s common for the high current setting to have a dedicated measurement jack, separate from those used for voltage, resistance and other measurements. Sometimes there are two dedicated current measurement jacks, with the second being for a low current range, while on other meters the low current range shares a jack with other functions. If you’re not sure which jack is which, RTFM!

Banana Jumper Method

Use your banana jumpers to connect the meter to the broken circuit’s jacks. The broken circuit as described here is designed for AC measurements, so you can’t hook it up “backwards.” AC current doesn’t have a polarity any more than AC voltage does.

(You could build a DC broken circuit on the same principles as the one I describe here, perhaps using barrel connectors instead of an AC extension cord. In that case, polarity would be an issue.)

Plug the DUT into the receptacle end of the broken circuit’s cable, then plug the other end into the wall. The DUT should power up, and you should get a current reading on your meter. If not, go back over the assembly steps!

This order of operations and the use of banana jumpers ensures that your hands and face are away from both the broken circuit and the DUT when you make the final connection, applying wall power. If something’s wrong enough to cause something to explode or catch fire, you are unlikely to be harmed while cowering under the desk like that. Get up, find a fire extinguisher, fix the immediate problem, then you can worry about the proximate problem.

Test Probe Method

The steps for using plain old test probes with the broken circuit are very similar to the banana jumper method above, except that you can’t really heal the break in the circuit with the meter’s test leads until after the AC wall and DUT connections are made.

The downside of this is that if something goes badly wrong, your hands and face are up above the work surface, exposed to this badness.

Standard test probes do have one advantage over banana plugs, however, which is that they’re designed to keep your fingers well back from the point of contact. If you are startled, you’re likely to jerk the probes back, breaking the circuit again.

Be careful anyway, and do reconsider building or buying yourself a set of banana jumpers!

I Get No Reading!

If everything appears to be connected properly but the DUT stays powered off and the meter reads 0 A, you’ve probably blown the current measurement fuse, if the meter has one, or the meter itself otherwise. There are so many ways to blow a fuse or fry a meter that I won’t bother to list them all here. Bottom line, you screwed up somewhere, and now have to figure out how.

It’s possible that you blew the fuse in the meter prior to embarking on this project, and are only now realizing it. So, take the fuse out of the meter, if it has one, and measure its resistance. The voltage and resistance meter sections are typically protected by a different fuse, which is why you can a) not realize the current section’s fuse is blown immediately, and b) use a meter to test its own current measurement fuse.

If the fuse’s resistance is low, the problem is not the fuse.

If it’s high, replacing it could fix the problem, or you could blow another fuse, which are expensive in the better class of meter. If you blow two fuses in a row, you’ve either got a bad connection somewhere, or the DUT draws more current than your meter can tolerate. In the latter case, what you really need is a shunt or clamp meter, not a broken circuit, but that’s a topic I cover elsewhere.

After the First Successful Reading

Above, I recommended setting the meter to its highest current measurement range, at least at first. Many meters have two ranges, a high one without very great accuracy, and a much more accurate one which will only tolerate some hundreds of milliamps, typically. If your meter is of this sort and your initial measurement says the device is only pulling, say, 0.02 A, you will now probably want to put the meter into its more accurate mode, which typically gives milliamp-level precision, or better.

If the measurement is right on the border of what your meter’s low current range will tolerate, you should stick to the high range. Almost every electrical device made has some sort of peak inrush current, or some operating mode where it takes more current than it typically does, either of which could damage the meter or blow its fuse.

When you’re done testing, unplug everything in reverse order. In the banana jumper case, that’s wall plug first, DUT second, banana jumpers last. For the test probe case, it’s test probes first, wall plug next, and DUT last.