A Beginner's ac Ammeter Project How to Measure Actual ac Current Without Cutting Your Power Cord Joel Chinkes c. 1992 Joel Chinkes Disclaimer: This project is designed for the complete beginner, but not for the complete klutz. It is possible to kill yourself and/or damage your property if you are not just a little bit careful. Be prepared to take responsibility for your own actions. All solar energy books tell you to first add up the power requirements of your lamps, tools, and radios. What they don't tell you is how to get an accurate current measurement without disassembling the power cord. Why not simply rely on product labels when adding up your loads? Surely the manufacturer has tested power draw at the factory. Well, yes and no. Every ac gizmo is labeled with some sort of power consumption number (watts or amps), but these numbers are often overstated or misleading. My method will also simplify finding "phantom" loads. A phantom load is an appliance which is switched off, yet continues to draw a small maintenance current. This may be to keep a memory chip alive, or for some timekeeping function. My video cassette player, for example, is a phantom load. It "stands by" using electricity 24 hours a day, waiting for somebody to stick a VCR tape in its mouth to suck on. What a waste! Custom probe An ordinary ammeter needs to be connected in series with the current it is measuring. Most appliance plugs are molded plastic, and the power cord disappears unbroken inside the gizmo. With lethal voltages at the plug, you have no safe place to attach an ammeter. Until now! Enter the Fiendishly Clever Extension Cord (FCEC): a custom wired dual plug that will accomplish our goal using only cheap, commonly available parts. The secret is the fact that ordinary duplex outlets have break-off ears connecting the two pairs of contact screws. These ears are provided on the off chance you want to separately control the two halves of a duplex plug. This could happen when half a duplex hall lamp outlet is controlled by a switch near the door, while the other half is live all the time. This is why there are four screws provided when two screws would do. In this case, we will break only one of the two ears off, which is probably a violation of some silly rule. Even more non-standard, we attach the incoming power cord to the two screws on the same side of the duplex outlet, whose Siamese Twin status has just been severed when you ripped its ear off. On a normal ac extension cord the power cord goes to either side of the plug. Here, both sides of the incoming power have gone to the same side of a plug with one jumper "ear" removed. Once you have this wacky wiring in place, button everything up in a safe receptacle box with lid, and you are ready to take some measurements. Very Important: Put a big red label on your project that says "Test only Ä not an extension cord!" Here's how you connect up in four steps in order: 1. Plug your appliance to be measured into one side of the (F.C.E.C.). 2. Stuff your ammeter probes into the other side of the F.C.E.C. 3. Plug the F.C.E.C. into a wall socket (Phantom loads show up here.) 4. Turn on the appliance and read your ammeter Electrons from the wall socket will now race up one side of the ac cord into your appliance, out the other side of the appliance, across the remaining ear inside the plug, into your ammeter, then out the other side of the ammeter back to the wall socket. This complete circuit puts your ammeter in series with the load being measured, simply by plugging them both in to an ordinary outlet box, wired a little funny on its insides. See figure 1. Error Conditions Suppose your ammeter is not set for ac amps. Or you forgot that the appliance might draw 20 amps and you left the ammeter on its 50 milliamp range. Or you only have a 10 ampere scale on your ammeter, but the toaster you are measuring has a 1500 watt sticker on it. These are user errors, so don't come crying to me about your fried meter. Suppose you can't make good contact with your ammeter probes inside the ac outlet? You might want to construct an ammeter probe with a regular ac plug on one end, and banana plugs (or whatever fits your ammeter) on the other end. Recall that you are dealing with lethal voltages, so you want to be particularly careful with the exposed banana plugs if the ac end gets plugged in some place it shouldn't be. If you plug your ammeter probes into a live outlet, expect to smell smoke! If your ammeter lacks an internal fuse, consider putting an external fuse into this circuit to protect your life and property. Fuses are always cheap insurance. Likewise, you should ground wires. If you choose to use three prong plugs and sockets, connect the ground wire in the usual way. It should be common between both sides of the duplex outlet. Keep this ordinary looking but non-standard extension cord in a safe place when it is not in use. Others may mistake it for what it isn't! Access Author: Joel Chinkes learned everything he vaguely remembers about electricity at The Bronx High School of Science and can be reached c/o Home Power, POB 520, Ashland OR, 97520