It’s possible to use a modern AC adapter to make a power supply for vintage machines. Sometimes this is desirable, because the modern supply may be more reliable than the old ones. But we have to know how to wire it. Here’s how to find positive and negative DC wires so you can wire the supply up properly when you change its plug.
To find the positive and negative wires, apply power to the AC adapter, set your multimeter to DC, then apply the red and black leads to the output and find a combination that gives you positive voltage. Whichever wire is connected to your multimeter’s red lead is positive, and the one on the black lead is negative or ground.
How to set your multimeter
Finding positive and negative DC wires starts with setting your multimeter properly. Most multimeters check a range of voltages. In my case, I wanted to change the plug on an AC adapter that provides 5 volts DC. My multimeter can has positions on its dial to check fractions of a volt, up to 20 volts, up to 200 volts, or up to 1,000 volts.
If your multimeter doesn’t say DC, the symbol for DC is a solid line with a dashed line under it. Use that one. The symbol for AC is a sine wave. You want the dotted and dashed line, not the wave.
I picked 20, since that’s closest to 5. I don’t do high voltage stuff. A more expensive multimeter probably would have a setting closer to 5 volts, but even my cheap Harbor Freight multimeter I got with a coupon is good enough for this job. I’m only worried about polarity, and whether the voltage is slightly higher than five volts. This setting gives me three decimal places so that’s plenty of precision for this use.
How to find positive wires
This AC adapter only has two wires going to its plug, so that makes it easy. With two wires, one of them will be positive and one will be ground. Attach the red lead from the multimeter to the wire you think is positive. If you have no idea, just guess. Attach the black lead to the other wire. If the voltage is a negative number, reverse the two leads. When you get the expected voltage, without a negative number, you know the wire your red lead is connected to is positive.
If you have multiple wires, each color of wire may output a different voltage. You’ll have to try different combinations of wires to see what they do. If you have +5, +12, and ground, you’ll get three different voltages depending on the combination: 5 volts, 12 volts, and 7 volts. Where’s the seven come from? Subtraction. When you connect two positive wires, the resulting voltage is the difference between the two. The two wires that give you 7 volts are positive.
Now, for a word about negative wires. Normally you don’t have a positive and a negative wire. Normally you have positive and ground. Or you have positive, ground, and negative. So let’s talk about that.
How to find ground wires
With two wires going to its plug, ground is easy too. With two wires, one of them will be positive and one will be ground. Attach the red lead from the multimeter to the wire you think is positive. If you have no idea, just guess. Attach the black lead to the other wire. If the voltage is a negative number, reverse the two leads. When you get the expected voltage, without a negative number, you know the wire your black lead is connected to is ground.
If you have multiple wires, each color of wire may output a different voltage. Try different combinations of wires to see what they do. If you have +5, +12, and ground, you’ll get three different voltages depending on the combination: 5 volts, 12 volts, and 7 volts. Look at the combinations that give you 5 and 12 volts. The wire both voltages have in common is the ground wire.
How to find negative wires
Negative wires can be trickier, but if you already know which wire is ground, it’s pretty easy. Any wire that gives you a negative voltage with a ground wire connected to the black lead is negative.
Alternatively, any wire that increases the voltage when you connect it to a positive wire is also negative. For example, some power supplies will output -5 and -12 volts. So if you connect a wire supplying +5 volts to another wire and get 10 or 17 volts, you found a negative wire.
I mention negative wires last because the easiest way to reliably find them is to map out your other wires first.
David Farquhar is a computer security professional, entrepreneur, and author. He started his career as a part-time computer technician in 1994, worked his way up to system administrator by 1997, and has specialized in vulnerability management since 2013. He invests in real estate on the side and his hobbies include O gauge trains, baseball cards, and retro computers and video games. A University of Missouri graduate, he holds CISSP and Security+ certifications. He lives in St. Louis with his family.
Another important point is to make sure that the test leads are plugged into the correct sockets. If you have them reversed, your indications of positive and negative will be backwards. The terminals will usually be clearly labeled, but both use the same type of jack so it’s possible to get it wrong.
Even an expensive digital multimeter probably won’t have a range that is closer to 5 volts. The most common display on a multimeter is a 3.5 digit display; that is, the high order digit only displays 0 or 1 and the other three are full range. More expensive high precision meters, mostly for bench use, may have a 4.5, 5.5, or 6.5 digit display, and a few companies have made meters with a 3.75 digit display where the high order digit has a range from 0 to 3.
In any of these, the ranges normally come in decades because that’s what makes sense for the display. In other words, you will have voltage ranges of 0.2, 2, 20, and 200 volts (or 0.4, 4, 40, and 400 on a 3.75 digit model). There is one higher range which would be 2000 volts in theory, but is usually limited to a lower number like 600 or 1000 volts because of limitations of the voltage ratings of the components in the meter (switches, resistors, circuit boards, wire insulation, etc.)
Old school analog VOMs and VTVMs are another matter. Those often have intermediate ranges because the human ability to get accurate readings from an analog display is limited, and you want to have a range that uses most of the scale of the meter for the voltages you are interested in. The classic Simpson 260 (which is still being made!) has ranges of 0.25, 1, 2.5, 10, 25, 50, 250, 500, and 1000 volts. For example, if the Simpson lacked that 10V range it would be a poor tool for measuring 5 volt logic and power supply levels because you would only be using 20% of the scale in the 25V range. A 5V range would also be poor for testing 5V equipment because there would be no headroom at all for overvoltage, and the usual 5 volt supplies have a 5% voltage tolerance. It would be a nice thing to have for testing modern 3.3 volt equipment, but the SImpson was designed long before that became a standard voltage for anything.