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

how to find positive and negative wires

This multimeter is dialed in to measure up to 20 volts DC, appropriate for most AC adapters. You can use the red and black leads to find positive and negative DC wires.

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.