How Are AC Adaptors Rated?

July 1, 1998 1:23 PM, Edited by Robert J. Lawrie

Q. I would like to know how AC adapters are rated, so I will not damage an electronic device. For example, I have a small calculator that operates on four AA batteries, or 6V DC and is rated at 4W. I would like to use an adapter that has a rated output of 13W at 6V and 600mADC. The voltage may be AC or DC. Can that wattage rating cause a problem? Also, I have a number of adapters rated at different values, such as an input rating of 120VAC, 5W, and an output rating of 3VDC, 300mA; another has a rated input of 120VAC, 18W with an output rating of 12AC, 1A. When can these be used? —R.E.

A. AC adapters and AC to DC converters (the small plug-in type) that R.E. refers to are rated in volts (output), amps (current capability), and watts in about any combination. Since the voltages and currents are so low, it is usually acceptable to use the formula: watts = volts x amps to find any unknown values. Note the nameplate ratings may not always come out right with this formula because there is a substantial reduction in output voltage with load on these devices. Most of these units are essentially a simple two-winding transformer to provide the required AC output. If they are UL-listed, they are usually thermally protected. Some may be furnished with a diode, half-wave rectifier to provide a DC output. Certain adapters are designed to supply special items, such as TV games, laptop computers, etc., and they more complex and often supply several voltages. If the adapter supplies AC power out, there is no concern for polarity. However, units supplying DC pose certain concerns. First, as in AC applications, the voltage and current must be compatible with the device. Second, the polarity of the plug must be correct.


A. R.E. should keep three basic principles in mind when selecting an adapter to use with an electronic device.

First, if the device requires AC, be sure to use an adapter that provides AC. Similarly, if the device needs DC, use an adapter that supplies DC. This sounds basic, but too often the wrong adapters are used. Be sure the adapter is marked with an “AC” or “DC” imprint. A device requiring DC power is polarity sensitive; the “+” power output conductor of the adapter must be connected to the “+” power terminal of the electronic device. The same is true for “-” conductors.

Second, the voltage output must be exactly equal to or greater than the voltage input requirements of the device. The only exception is if an adapter is specifically labeled by the manufacturer as suitable for use with a particular device.

Third, the current output of the adapter must be equal to or greater than the current input requirement of the device. For R.E.’s question, his calculator requires 6VDC at 4W. Using Ohms Law, we find 4W divided by 6V = 0.667A or 667mA. The adapter he wants to use is rated at only 600mA and 6VDC. Thus, that specific adapter is too small to use with this device. He should use an adapter that will supply at least 700mA at the required 6V level. — F.M.P.