They're designed to run on the 120-volt AC power supplied to homes in the U.S. Alternating current or AC, constantly changes polarity, sending current one way through the circuit, then reversing and sending it the other way. It does this very quickly -- 60 times per second in most U.S. electrical systems.
AC will display as zero voltage on a DC setting, regardless of the polarity. Reversing the leads and still reading zero will distinguish this from DC with reversed polarity if the meter displays zero for reversed polarity DC. Measuring DC on an AC setting will depend on the meter. The meter rectifies AC to turn it into DC for a reading. It includes 3 AC outlets, as well as a 2 Amp USB port perfect for all of your mobile devices. Connect the unit directly to the vehicle battery with ring connectors. Protection against power surges and overheating is built into every unit. Converts 12-Volt battery power to 120-Volt AC household power. Easy to use connects directly to battery. First stage of SMPS rectifies 250V AC to maybe around 300V DC. This voltage is then used to supply another stage of SMPS, but that's not important for this question. So basically from 2 stage of SMPS onwards, SMPS knows no difference between AC and DC supply voltage. Advantage: 0 cost of inverter, 0% losses on DC to AC inverter Disadvantage:voltage; type of voltage: AC or DC ; current; If your device or broken wall wart says 9 V DC, get a 9 V DC replacement. The right voltage and type of voltage are important: a too high voltage may damage your device, a too low voltage too, though that's less common. But at a too low voltage it may not work properly.AllanJ. 10774 posts · Joined 2007. #2 · Apr 30, 2010 (Edited) What comes directly out of a 120 to 12 volt step down transformer is roughly 12 volts AC. The actual voltage measured may vary depending on the load. For model trains, computers, and various other uses the AC may be rectified (into DC) using other components in the transformer 2. Use the single phase equation. The single phase AC power equation that relates volts, amps, and watts is similar to the equation used for DC power. The difference is the use of the power factor. [10] Amps = Watts / (PF X Volts) where the power factor (PF) is a unitless value. 3.