This can explain better than I can http://www.bcae1.com/voltages.htm Some meters will read a DC voltage even though the meter is set to AC. A meter that reads TRUE RMS AC VOLTAGE when set to AC will so no voltage when a DC voltage is applied. The meter looks for the voltage swing and AC voltage swings (cycles) and DC voltage does not. The words TRUE RMS will show up somewhere on the box or paper work for the meter. Use a GOOD name brand meter like Fluke and you will be OK. A lot of the cheap off brand meters are not TRUE RMS meters. And a cheap meter can not see a AC voltage riding on a DC voltage. Trust me on this one I have learned the hard way. When you are looking for a DC and AC voltage on the same line you need a meter that can tell the two apart. Lets say you have a 12 volt DC and a 20 volt AC on the same line. You set a cheap meter that is not TRUE RMS on AC it will read 20 volts AC but set it to DC and it will read 20 volts DC. OK lets say the voltages are switched and now you have 12 volts AC and 20 volts DC on the same line. The cheap meter will show 20 volts AC even though the AC voltage is only 12 volts and it is the DC that is 20. I have learn this the hard way.