Digial Voltmeter DC and AC mode

Usually, yes. On most DVMs, the DC range is heavily averaged/lowpass filtered, so the AC component is mostly ignored. And on the AC range, it's AC-coupled, and response drops off below 10 Hz or something like that, so the AC range ignores any DC component entirely.

It needn't be so, though: some meters read the *true* RMS voltage on their AC range, which includes any DC component; but that's rare.

I think the old analog Simpson-type VOMs may have responded to DC on their AC range. Anybody know for sure? Did they have a series cap, or just a rectifier and the meter movement?

John

Probably - although there may be some meters where this is not so.

Cool: the AC range is DC coupled, but the OUTPUT range adds a series cap!

John

aman wrote:

no. the DMM (digital Multimeter) will give you DC (direct current) from a steady source of energy , like a Battery for example. AC voltages are alternating types of energy meaning, take the battery and keep reversing the leads on the meter at 60 times per second or 50 times depending from what country your coming from. this Voltage is changing polarity.. lets look at it from a more simpler view. a common cell(1 part of a battery) (carbon) puts out aprox 1.56 volts new. imagine putting this cell on a rotating plate on it's side. now imagine the probes are on the edge of the plate equally spaced apart. as the plate turns with the cell lying on it, the - + terminals are getting closer to the probe tips, lets say the meter is reading the approaching terminals of the cell giving you a stronger reading until they are perfectly lined up with the 2 probes! at this point the reading is at its strongest. note that the DMM may have the - or + sign on the left of the reading indicating the polarity. now the cell is still turning and as it does the voltage is now going down. when the 2 cell tips are at the far points of each probe tip, the voltage reading will become 0. the cell keeps turning and you then will see voltage start increasing again ! but this time the polarity is now the opposite as it was before. this continues until we get back to the starting point. this is call alternating current because the voltage is raising and falling back and forth alternating its polarity as it does so. Hence the need for a meter to have a different mode to display to you the Average or RMS. (root mean Square) reading which is really not the highest voltage of the sine wave (PEAK). for example. 10 Volts RMS. would be something like 10.4 if this AC wave was converted to a clean DC to represent the peak.. so when measuring that 120 Volts of house current , the actual peak voltage is around 160 volts. any ways.. enough of this.